Astronomy. (continued 2)

Diogenes Laertius thought aerolites came from the sun; but Pliny derides this theory. The fall of aerolites in bright sunshine, and when the moon’s disc was invisible, probably led to the idea of sun-stones. Moreover Anaxagoras regarded the sun as “a molten fiery mass;” and Euripides, in Phaëton, terms the sun “a golden mass,” that is to say, a fire-coloured, brightly-shining matter, but not leading to the inference that aerolites are golden sun-stones. The Greek philosophers had four hypotheses as to their origin: telluric, from ascending exhalations; masses of stone raised by hurricanes; a solar origin; and lastly, an origin in the regions of space, as heavenly bodies which had long remained invisible: the last opinion entirely according with that of the present day.

Chladni states that an Italian physicist, Paolo Maria Terzago, on the occasion of the fall of an aerolite at Milan, in 1660, by which a Franciscan monk was killed, was the first who surmised that aerolites were of selenic origin. Without any previous knowledge of this conjecture, Olbers was led, in 1795 (after the celebrated fall at Siena, June 16th, 1794), to investigate the amount of the initial tangential force that would be required to bring to the earth masses projected from the moon. Olbers, Brandes, and Chaldni thought that “the velocity of 16 to 32 miles, with which fire-balls and shooting-stars entered our atmosphere,” furnished a refutation to the view of their selenic origin. According to Olbers, it would require to reach the earth, setting aside the resistance of the air, an initial velocity of 8292 feet in the second; according to Laplace, 7862; to Biot, 8282; and to Poisson, 7595. Laplace states that this velocity is only five or six times as great as that of a cannon-ball; but Olbers has shown that “with such an initial velocity as 7500 or 8000 feet in a second, meteoric stones would arrive at the surface of our earth with a velocity of only 35,000 feet.” But the measured velocity of meteoric stones averages upwards of 114,000 feet to a second; consequently the original velocity of projection from the moon must be almost 110,000 feet, and therefore 14 times greater than Laplace asserted. It must, however, be recollected, that the opinion then so prevalent, of the existence of active volcanoes in the moon, where air and water are absent, has since been abandoned.

Laplace elsewhere states, that in all probability aerolites “come from the depths of space;” yet he in another passage inclines to the hypothesis of their lunar origin, always, however, assuming that the stones projected from the moon “become satellites of our earth, describing around it more or less eccentric orbits, and thus not reaching its atmosphere until several or even many revolutions have been accomplished.”

In Syria there is a popular belief that aerolites chiefly fall on clear moonlight nights. The ancients (Pliny tells us) looked for their fall during lunar eclipses.—Abridged from Humboldt’s Cosmos, vol. i. (Bohn’s edition).

Dr. Laurence Smith, U.S., accepts the “lunar theory,” and considers meteorites to be masses thrown off from the moon, the attractive power of which is but one-sixth that of the earth; so that bodies thrown from the surface of the moon experience but one sixth the retarding force they would have when thrown from the earth’s surface.

Look again (says Dr. Smith) at the constitution of the meteorite, made up principally of pure iron. It came evidently from some place where there is little or no oxygen. Now the moon has no atmosphere, and no water on its surface. There is no oxygen there. Hurled from the moon, these bodies,—these masses of almost pure iron,—would flame in the sun like polished steel, and on reaching our atmosphere would burn in its oxygen until a black oxide cooled it; and this we find to be the case with all meteorites,—the black colour is only an external covering.

Sir Humphry Davy, from facts contained in his researches on flame, in 1817, conceives that the light of meteors depends, not upon the ignition of inflammable gases, but upon that of solid bodies; that such is their velocity of motion, as to excite sufficient heat for their ignition by the compression even of rare air; and that the phenomena of falling stars may be explained by regarding them as small incombustible bodies moving round the earth in very eccentric orbits, and becoming ignited only when they pass with immense rapidity through the upper regions of the atmosphere; whilst those meteors which throw down stony bodies are, similarly circumstanced, combustible masses.

Masses of iron and nickel, having all the appearance of aerolites or meteoric stones, have been discovered in Siberia, at a depth of ten metres below the surface of the earth. From the fact, however, that no meteoric stones are found in the secondary and tertiary formations, it would seem to follow that the phenomena of falling stones did not take place till the earth assumed its present conditions.

VAST SHOWER OF METEORS.

The most magnificent Shower of Meteors that has ever been known was that which fell during the night of November 12th, 1833, commencing at nine o’clock in the evening, and continuing till the morning sun concealed the meteors from view. This shower extended from Canada to the northern boundary of South America, and over a tract of nearly 3000 miles in width.

IMMENSE METEORITE.

Mrs. Somerville mentions a Meteorite which passed within twenty-five miles of our planet, and was estimated to weigh 600,000 tons, and to move with a velocity of twenty miles in a second. Only a small fragment of this immense mass reached the earth. Four instances are recorded of persons being killed by their fall. A block of stone fell at Ægos Potamos, B.C. 465, as large as two millstones; another at Narni, in 921, projected like a rock four feet above the surface of the river, in which it was seen to fall. The Emperor Jehangire had a sword forged from a mass of meteoric iron, which fell in 1620 at Jahlinder in the Punjab. Sixteen instances of the fall of stones in the British Isles are well authenticated to have occurred since 1620, one of them in London. It is very remarkable that no new chemical element has been detected in any of the numerous meteorites which have been analysed.

NO FOSSIL METEORIC STONES.

It is (says Olbers) a remarkable but hitherto unregarded fact, that while shells are found in secondary and tertiary formations, no Fossil Meteoric Stones have as yet been discovered. May we conclude from this circumstance, that previous to the present and last modification of the earth’s surface no meteoric stones fell on it, though at the present time it appears probable, from the researches of Schreibers, that 700 fall annually?24

THE END OF OUR SYSTEM.

While all the phenomena in the heavens indicate a law of progressive creation, in which revolving matter is distributed into suns and planets, there are indications in our own system that a period has been assigned for its duration, which, sooner or later, it must reach. The medium which fills universal space, whether it be a luminiferous ether, or arise from the indefinite expansion of planetary atmospheres, must retard the bodies which move in it, even were it 360,000 millions of times more rare than atmospheric air; and, with its time of revolution gradually shortening, the satellite must return to its planet, the planet to its sun, and the sun to its primeval nebula. The fate of our system, thus deduced from mechanical laws, must be the fate of all others. Motion cannot be perpetuated in a resisting medium; and where there exist disturbing forces, there must be primarily derangement, and ultimately ruin. From the great central mass, heat may again be summoned to exhale nebulous matter; chemical forces may again produce motion, and motion may again generate systems; but, as in the recurring catastrophes which have desolated our earth, the great First Cause must preside at the dawn of each cosmical cycle; and, as in the animal races which were successively reproduced, new celestial creations of a nobler form of beauty and of a higher form of permanence may yet appear in the sidereal universe. “Behold, I create new heavens and a new earth, and the former shall not be remembered.” “The new heavens and the new earth shall remain before me.” “Let us look, then, according to this promise, for the new heavens and the new earth, wherein dwelleth righteousness.”—North-British Review, No. 3.

BENEFITS OF GLASS TO MAN.

Cuvier eloquently says: “It could not be expected that those Phœnician sailors who saw the sand of the shores of Bætica transformed by fire into a transparent Glass, should have at once foreseen that this new substance would prolong the pleasures of sight to the old; that it would one day assist the astronomer in penetrating the depths of the heavens, and in numbering the stars of the Milky Way; that it would lay open to the naturalist a miniature world, as populous, as rich in wonders as that which alone seemed to have been granted to his senses and his contemplation: in fine, that the most simple and direct use of it would enable the inhabitants of the coast of the Baltic Sea to build palaces more magnificent than those of Tyre and Memphis, and to cultivate, almost under the polar circle, the most delicious fruit of the torrid zone.”

THE GALILEAN TELESCOPE.

Galileo appears to be justly entitled to the honour of having invented that form of Telescope which still bears his name; while we must accord to John Lippershey, the spectacle-maker of Middleburg, the honour of having previously invented the astronomical telescope. The interest excited at Venice by Galileo’s invention amounted almost to frenzy. On ascending the tower of St. Mark, that he might use one of his telescopes without molestation, Galileo was recognised by a crowd in the street, who took possession of the wondrous tube, and detained the impatient philosopher for several hours, till they had successively witnessed its effects. These instruments were soon manufactured in great numbers; but were purchased merely as philosophical toys, and were carried by travellers into every corner of Europe.

WHAT GALILEO FIRST SAW WITH HIS TELESCOPE.

The moon displayed to him her mountain-ranges and her glens, her continents and her highlands, now lying in darkness, now brilliant with sunshine, and undergoing all those variations of light and shadow which the surface of our own globe presents to the alpine traveller or to the aeronaut. The four satellites of Jupiter illuminating their planet, and suffering eclipses in his shadow, like our own moon; the spots on the sun’s disc, proving his rotation round his axis in twenty-five days; the crescent phases of Venus, and the triple form or the imperfectly developed ring of Saturn,—were the other discoveries in the solar system which rewarded the diligence of Galileo. In the starry heavens, too, thousands of new worlds were discovered by his telescope; and the Pleiades alone, which to the unassisted eye exhibit only seven stars, displayed to Galileo no fewer than forty.—North-British Review, No. 3.

The first telescope “the starry Galileo” constructed with a leaden tube a few inches long, with a spectacle-glass, one convex and one concave, at each of its extremities. It magnified three times. Telescopes were made in London in February 1610, a year after Galileo had completed his own (Rigaud, On Harriot’s Papers, 1833). They were at first called cylinders. The telescopes which Galileo constructed, and others of which he made use for observing Jupiter’s satellites, the phases of Venus, and the solar spots, possessed the gradually-increasing powers of magnifying four, seven, and thirty-two linear diameters; but they never had a higher power.—Arago, in the Annuaire for 1842.

Clock-work is now applied to the equatorial telescope, so as to allow the observer to follow the course of any star, comet, or planet he may wish to observe continuously, without using his hands for the mechanical motion of the instrument.

ANTIQUITY OF TELESCOPES.

Long tubes were certainly employed by Arabian astronomers, and very probably also by the Greeks and Romans; the exactness of their observations being in some degree attributable to their causing the object to be seen through diopters or slits. Abul Hassan speaks very distinctly of tubes, to the extremities of which ocular and object diopters were attached; and instruments so constructed were used in the observatory founded by Hulagu at Meragha. If stars be more easily discovered during twilight by means of tubes, and if a star be sooner revealed to the naked eye through a tube than without it, the reason lies, as Arago has truly observed, in the circumstance that the tube conceals a great portion of the disturbing light diffused in the atmospheric strata between the star and the eye applied to the tube. In like manner, the tube prevents the lateral impression of the faint light which the particles of air receive at night from all the other stars in the firmament. The intensity of the image and the size of the star are apparently augmented.—Humboldt’s Cosmos, vol. iii. p. 53.

NEWTON’S FIRST REFLECTING TELESCOPE.

The year 1668 may be regarded as the date of the invention of Newton’s Reflecting Telescope. Five years previously, James Gregory had described the manner of constructing a reflecting telescope with two concave specula; but Newton perceived the disadvantages to be so great, that, according to his statement, he “found it necessary, before attempting any thing in the practice, to alter the design, and place the eye-glass at the side of the tube rather than at the middle.” On this improved principle Newton constructed his telescope, which was examined by Charles II.; it was presented to the Royal Society near the end of 1671, and is carefully preserved by that distinguished body, with the inscription:

“The first Reflecting Telescope; invented by Sir Isaac Newton, and made with his own hands.”

Sir David Brewster describes this telescope as consisting of a concave metallic speculum, the radius of curvature of which was 12-2/3 or 13 inches, so that “it collected the sun’s rays at the distance of 6-1/3 inches.” The rays reflected by the speculum were received upon a plane metallic speculum inclined 45° to the axis of the tube, so as to reflect them to the side of the tube in which there was an aperture to receive a small tube with a plano-convex eye-glass whose radius was one-twelfth of an inch, by means of which the image formed by the speculum was magnified 38 times. Such was the first reflecting telescope applied to the heavens; but Sir David Brewster describes this instrument as small and ill-made; and fifty years elapsed before telescopes of the Newtonian form became useful in astronomy.

SIR WILLIAM HERSCHEL’S GREAT TELESCOPE AT SLOUGH.

The plan of this Telescope was intimated by Herschel, through Sir Joseph Banks, to George III., who offered to defray the whole expense of it; a noble act of liberality, which has never been imitated by any other British sovereign. Towards the close of 1785, accordingly, Herschel began to construct his reflecting telescope, forty feet in length, and having a speculum fully four feet in diameter. The thickness of the speculum, which was uniform in every part, was 3½ inches, and its weight nearly 2118 pounds; the metal being composed of 32 copper, and 10·7 of tin: it was the third speculum cast, the two previous attempts having failed. The speculum, when not in use, was preserved from damp by a tin cover, fitted upon a rim of close-grained cloth. The tube of the telescope was 39 ft. 4 in. long, and its width 4 ft. 10 in.; it was made of iron, and was 3000 lbs. lighter than if it had been made of wood. The observer was seated in a suspended movable seat at the mouth of the tube, and viewed the image of the object with a magnifying lens or eye-piece. The focus of the speculum, or place of the image, was within four inches of the lower side of the mouth of the tube, and came forward into the air, so that there was space for part of the head above the eye, to prevent it from intercepting many of the rays going from the object to the mirror. The eye-piece moved in a tube carried by a slider directed to the centre of the speculum, and fixed on an adjustible foundation at the mouth of the tube. It was completed on the 27th August 1789; and the very first moment it was directed to the heavens, a new body was added to the solar system, namely, Saturn and six of its satellites; and in less than a month after, the seventh satellite of Saturn, “an object,” says Sir John Herschel, “of a far higher order of difficulty.”—Abridged from the North-British Review, No. 3.

This magnificent instrument stood on the lawn in the rear of Sir William Herschel’s house at Slough; and some of our readers, like ourselves, may remember its extraordinary aspect when seen from the Bath coach-road, and the road to Windsor. The difficulty of managing so large an instrument—requiring as it did two assistants in addition to the observer himself and the person employed to note the time—prevented its being much used. Sir John Herschel, in a letter to Mr. Weld, states the entire cost of its construction, 4000l., was defrayed by George III. In 1839, the woodwork of the telescope being decayed, Sir John Herschel had it cleared away; and piers were erected, on which the tube was placed, that being of iron, and so well preserved that, although not more than one-twentieth of an inch thick, when in the horizontal position it contained within all Sir John’s family; and next the two reflectors, the polishing apparatus, and portions of the machinery, to the amount of a great many tons. Sir John attributes this great strength and resistance to the internal structure of the tube, very similar to that patented under the name of corrugated iron-roping. Sir John Herschel also thinks that system of triangular arrangement of the woodwork was upon the principle to which “diagonal bracing” owes its strength.

THE EARL OF ROSSE’S GREAT REFLECTING TELESCOPE.

Sir David Brewster has remarked, that “the long interval of half a century seems to be the period of hybernation during which the telescopic mind rests from its labours in order to acquire strength for some great achievement. Fifty years elapsed between the dwarf telescope of Newton and the large instruments of Hadley; other fifty years rolled on before Sir William Herschel constructed his magnificent telescope; and fifty years more passed away before the Earl of Rosse produced that colossal instrument which has already achieved such brilliant discoveries.”25

In the improvement of the Reflecting Telescope, the first object has always been to increase the magnifying power and light by the construction of as large a mirror as possible; and to this point Lord Rosse’s attention was directed as early as 1828, the field of operation being at his lordship’s seat, Birr Castle at Parsonstown, about fifty miles west of Dublin. For this high branch of scientific inquiry Lord Rosse was well fitted by a rare combination of “talent to devise, patience to bear disappointment, perseverance, profound mathematical knowledge, mechanical skill, and uninterrupted leisure from other pursuits;”26 all these, however, would not have been sufficient, had not a great command of money been added; the gigantic telescope we are about to describe having cost certainly not less than twelve thousand pounds.

Lord Rosse ground and polished specula fifteen inches, two feet, and three feet in diameter before he commenced the colossal instrument. It is impossible here to detail the admirable contrivances and processes by which he prepared himself for this great work. He first ascertained the most useful combination of metals for specula, both in whiteness, porosity, and hardness, to be copper and tin. Of this compound the reflector was cast in pieces, which were fixed on a bed of zinc and copper,—a species of brass which expanded in the same degree by heat as the pieces of the speculum themselves. They were ground as one body to a true surface, and then polished by machinery moved by a steam-engine. The peculiarities of this mechanism were entirely Lord Rosse’s invention, and the result of close calculation and observation: they were chiefly, placing the speculum with the face upward, regulating the temperature by having it immersed in water, usually at 55° Fahr., and regulating the pressure and velocity. This was found to work a perfect spherical figure in large surfaces with a degree of precision unattainable by the hand; the polisher, by working above and upon the face of the speculum, being enabled to examine the operation as it proceeded without removing the speculum, which, when a ton weight, is no easy matter.

The contrivance for doing this is very beautiful. The machine is placed in a room at the bottom of a high tower, in the successive floors of which trap-doors can be opened. A mast is elevated on the top of the tower, so that its summit is about ninety feet above the speculum. A dial-plate is attached to the top of the mast, and a small plane speculum and eye-piece, with proper adjustments, are so placed that the combination becomes a Newtonian telescope, and the dial-plate the object. The last and most important part of the process of working the speculum, is to give it a true parabolic figure, that is, such a figure that each portion of it should reflect the incident ray to the same focus. Lord Rosse’s operations for this purpose consist—1st, of a stroke of the first eccentric, which carries the polisher along one-third of the diameter of the speculum; 2d, a transverse stroke twenty-one times slower, and equal to 0·27 of the same diameter, measured on the edge of the tank, or 1·7 beyond the centre of the polisher; 3d, a rotation of the speculum performed in the same time as thirty-seven of the first strokes; and 4th, a rotation of the polisher in the same direction about sixteen times slower. If these rules are attended to, the machine will give the true parabolic figure to the speculum, whether it be six inches or three feet in diameter. In the three-feet speculum, the figure is so true with the whole aperture, that it is thrown out of focus by a motion of less than the thirtieth of an inch, “and even with a single lens of one-eighth of an inch focus, giving a power of 2592, the dots on a watch-dial are still in some degree defined.”

Thus was executed the three-feet speculum for the twenty-six-feet telescope placed upon the lawn at Parsonstown, which, in 1840, showed with powers up to 1000 and even 1600; and which resolved nebulæ into stars, and destroyed that symmetry of form in globular nebulæ upon which was founded the hypothesis of the gradual condensation of nebulous matter into suns and planets.27

Scarcely was this instrument out of Lord Rosse’s hands, when he resolved to attempt by the same processes to construct another reflector, with a speculum six feet in diameter and fifty feet long! and this magnificent instrument was completed early in 1845. The focal length of the speculum is fifty-four feet. It weighs four tons, and, with its supports, is seven times as heavy as the four-feet speculum of Sir William Herschel. The speculum is placed in one of the sides of a cubical wooden box, about eight feet wide, and to the opposite end of this box is fastened the tube, which is made of deal staves an inch thick, hooped with iron clamp-rings, like a huge cask. It carries at its upper end, and in the axis of the tube, a small oval speculum, six inches in its lesser diameter.

The tube is about 50 feet long and 8 feet in diameter in the middle, and furnished with diaphragms 6½ feet in aperture. The late Dean of Ely walked through the tube with an umbrella up.

The telescope is established between two lofty castellated piers 60 feet high, and is raised to different altitudes by a strong chain-cable attached to the top of the tube. This cable passes over a pulley on a frame down to a windlass on the ground, which is wrought by two assistants. To the frame are attached chain-guys fastened to the counterweights; and the telescope is balanced by these counterweights suspended by chains, which are fixed to the sides of the tube and pass over large iron pulleys. The immense mass of matter weighs about twelve tons.

On the eastern pier is a strong semicircle of cast-iron, with which the telescope is connected by a racked bar, with friction-rollers attached to the tube by wheelwork, so that by means of a handle near the eye-piece, the observer can move the telescope along the bar on either side of the meridian, to the distance of an hour for an equatorial star.

On the western pier are stairs and galleries. The observing gallery is moved along a railway by means of wheels and a winch; and the mechanism for raising the galleries to various altitudes is very ingenious. Sometimes the galleries, filled with observers, are suspended midway between the two piers, over a chasm sixty feet deep.

An excellent description of this immense Telescope at Birr Castle will be found in Mr. Weld’s volume of Vacation Rambles.

Sir David Brewster thus eloquently sketches the powers of the telescope at the close of his able description of the instrument, which we have in part quoted from his Life of Sir Isaac Newton.

We have, in the mornings, walked again and again, and ever with new delight, along its mystic tube, and at midnight, with its distinguished architect, pondered over the marvellous sights which it dis-closes,—the satellites and belts and rings of Saturn,—the old and new ring, which is advancing with its crest of waters to the body of the planet,—the rocks, and mountains, and valleys, and extinct volcanoes of the moon,—the crescent of Venus, with its mountainous outline,—the systems of double and triple stars,—the nebulæ and starry clusters of every variety of shape,—and those spiral nebular formations which baffle human comprehension, and constitute the greatest achievement in modern discovery.

The Astronomer Royal, Mr. Airy, alludes to the impression made by the enormous light of the telescope,—partly by the modifications produced in the appearance of nebulæ already figured, partly by the great number of stars seen at a distance from the Milky Way, and partly from the prodigious brilliancy of Saturn. The account given by another astronomer of the appearance of Jupiter was that it resembled a coach-lamp in the telescope; and this well expresses the blaze of light which is seen in the instrument.

The Rev. Dr. Scoresby thus records the results of his visits:

The range opened to us by the great telescope at Birr Castle is best, perhaps, apprehended by the now usual measurement—not of distances in miles, or millions of miles, or diameters of the earth’s orbit, but—of the progress of light in free space. The determination within, no doubt, a small proportion of error of the parallax of a considerable number of the fixed stars yields, according to Mr. Peters, a space betwixt us and the fixed stars of the smallest magnitude, the sixth, ordinarily visible to the naked eye, of 130 years in the flight of light. This information enables us, on the principles of sounding the heavens, suggested by Sir W. Herschel, with the photometrical researches on the stars of Dr. Wollaston and others, to carry the estimation of distances, and that by no means on vague assumption, to the limits of space opened out by the most effective telescopes. And from the guidance thus afforded us as to the comparative power of the six feet speculum in the penetration of space as already elucidated, we might fairly assume the fact, that if any other telescope now in use could follow the sun if removed to the remotest visible position, or till its light would require 10,000 years to reach us, the grand instrument at Parsonstown would follow it so far that from 20,000 to 25,000 years would be spent in the transmission of its light to the earth. But in the cases of clusters of stars, and of nebulæ exhibiting a mere speck of misty luminosity, from the combined light of perhaps hundreds of thousands of suns, the penetration into space, compared with the results of ordinary vision, must be enormous; so that it would not be difficult to show the probability that a million of years, in flight of light, would be requisite, in regard to the most distant, to trace the enormous interval.

GIGANTIC TELESCOPES PROPOSED.

Hooke is said to have proposed the use of Telescopes having a length of upwards of 10,000 feet (or nearly two miles), in order to see animals in the moon! an extravagant expectation which Auzout considered it necessary to refute. The Capuchin monk Schyrle von Rheita, who was well versed in optics, had already spoken of the speedy practicability of constructing telescopes that should magnify 4000 times, by means of which the lunar mountains might be accurately laid down.

Optical instruments of such enormous focal lengths remind us of the Arabian contrivances of measurement: quadrants with a radius of about 190 feet, upon whose graduated limb the image of the sun was received as in the gnomon, through a small round aperture. Such a quadrant was erected at Samarcand, probably constructed after the model of the older sextants of Alchokandi, which were about sixty feet in height.

LATE INVENTION OF OPTICAL INSTRUMENTS.

A writer in the North-British Review, No. 50, considers it strange that a variety of facts which must have presented themselves to the most careless observer should not have led to the earlier construction of Optical Instruments. The ancients, doubtless, must have formed metallic articles with concave surfaces, in which the observer could not fail to see himself magnified; and if the radius of the concavity exceeded twelve inches, twice the focal distance of his eye, he had in his hands an extempore reflecting telescope of the Newtonian form, in which the concave metal was the speculum, and his eye the eye-glass, and which would magnify and bring near him the image of objects nearly behind him. Through the spherical drops of water suspended before his eye, an attentive observer might have seen magnified some minute body placed accidentally in its anterior focus; and in the eyes of fishes and quadrupeds which he used for his food, he might have seen, and might have extracted, the beautiful lenses which they contain, and which he could not fail to regard as the principal agents in the vision of the animals to which they belonged. Curiosity might have prompted him to look through these remarkable lenses or spheres; and had he placed the lens of the smallest minnow, or that of the bird, the sheep, or the ox, in or before a circular aperture, he would have produced a microscope or microscopes of excellent quality and different magnifying powers. No such observations seem, however, to have been made; and even after the invention of glass, and its conversion into globular vessels, through which, when filled with any fluid, objects are magnified, the microscope remained undiscovered.

A TRIAD OF CONTEMPORARY ASTRONOMERS.

It is a remarkable fact in the history of astronomy (says Sir David Brewster), that three of its most distinguished professors were contemporaries. Galileo was the contemporary of Tycho during thirty-seven years, and of Kepler during the fifty-nine years of his life. Galileo was born seven years before Kepler, and survived him nearly the same time. We have not learned that the intellectual triumvirate of the age enjoyed any opportunity for mutual congratulation. What a privilege would it have been to have contrasted the aristocratic dignity of Tycho with the reckless ease of Kepler, and the manly and impetuous mien of the Italian sage!—Brewster’s Life of Newton.

A PEASANT ASTRONOMER.

At about the same time that Goodricke discovered the variation of the remarkable periodical star Algol, or β Persei, one Palitzch, a farmer of Prolitz, near Dresden,—a peasant by station, an astronomer by nature,—from his familiar acquaintance with the aspect of the heavens, was led to notice, among so many thousand stars, Algol, as distinguished from the rest by its variation, and ascertained its period. The same Palitzch was also the first to re-discover the predicted comet of Halley in 1759, which he saw nearly a month before any of the astronomers, who, armed with their telescopes, were anxiously watching its return. These anecdotes carry us back to the era of the Chaldean shepherds.—Sir John Herschel’s Outlines.

SHIRBURN-CASTLE OBSERVATORY.

Lord Macclesfield, the eminent mathematician, who was twelve years President of the Royal Society, built at his seat, Shirburn Castle in Oxfordshire, an Observatory, about 1739. It stood 100 yards south from the castle-gate, and consisted of a bed-chamber, a room for the transit, and the third for a mural quadrant. In the possession of the Royal Astronomical Society is a curious print representing two of Lord Macclesfield’s servants taking observations in the Shirburn observatory; they are Thomas Phelps, aged 82, who, from being a stable-boy to Lord-Chancellor Macclesfield, rose by his merit and genius to be appointed observer. His companion is John Bartlett, originally a shepherd, in which station he, by books and observation, acquired such a knowledge in computation, and of the heavenly bodies, as to induce Lord Macclesfield to appoint him assistant-observer in his observatory. Phelps was the person who, on December 23d, 1743, discovered the great comet, and made the first observation of it; an account of which is entered in the Philosophical Transactions, but not the name of the observer.

LACAILLE’S OBSERVATORY.

Lacaille, who made more observations than all his contemporaries put together, and whose researches will have the highest value as long as astronomy is cultivated, had an observatory at the Collège Mazarin, part of which is now the Palace of the Institute, at Paris.

For a long time it had been without observer or instruments; under Napoleon’s reign it was demolished. Lacaille never used to illuminate the wires of his instruments. The inner part of his observatory was painted black; he admitted only the faintest light, to enable him to see his pendulum and his paper: his left eye was devoted to the service of looking to the pendulum, whilst his right eye was kept shut. The latter was only employed to look to the telescope, and during the time of observation never opened but for this purpose. Thus the faintest light made him distinguish the wires, and he very seldom felt the necessity of illuminating them. Part of these blackened walls were visible long after the demolition of the observatory, which took place somewhat about 1811.—Professor Mohl.

NICETY REQUIRED IN ASTRONOMICAL CALCULATIONS.

In the Edinburgh Review, 1850, we find the following illustrations of the enormous propagation of minute errors:

The rod used in measuring a base-line is commonly about ten feet long; and the astronomer may be said truly to apply that very rod to mete the distance of the stars. An error in placing a fine dot which fixes the length of the rod, amounting to one-five-thousandth of an inch (the thickness of a single silken fibre), will amount to an error of 70 feet in the earth’s diameter, of 316 miles in the sun’s distance, and to 65,200,000 miles in that of the nearest fixed star. Secondly, as the astronomer in his observatory has nothing further to do with ascertaining lengths or distances, except by calculation, his whole skill and artifice are exhausted in the measurement of angles; for by these alone spaces inaccessible can be compared. Happily, a ray of light is straight: were it not so (in celestial spaces at least), there would be an end of our astronomy. Now an angle of a second (3600 to a degree) is a subtle thing. It has an apparent breadth utterly invisible to the unassisted eye, unless accompanied with so intense a splendour (e. g. in the case of a fixed star) as actually to raise by its effect on the nerve of sight a spurious image having a sensible breadth. A silkworm’s fibre, such as we have mentioned above, subtends an angle of a second at 3½ feet distance; a cricket-ball, 2½ inches diameter, must be removed, in order to subtend a second, to 43,000 feet, or about 8 miles, where it would be utterly invisible to the sharpest sight aided even by a telescope of some power. Yet it is on the measure of one single second that the ascertainment of a sensible parallax in any fixed star depends; and an error of one-thousandth of that amount (a quantity still unmeasurable by the most perfect of our instruments) would place the star too far or too near by 200,000,000,000 miles; a space which light requires 118 days to travel.

CAN STARS BE SEEN BY DAYLIGHT?

Aristotle maintains that Stars may occasionally be seen in the Daylight, from caverns and cisterns, as through tubes. Pliny alludes to the same circumstance, and mentions that stars have been most distinctly recognised during solar eclipses. Sir John Herschel has heard it stated by a celebrated optician, that his attention was first drawn to astronomy by the regular appearance, at a certain hour, for several successive days, of a considerable star through the shaft of a chimney. The chimney-sweepers who have been questioned upon this subject agree tolerably well in stating that “they have never seen stars by day, but that when observed at night through deep shafts, the sky appeared quite near, and the stars larger.” Saussure states that stars have been seen with the naked eye in broad daylight, on the declivity of Mont Blanc, at an elevation of 12,757 feet, as he was assured by several of the alpine guides. The observer must be placed entirely in the shade, and have a thick and massive shade above his head, else the stronger light of the air will disperse the faint image of the stars; these conditions resembling those presented by the cisterns of the ancients, and the chimneys above referred to. Humboldt, however, questions the accuracy of these evidences, adding that in the Cordilleras of Mexico, Quito, and Peru, at elevations of 15,000 or 16,000 feet above the sea-level, he never could distinguish stars by daylight. Yet, under the ethereally pure sky of Cumana, in the plains near the sea-shore, Humboldt has frequently been able, after observing an eclipse of Jupiter’s satellites, to find the planet again with the naked eye, and has most distinctly seen it when the sun’s disc was from 18° to 20° above the horizon.

LOST HEAT OF THE SUN.

By the nature of our atmosphere, we are protected from the influence of the full flood of solar heat. The absorption of caloric by the air has been calculated at about one-fifth of the whole in passing through a column of 6000 feet, estimated near the earth’s surface. And we are enabled, knowing the increasing rarity of the upper regions of our gaseous envelope, in which the absorption is constantly diminishing, to prove that about one-third of the solar heat is lost by vertical transmission through the whole extent of our atmosphere.—J. D. Forbes, F.R.S.; Bakerian Lecture, 1842.

THE LONDON MONUMENT USED AS AN OBSERVATORY.

Soon after the completion of the Monument on Fish Street Hill, by Wren, in 1677, it was used by Hooke and other members of the Royal Society for astronomical purposes, but abandoned on account of the vibrations being too great for the nicety required in their observations. Hence arose the report that the Monument was unsafe, which has been revived in our time; “but,” says Elmes, “its scientific construction may bid defiance to the attacks of all but earthquakes for centuries to come.” This vibration in lofty columns is not uncommon. Captain Smythe, in his Cycle of Celestial Objects, tells us, that when taking observations on the summit of Pompey’s Pillar, near Alexandria, the mercury was sensibly affected by tremor, although the pillar is a solid.