Note Note (a). Chap. 5. Book 1 — TO THE READER.

[a] That there is such a Thing as Gravity, is manifest from its Effects here upon Earth; and that the Heavenly Bodies attract or gravitate to one another, when placed at due Distances, is made highly probable by Sir Isaac Newton. This attractive or gravitating Power, I take to be congenial to Matter, and imprinted on all the Matter of the Universe by the Creator’s Fiat at the Creation. What the Cause of it is, the Newtonian Philosophy doth not pretend to determine for want of Phænomena, upon which Foundation it is that that Philosophy is grounded, and not upon chimerical and uncertain Hypotheses: But whatever the Cause is, that Cause penetrates even to the Centers of the Sun and Planets, without any Diminution of its Virtue; and it acteth not according to the Superficies of Bodies (as Mechanical Causes do) but in proportion to the Quantity of their solid Matter; and lastly, it acteth all round it at immense Distances, decreasing in duplicate proportion to those Distances, as Sir Isaac Newton saith, Princip. pag. ult. What useful Deductions, and what a rational Philosophy have been drawn from hence, may be seen in the same Book.

This Attraction, or Gravity, as its Force is in a certain proportion, so makes the Descent of Bodies to be at a certain rate. And was it not for the Resistence of the Medium, all Bodies would descend to the Earth at the same rate; the lightest Down, as swiftly as the heaviest Mineral: As is manifest in the Air-Pump, in which the lightest Feather, Dust, &c. and a piece of Lead, drop down seemingly in the same Time, from the top to the bottom of a tall exhausted Receiver.

The rate of the Descent of heavy Bodies, according to Galileo, Mr. Huygens, and Dr. Halley (after them) is 16 Feet one Inch in one Second of Time; and in more Seconds, as the Squares of those Times. But in some accurate Experiments made in St. Paul’s Dome, June 9. 1710, at the Height of 220 Feet, the Descent was scarcely 14 Feet in the first Second. The Experiments were made in the Presence of some very considerable Members of the Royal Society, by Mr. Hawksbee, their Operator, with glass, hollow Balls, some empty, some filled with Quick-silver, the Barometer at 297, the Thermometer 60 Degrees above Freezing. The Weight of the Balls, their Diameters, and Time of the Descent is in this Table.

Balls filled with ☿. Empty Balls.

Weight. Diameter. Time. Weight. Diameter. Time.

Grains. Tenth inch. ½ Secᵈˢ. Grains. Inch. Tenth. ½ Secᵈˢ.

908 8 8 510 5 1 17

993 8 8 less. 642 5 2 16

866 8 8 599 5 1 16

747 7½ 8 more. 515 5 nearly 16½

808 7½ 8 483 5 nearly 17

784 7½ 8 more. 641 5 2 16

The Reason why the heavy, full Balls fell in half the Time of the hollow ones, was the Resistence of the Air: Which Resistence is very ingeniously and accurately assigned by Dr. Wallis, in Philos. Trans. Nᵒ. 186. And the cause of the Resistence of all Fluids, (as Sir Isaac Newton, Opt. Q. 20.) is partly from the Friction of the Parts of the Fluid, partly from the Inertia thereof. The Resistence a spherical Body meets with from Friction, is as the right Angle under the Diameter, and the Velocity of the moving Body: And the Resistence from the Vis Inertia, is as the Square of that Product.

For a farther Account of the Properties and Proportions, &c. of Gravity in the Fall or Projection of Bodies, I shall refer to the larger Accounts of Galilæus, Torricellius, Huygens, Sir Isaac Newton, &c. or to the shorter Accounts of Dr. Halley in Philos. Trans. abridged by Mr. Lowthorp, Vol. I. p. 561. or Dr. Clarke in his Notes on Rohault, Phys. 2. c. 28. §. 13, 16. And for the Resistence of Fluids, I refer to Dr. Wallis before-cited, and the Act. Erudit. Lips. May 1693. where there is a way to find the Force of Mediums upon Bodies of different Figures.