Note g — BOOK I.

I have before in Note (a), Chap. 3. shewn what Vapours are, and how they are rais’d. That which I shall here note, is their Quantity: Concerning which the before-commended Dr. Halley hath given us some curious Experiments in our Phil. Transact. which may be met with together in Mr. Lowthorp’s Abridg. Vol. II. p. 108. and 126. Mr. Sedileau also at Paris observed it for near three Years. By all their Observations it appears, that in the Winter Months the Evaporations are least, and greatest in Summer, and most of all in windy Weather. And by Monsieur Sedileau’s Observations it appears, that what is raised in Vapours, exceeds that which falleth in Rain. In the seven last Months of the Year 1688, the Evaporations amounted to 22 Inches 5 Lines; but the Rain only to Inches 6⅓ Lines: In 1689, the Evaporations were 32 Inches 10½ Lines; but the Rain 18 Inches 1 Line: In 1690, the Evaporations 30 Inches 11 Lines; the Rain 21 Inches ⅓ of a Line. Vid. Mem. de Math. Phys. Ann. 1692. p. 25.

If it be demanded, What becomes of the Overplus of Exhalations that descend not in Rain? I answer, They are partly tumbled down and spent by the Winds, and partly descend in Dews, which amount to a greater quantity than is commonly imagined. Dr. Halley found the descent of Vapours in Dews so prodigious at St. Helena, that he makes no doubt to attribute the Origine of Fountains thereto. And I my self have seen in a still, cool Evening, large thick Clouds hanging, without any Motion in the Air, which in two or three Hours Time have been melted down by Degrees, by the cold of the Evening, so that not any the least Remains of them have been left.