FOOTNOTES:
[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.
[] That the heavenly Bodies move round their own Axes, is, beyond all doubt, manifest to our Eye, in some of them, from the Spots visible on them. The Spots on the Sun (easily visible with an ordinary Glass) do manifest him to revolve round his own Axis in about 25¼ Days. The Spots on ♃ and ♂ prove those two Planets to revolve also from East to West, as Dr. Hook discover’d in 1664, and 1665. And ♀ also (although near the strong Rays of the Sun) hath, from some Spots, been discovered by Mr. Cassini, in 1666, and 1667, to have a manifest Rotation. V. Lowth. Abridg. Vol. 1. p. 382, and 423, 425. And such Uniformity hath the Creator observ’d in the Works of Nature, that what is observable in one, is generally to be found in all others of the same kind. So that since ’tis manifest the Sun, and three of his Planets whirl round, it is very reasonable to conclude all the rest do so too, yea, every Globe of the Universe.
[c] The Earth’s Circumference being 25031½ Miles, (according to [Book II. Chap. 2. Note (a).]) if we divide that into 24 Hours, we shall find the Motion of the Earth to be nearly 1043 Miles in an Hour. Which, by the by, is a far more reasonable and less rapid Rate, than that of the Sun would be, if we suppose the Earth to stand still, and the Sun to move round the Earth. For according to the Proportions in [Note (e)], of the preceding Chapter, the Circumference of the Magnus Orbis is 540686225 English Miles, which divided by 24 Hours, gives 22528364 Miles in an Hour. But what is this to the Rapidity of the fixt Stars, if we suppose them; not the Earth, to move? Which is a good Argument for the Earth’s Motion.
[d] Nihil majus, quàm quòd ita stabilis est Mundus, atque ita cohæret ad permanendum, ut nihil nè excogitari quidem possit aptius. Omnes enim partes ejus undique medium locum capessentes, nituntur æqualiter: maximè autem corpora inter se juncta permanent, cum quodam quasi vinculo circumdata colligantur: quod facit ea natura, quæ per omnem mundum omnia Mente, & Ratione conficiens, funditur, & ad medium rapit, & convertit extrema, Cic. de Nat. Deor. l. 2. c. 45.
[e] Eâdem ratione Mare, cùm supra terram sit, medium tamen terræ locum expetens, conglobatur undique æqualiter, neque redundat unquam, neque effunditur. Id. paulo post.
[f] That there is no such Thing as positive Levity, but that Levity is only a less Gravity, is abundantly manifested by the acute Seig. Alph. Borelli de Mot. à Grav. pend. cap. 4. See also the Annotations of the learned and ingenious Dr. Clark on Rohaulti Phys. p. 1. c. 16. Note 3. Also the Exper. of the Acad. del Cimento, p. 118, &c. Dr. Wallis’s Disc. of Gravity and Gravitation before the Royal Society, Nov. 12. 1674. p. 28, &c.
[g] 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.
[h] See [Book II. Chap. 3. Note (c).]
