P. 455, that the Rays of the Full-Moon in OƐteber, collected in the Focus of a Burning-Glass of 35 Inches broad, and into a Place three hundred and fix times leffer than what the fame would have filled in their natural State, did not produce the leaft Alteration in a Thermometer which was moved by Air, and which discover'd the smallest Increase of the Warmth thereof, tho' the Ball of it was held for a while in the faid Focus: And we likewife find by the faid AEF. Lipf. 1697, p. 429, that the fo famous BurningGlaffes of Mr. Tfchirnhaus produced indeed a greater Brightness by the Rays of the Moon in the Focus, but not the leaft Warmth. Now could ever any Body have imagined that the Rays of the Sun collected by a Burning-Glafs, or reflected from the fame, will burn fo terribly; and yet that the fame Rays rebounding from the Moon to us, do not appear to bring the leaft Warmth along with them, notwithstanding that by this kind of Glaffes the Brightness of the MoonLight is very much augmented in the Focus as well as that of the Sun.

But we fhall have occafion to fay fomething more of this, when we come to the Contemplation of Unknown Things.

Now how advantagious it is to hot Countries, that this Light of the Moon produces no Heat, is obvious to every one who knows that if it was otherwife, and that the Rays of the Moon were likewife hot, that part of the Earth would be barren, and foon burnt up, fince the defcending Dews of the Night, by which it is now moiftned, would then ceafe to fall down, and this Fire of the Moon would draw its Vapours upwards. Moreover, if the Night-Air were not fresher and cooler, and that the Rays of the Moon kept the fame in a continual Warmth,

it

it is plain enough how prejudicial it would be to the Health of all Men, and the hot Parts of the World would fuffer great Inconveniencies thereby.

SECT. XLV. The Moon's Magnitude and Diftance from the Earth.

I KNOW not whether it is neceffary to fhew in this Place, how much the Moon is fmaller than the Earth, and how much the Light thereof (which in itself is hot, yet) by reafon of the Diftance is weaker, and lefs warm upon the Earth, than at the Moon itself. But fince the preceding Figures may be of use to us in this Matter, and the Grounds of this Calculation depend on that of the Sun, we fhall briefly touch upon the fame.

1. Let A B (as before in Tab. XX. Fig. 1.) be the Semidiameter of the Earth, and D C now be that of the Moon; then will the Angle A CB of the Moon's horizontal Parallax, when he is New, or Full, according to Sir J. Newton, be at a Medium.57 Min. 30 Sec.

And its apparent Diameter DCG, according to the fame.

31 Min. 30 Sec.

15 Min. 45 Sec.

The half of which therefore? for the Angle DB C, is In the Fourth or laft Quarter of the Moon, and at a middle Distance likewife from the Earth, Sir J. N. again fuppofes,

or? 56 Min. 40 Sec.

The Horizontal Parallax or
the Angle AC B

The apparent Diameter, or the
Angle DB G

The half of which for the An-
gle D BC iş

31 Min. 3 Sec.

15 Min. 3x Sec.

Ccc 3

Now

1

Now fince A and D are Right Angles, made by a Tangent and a Semidiameter, if we take the Earth's Semidiameter A B for an Unit, we fhall, by plain Trigonometry, find B C, or the Moon's Distance from the Earth to amount to;

At New or Full barely 60 Semidiameters of the
Earth.

And at the Quarters, barely 61 of the fame.
So that the middle Distance is about 60%
thereof.

945

2. Now to difcover the Magnitude_of_the Moon; we firft find its Semidiameter DC by Trigonometry aforefaid, amounting at Full or New to or, and at the Quarters to Parts of AB, or of the Earth's Semidiameter; both which do not differ much from Parts of the fame.

3400

From whence it therefore follows, That the Earth's Diameter A B, Is to that of the Moon CD As about 11 To 3; Confequently the Body of the Earth Is to that of the Moon, As the Cube of 11 or 1331 To the Cube of 3 or 27, according to what we have faid about the Sun. And therefore 27 Globes of the Earth, are equal to 133 Globes of the Moon, or the Moon is 49, barely 50 times (to compute in Round Numbers) leffer than the Earth.

This Calculation is accurate enough to build upon in Things of fuch a Nature; and if it be not entirely exact, neither is it far fhort of the Truth.

With this likewise agrees the Conclufion of that Great Aftronomer Mr. Flamstead, who makes the Diameter of the Earth (as Mr. Whifton fays in his Pralect. Phyfic. p. 292.) to amount to 7935 English Miles, and that of the Moon 2175 of the fame: Which Proportion of 7935 to 2175, varies but little from the above-mentioned of 11 to 3.

SECT

SECT. XLVI. Why the Light of the Moon is not Warm.

Now if we fuppofe the Point S in the Moon, and the Point B on the Earth, (Tab. XXI. Fig. 2.) and farther, the Length S% as the Moon's Seinidiameter; and if we look back upon what has been faid above concerning the Diverging and Spreading of Light at various Distances, as well with refpect to its Heat as Shining; we shall find that in both these Cafes the Force of the Light at b, Is to that at B, As the Square of S B, To the Square of Sb.

Now we have shown above, that as S Bis 60 Semidiameters of the Earth, or the Moon's Diftance; fo St is parts of one Semidiameter of the Earth, when it reprefents that of the Moon.

Now is the Square of or Sb, and 36604 that of 60 or S B; and confequently the Firft is to the Second, As 9 To 442890, or, As 1 To 49210, omitting the Fraction. From whence it appears, that the Warmth of the Light which comes down from the Moon, is about 50,000 times lefs when it has reach'd us upon the Earth B, than it is at the Point b, when it has proceeded no farther than the length of one Semidiameter of the Moon, or Sb.

And this is the' Reason, according to Mr. Whi fton, Prælect. Aftron. p. 108. why the Light of the Moon is not attended with any fenfible Warmth by the time it has reach'd us here upon Earth. But forafmuch as Dr. Hook has collected the fame into a Space 500 times fmaller, and confequently render'd it 500 times as ftrong as the faid Light is in its natural State; and therefore in fuch a Focus it is no more than 500 times weaker than at the Moon itself; yet, even in that cafe this LearnCcc 4

ed

ed Man could not perceive any fign of Warmth, notwithstanding that the Shining or Light of the Moon (which deferves to be taken notice of) was increased proportionably. I leave it to the Judgment and Obfervations of others, whether

part of the Warmth of a hot Summers-Day with us, would be able to make any Impreffion even on the best Thermometer: For that the Rays of the Sun have much the fame Power on the Moon, as on the Earth, is plain enough from the little difference there is of both their Distances from the Sun.

And in this Experiment the Thermometer being moved by the Light of the Sun, it fhould feem that we are obliged to feek for fome other Cause than the fimple Distance, to which we fhould afcribe, that the Light or Rays of the Sun are reflected to us from the Moon, without bringing any Heat along with them, but leaving it all behind them.

Be that as it will, this is plain, that if the Moon fhould tranfmit to us fo much Heat with its Rays, we fhould fuffer many Inconveniencies from thence; which is now prevented by our great Creator, who directs all Things with infinite Wifdom and Order.

SECT. XLVII. The Caufe of Ebbing and Flowing not inquired into.

Now whether the Moon is the Caufe of the Ebbing and Flowing of the Great Seas, and of the continual Motion of their Waves to the very bottom thereof, we fhall neither difpute nor inquire into here; because we are not yet fo far advanced in this Knowledge, as to be able to fay any thing concerning it that may be free from every ones Objections, tho we seem to be very far advanced in that Matter.