LECTURE III.

LADIES and GENTLEMEN,

Am happy to reflect, that in your walk to this affembly

advantage we derive from that fair planet which is the fubject of our prefent difcourfe: I need not relate to you the pleasures of a moonlight walk, or remark the cool filvery beam and the fober refplendence, fhed by our attendant fatellite; yourselves are witneffes, that its ultimate degree of brightness is no ill fubftitute for the fun himself, and that, after the fultry heat of an autumnal day, we enjoy no less the rifing of the moon, than after the season of repofe, we enjoy the beams of morning.

Is it not laudable to inquire into the regulations, and the propertiesof this gentle planet ? is it not natural to wish for information with regard to its changes, of which we are from time to time fpectators with regard to its influences, which cannot but be confiderable? Such curiofity is highly honourable, and I hope in the course of this evening to enliven and gratify it. You are fenfible, that although at this time the lunar orb fhines with full refplendence, yet that it has not long done fo, nor will long continue to do so, but “ nightly changes in "her circling orb :" the reafon of this variation of appearance will quickly engage our attention. L

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We might, to be fure, take the moon as at this time, and by confidering her relative fituation to the fun and earth, defcribe the causes of her prefent plenitude; but, I think it will be more regular, previously to obferve, that she is at no very great DISTANCE from the earth (as diftances are found among heavenly bodies), but is a clofe attendant on our fervice, and no further off than about fixty femi-diameters of the earth, or 240,000 miles; and revolves round the earth from weft to eaft, in about twenty-feven days, which is called a periodical month. According to that part of the orbit of her revolution wherein fhe is when we obferve her, fuch is the afpect the prefents to our view.

If the moon was a body poffeffing native light, we should never perceive in her any diverfity of form; but as the fhines entirely by light received from the fun, and reflected by her opacity, it follows, that according to the fituation of the beholder with refpect to her illuminated parts, he will behold more or lefs of her reflected beams: for only one half of a globe can be enlightened at one time.

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The affair is rendered familiar by a fimple experiment: I fet this candle at the further end of the table; that lady will lend me her pin-cufhion, I place it at fome distance from the candle to reprefent the earth; and the moon is watch, if you please. Obferve that I place the watch between the light and the pin-cufhion (this is termed conjunction): the dial-plate is turned to the fun, indicating a new moon, or, the moon in fuch a situation that its enlightened parts are turned from the earth. I fay the earth being unable to look through the body of this orb, can fee no light

whatever

whatever reflected from it; but as I move it a little to the left, the dial-plate comes in fight; and the further I move it to the left (always preferving its circular motion round the pin-cushion) the more of the dial-plate becomes visible, till at length having got half through the revolution, the dial-plate is exactly oppofite to our fuppofed earth, and vifible diftinctly and entirely to the oppofite part of it.

Obferve, if you pleafe, that when we fet off, the earth and the moon were in a direct line with the fun, the moon being between the other two; but now the moon and the earth are again in a direct line with the fun, but the moon is without the other two, and the earth is between; (this is termed oppofition :) thus, to-night (it being full moon) where are we to suppose the fun, or in what quarter of the heavens should we feek him?-Diametrically oppofite to the moon. There remains now only to continue the course of the moon round the earth, to complete a revolution, and bring her again to the station fhe quitted.

This would be an exact reprefentation of these planets, were the earth always at reft; but as, while the moon has been thus engaged in her course, the earth also, which has a course of her own, has minded her own business, and advanced during one lunar revolution (or month) in her proper orbit, the lays the moon under the neceffity of adding fomewhat to her progrefs, in order to come again exactly into a direct line between the fun and the earth, from which direct line she is to begin her courfe anew.. This additional time is upwards of two days, and five hours, making in

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