37G CONDITION OF THE MOON ’S SUBFACE.

look brighter than they are in reality? We haveonly to suppose that De la Rue’s photographic resultsrepresent pretty accurately the true relative luminosityof different parts of the moon to answer this questionat once in the affirmative.

It seems to accord with this view, that the greaterdarkness of the floor of Plato agrees, according to Mr.Birt’s light-curves, with the time when the sun attainshis greatest elevation above the level of the floor.For if the action of the sun were the cause of thedarkening, we should expect the greatest effect toappear some considerable time after the sun hadculminated (as supposed to be seen from the floor ofPlato ). We know that on our own earth all diurnalsolar effects, except those which may be describedas optical, attain their maximum after the sun hasreached his highest point on the heavens, while allannual solar effects attain their maximum aftermidsummer. If an observer on Venus could watchthe forests of our north temperate zones asthey became clothed of vegetation, and were after-wards disrobed of their leafy garment during theprogress of the year, it would not be on the 21st ofJune that he would recognize the most abundantsigns of vegetation. In July and August vegetationmost richly clothes the northern lands of our earth.It is then also that the heat is greatest; that is thetime of true midsummer as distinguished from astro-nomical midsummer.’ And in like manner the trueheat-noon is at about two o’clock in the afternoon,