52 Condition of the Moon's Surface. [January, 



It seems to accord with this view, that the greater darkness 

 of the floor of Plato agrees, according to Mr. Birt's light 

 curves, with the time when the sun attains his greatest eleva- 

 tion above the level of the floor. For if the action of the sun 

 were the cause of the darkening we should expect the 

 greatest effect to appear, some considerable time after the 

 sun had culminated (as supposed to be seen from the floor 

 of Plato). We know th^t on our own earth all diurnal solar 

 effects, except those which may be described as optical, 

 attain their maximum after the sun has reached his highest 

 point on the heavens, while all annual solar effects attain 

 their maximum after midsummer. If an observer on Venus 

 could watch the forests of our north temperate zones as 

 they became clothed with vegetation and were afterwards 

 disrobed of their leafy garment during the progress of the 

 year, it would not be on the 21st of June that he would 

 recognise the most abundant signs of vegetation. In July 

 and August vegetation most richly clothes the northern 

 lands of our earth. It is then also that the heat is greatest; 

 that is the time of true midsummer as distinguished from 

 astronomical midsummer. And in like manner the true 

 heat-noon is at about two o'clock in the afternoon, not at 

 the epoch when the sun is highest, or at astronomical noon. 

 The difference in either case amounts to about one-twelfth 

 part of the complete period in question : in one case we find 

 the maximum of heat a month or twelfth part of the year 

 after the time of the sun's greatest northerly declination; in 

 the other we find the time of greatest heat two hours or one- 

 twelfth part of a day after the time of the sun's greatest 

 elevation. If we take a corresponding portion of the lunar 

 month, we find that the greatest effect of any solar action on 

 the floor of Plato might be expected to take place about two- 

 and-a-half days after the sun had attained his greatest eleva- 

 tion. This differs to a sufficient degree from Mr. Birt's 

 estimate to justify the suspicion that either the effect is 

 physiological, or that it is purely an optical peculiarity, that 

 is, due to the manner in which the light falls on a surface 

 of peculiar configuration. 



It does not appear to me, I may remark further, that Mr. 

 Birt has demonstrated the occurrence of real variations in 

 the condition of the spots upon the floor of Plato. He has 

 ascertained that some of these are at times relatively 

 darker or brighter than at others, and that this is not a mere 

 physiological effect is proved by the fact that the result has 

 been obtained by comparing the spots inter se. Nevertheless 

 it must not be forgotten how largely the presentation of the 



