CONDITION OF THE HOON’s SURFACE. 377

not at the epoch, when the sun is highest, or at astro-nomical noon. The difference in either case amountsto about one-twelfth part of the complete period inquestion : in one case we find the maximum of heat amonth or twelfth part of the year after the time of thesun’s greatest northerly declination; in the other wefind the time of greatest heat two hours, or one-twelfth part of a day, after the time of the sun’sgreatest elevation. If we take a corresponding por-tion of the lunar month, we find that the greatesteffect of any solar action on the floor of Plato mightbe expected to take place about two and a half daysafter the sun had attained his greatest elevation.This differs to a sufficient degree from Mr. Birt’sestimate to justify the suspicion that either the effectis physiological, or that it is purely an optical pecu-liarity, that is, due to the manner in which the lightfalls on a surface of peculiar configuration.

It does not appear to me, I may remark further,that Mr. Birt has demonstrated the occurrence of realvariations in the condition of the spots upon the floorof Plato . He has ascertained that some of these areat times relatively darker or brighter than at others,and that this is not a mere physiological effect isproved by the fact that the result has been obtainedby comparing the spots inter se. Nevertheless itmust not be forgotten how largely the presentation ofthe floor of Plato towards the terrestrial observer isaffected by libration, now tilting the floor more fullytowards the observer and presently tilting it away