1886.] NEW YORK ACADEMY OF SCIENCES. 237 



Hie Moon. 



During the past ten years there has been no work upon the 

 lunar theory quite on a level with that of Hansen, Delaunay, 

 Piantamour, and Adams in the years preceding; but tlie labors 

 of Neison, Hill, and Newcomb well deserve mention. The 

 former especially has carried his approximations to a consider- 

 ably higher point than any of his predecessors, though not with- 

 out making a few numerical mistakes, which have been detected 

 and corrected by Hill. The investigation of ancient and mediaeval 

 observations of the moon by Newcomb is also a very important 

 work, as showing clearly that the lunar theory is still incom- 

 plete, and that it is impossible by any tables yet made to represent 

 accurately the whole series of observations, A value of the secu- 

 lar acceleration, which suits the observations of the last two hun- 

 dred years, will not fit the Arabian observations made one 

 thousand years ago, nor will it satisfy the eclipse observations of 

 still more ancient date; unless at least the received interpreta- 

 tion of those ancient eclipses be admitted to be wrong, as Prof. 

 Newcomb seems to consider rather probable. From his discus- 

 sion he derives for the secular acceleration a value of 8". 4 as 

 against the value of 12". 1 deduced by Hansen. 



It will be remembered probably by every one present that the 

 theoretical value of this quantity is about 6", and that Ferrel, 

 Adams, Delaunay, and others attributed its apparent increase 

 to 12" to the action of the tides in retarding the earth's rotation 

 and so lengthening the day; if Newcomb's value is correct, this 

 tidal retanlation is cut down from G" to about to 2". 5. 



The study of the moon's surface has been carried on with as- 

 siduity, but I do not know that any remarkable results have 

 been reached, though Klein's observation, in 1877, of what he 

 supposed to be a newly-formed crater (Hyginus N.) excited a 

 good deal of interest and discussion for a number of years; and 

 the most eminent selenograpliers are still divided in opinion on 

 the question. 



The publication by the Grerman Government of Schmidt's great 

 map of the moon, in 18T8, unquestionably marks an epoch in 

 selenography; and the photographic work of Pritchard, and the 

 heliometric determination of the moon's physical libration by 

 Hartwig, must not pass unnoticed. 



Probably, however, the lunar work which has drawn to itself 

 most attention and interest is the investigation of the moon's 

 heat by Lord Rosse, and Professor Langley, 



The earliest observations of the kind date back now forty 

 years, when Melloni, in 1846, first detected the moon's heat by 

 means of the then newly invented thermopile. But the first 



