SIMON NEWCOMB—STONE. 241 
from the latest observations made at Greenwich and Washington. 
The elements used in Hansen’s tables were based on observations made 
between 1750 and 1850. Having found that these tables failed to 
satisfy later observations, Newcomb compared them with all known 
observations made before 1750. This investigation was aided by a 
visit to the principal observatories of Europe which led to the dis- 
covery, especially in Paris, of numerous and valuable unpublished 
observations of eclipses and occultations. Also, a large part of the 
published observations had not before been used for determining the 
moon’s place. The comparison when completed disclosed discrep- 
ancies that could be explained in two ways: (1) By supposing the 
‘discrepancies to be only apparent, arising from inequalities in the 
axial rotation of the earth; (2) by assuming empirically a correction 
to Hansen’s value of a term depending on the action of Venus and 
having a period of two hundred and seventy-three years. Later, from 
the exhaustive study of the transits of Mercury from 1677 to 1881, 
already referred to, it was inferred that the discrepancies between the 
observed and the computed positions of the moon could not be ac- 
counted for on the assumption of inequalities in the axial rotation of 
the earth, and that “ inequalities in the motion of the moon not ac- 
counted for by the theory of gravitation really exist.” The last work 
performed by Professor Newcomb, while his life was nearing its end, 
was a comparison with Hansen’s tables of all the observations of the 
moon to date, made with the aid of a grant from the Carnegie Insti- 
tution, a result of which was the confirmation of the existence of devi- 
ations apparently not accounted for by the law of gravitation. The 
larger part of these he found could be reduced to a single term, as 
previously suggested, but the existence of well-marked smaller out- 
standing deviations of an apparently irregular character was also 
clearly shown. 
While the comparisons of Hansen’s tables with observations is 
probably the permanent result of greatest value that Professor New- 
comb contributed to the study of the moon’s motion, laying as it does 
a firm foundation upon which to base the determination of the numer- 
ical values of the constants employed, whatever method may ulti- 
mately be adopted for the analytical discussion, nevertheless, as in 
the case of his study of planetary theory, his theoretical study of the 
lunar problem would by itself have been sufficient to have secured 
for him a high and enduring place in the history of the subject. His 
most important contribution to lunar theory related to the action of 
the planets on the moon. His first memoir on this subject appeared 
in Liouville’s Journal in 1871. Afterward he published a rediscus- 
sion of the problem in the Astronomical Papers. Only a few years 
before his death he took up the whole subject again and published a 
final memoir in 1907 under the auspices of the Carnegie Institution. 
