64 PROCEEDINGS OF SECTION A. 
which he calculated at 186 feet. Using the data furnished 
by the computations of Laplace, the difference between the 
two diameters comes out greater than the estimate of 
Newton, being about 426 feet. The probable form of the 
moon is thus an ellipsoid, having its greatest axis directed 
towards the earth, and its least axis through the poles. 
Although the difference in the diameters is too small for 
direct measurement, the fact that the moon’s figure is ellip- 
soidal, or at any rate that the distribution of its mass is 
like that of a uniform ellipsoid, is shown by the existence of 
an observable real libration, and also, as pointed out by La 
grange, by the continuance of the co-incidence which exists 
between the descending or ascending node of the lunar 
equator with the ascending or descending node of her rela- 
tive orbit. Hansen, basing his calculations upon the dis 
crepancies between the observed and computed longitudes 
of the moon, inferred that the moon’s centre of figure was as 
much as 59 kilometres nearer to us than its centre of 
gravity; and Gussew, from measurements made on two of 
De la Rue’s photographs, estimated pet the elongation 
towards the earth was as great as 5°5 per cent. These 
results are quite at variance with the tidal theory, and also 
with estimates based upon the amount of the real libration, 
the elongation being far too great, and they were disputed 
by Newcomb. Recently, in order to determine this point, 
Dr. Franz has made a series of very careful measurements 
of a set of five photographs of the moon near the full, taken 
at the Lick Observatory. The photographs, taken under 
different librations, gave pictures of the lunar surface, such 
that in the intervening intervals of time the moon had ap- 
parently rocked through angles varying from 10° to 14°. 
M easurements upon the relative positions of objects near the 
moon’s equator would suffice, under these conditions, to de- 
termine the departure of the moon’s shape from the spheri- 
cal,if it were anything like so marked as the work of Gussew 
and Hansem would lead us to suppose. The result of the 
_ very careful and elaborate measurements of Dr. Franz is 
quite contrary to that of these observers. and proves that the 
moon is sensibly spherical, in agreement with the tidal 
theory and the estimates formed from its real libration. 
The investigations and speculations of G. H. Darwin with 
regard to the effect of tidal friction upon the earth and moon 
form one of the most interesting chapters in. our present 
subject... The moon’s attraction upon the waters of the 
earth, when disturbed by tidal action modified by friction, 
tends to slacken the speed of the earth’s rotation, and, conse- 
