206 Q. A. Young—Diameters of Mars. 
ArT. XX VI.—Measures of the Polar and Equatorial Diameters 
‘of Mars, made at Princeton, New Jersey, U. 8. ; by Professor 
C. A. Youne. 
THE polar compression of Mars has never yet been satis- 
factorily determined, the results of different observers ranging 
from that of Sir W. Herschel, who made it ;,, to that of Bessel, 
who found it insensible. The value j,, deduced by Main at 
Oxford, from his measures in 1862-8, has probably been of late 
more generally accepted than any other, though by no means 
without reserve. 
Hartwig, in his recently published investigation upon the 
diameters of Venus and Mars, gives +, as the result obtained 
by combining all the double-image measurements made at 
admit the presence of water upon its surface, as the polar 
“snow caps” seem to indicate, except upon the almost absurd 
assumption of a density rapidly increasing from the center 
toward the surface. 
It has seemed to the writer quite possible that the difference 
of illumination of the limbs of the planet, caused by phase, 
may lie at the bottom of the difficulty. Except on rare occa- 
sions there is phase enough, even at the moment of opposition, 
to produce a notable difference of appearance between the 
fully illuminated edge of the planet’s dise and that opposite, 4 
difference which can hardly fail to be felt in micrometric 
measurements. Unexceptionable observations for determining 
the polar compression can therefore be made only when the 
planet reaches opposition and its node together. ‘This was so 
nearly the case last season, that on the night of November 
12th, an observer on the planet would have witnessed a Transit 
of the Earth. At this time, and for a few days before and 
after, the phase was extremely small, and an opportunity was 
presented for determining the planet’s ellipticity such as will 
not be available again for nearly half a century. : 
e measures detailed below were made by myself with 4 
filar position micrometer attached to the nine and one-half inch 
Equatorial of the School of Science Observatory. The object 
glass of this instrument (by Clark) is constructed substantially 
upon the Gaussian curves, and is of the highest excellence. 
During the past year it has shown repeatedly both of the satel 
lites of Mars, the two outer satellites of Uranus, and the Satur- 
nian satellite Mimas, the last, for my eye, which is not extremely 
sensitive, being just at the limit of visibility. The teles- 
