MAES. 
229 
that of the other bodies that circulate about the Sun. This 
does not appeal* so prominently, however, when looked at 
with a telescope, as when seen by unaided vision. Still, how- 
ever, even with the former, the orang’e light is very decided, 
and if compared with the Moon or a neighbouring white star, 
the contrast is sufficiently striking. Viewed when the whole disc 
of the planet is illuminated, its form appears quite circular, and 
no suspicion is aroused of a flattening at the poles, or bulging 
forth of the equatorial regions. But when the micrometer is 
applied, and careful measurements are made of its polar and 
equatorial diameters, several observers have agreed that there 
is a slight variation from the circular form, although the results 
which they have obtained are very discordant. Herschel was 
the first who suspected the elliptical form of the planet, and 
who patiently set about to determine the amount of this varia- 
tion. To arrive at a knowledge of the figure of the Earth 
requires long and arduous labour ; but in the case of the 
planets the method is more simple, and the diameters at dif- 
ferent parts of the disc may be said to be measured with the 
same facility as if it were a palpable object. Herschel found 
that the proportions of the equatorial diameters of Mars were 
as 1355 to 1272, or near as 16 to 15. Schroeter could not per- 
ceive any such elhpticity, and was of opinion that the two 
diameters were in the proportion of 81 to 80. Arago found 
them to vary in the proportion of 31 to 30. The Greenwich 
observations of late years give this variation as 52 to 51, and as 
62 to 61. Other observers, among whom is Bessel, have not 
been able to detect the slightest difference between the dia- 
meters. Herschel, however, states, that on one occasion he 
showed the planet to some scientific friends, one of whom con- 
sidered that it was as considerably bulged out at the equator as 
the globe of Jupiter. At certain times, as the whole surface 
of Mars is not illuminated, it will appear of the same figure as 
the Moon when three or four days before or after full ; but even 
when this was the case, the flattening at the poles was still readily 
perceived by Herschel. There are a few circumstances which 
militate against the correctness of those measures ; sometimes 
the white cap of snow seems to project over the edge of the 
planet, at others the equatorial margins are exceedingly bright 
and radiating. According to theory, the proportion of the 
polar to the equatorial zone should be as 192 to 193 ; but dif- 
ferent degrees of density at various parts of the globe would, 
of course, alter this. By observations made in September and 
October of 1862, Mr. Main concludes its polar diameter to be 
4,221 miles, and its equatorial 4,332 miles, — a great difference 
for a small body like Mars, which has almost the same density 
as the Earth. In the latter body, the difference between the 
