C. A. Young—Diameters of Mars. 211 
The introduction of the correction I reduces the sum of 
pv’ from 13994 to 1:2612. 
The values of the diameters are of course not very reliable, 
being subject to the considerable constant error which a ways 
affects filar micrometer measures. If we accept Hartwig’s 
determination, 9’’:352,* for the diameter at distance unity, a 
value which depends upon the whole body of heliometer and 
double-image micrometer measures up to 1877, we get for the 
Opposition diameter of 1879, 19/128. Comparing this with 
my mean, we find 1-46 as the constant error o y filar 
micrometer measures, a value rather unexpectedly large, but 
not unprecedented. ; 
This constant error can however have but very slight, if any, 
influence upon the measured difference of diameters, and we 
find for the ellipticity of the visible periphery of the planet, 
00818 1 
the = 
value 19-128 934° 
1 —2é sin’ p+ é sin’ — 
1 — é*sin"@ 
p=a ? 
A gb ot ; satigot 
and putting ‘—= 534 We easily find e, the eccentricity of a 
a . 
planetary meridian, and from it O,, the lar compression. 
Performing the solution we get C,=gty, which may be taken 
as the final result of the work here detailed. 
The limits of probable error extend from yf tO tz, and so 
far as can be judged from the observations themselves, the 
chances are more than two to one that the ellipticity lies be- 
tween +1 : 
The work of reduction was nearly finished before the writer 
had seen Professor Adams's recent paper upon the orbits of the 
satellites of Mars, in which he gives $y as the ellipticity which 
the planet ought to have if it follows the same law of centra 
density as the earth. The closeness of the accordance is prob- 
ably in considerable degree accidental, but it is quite satis- 
factory. 
Princeton, New Jersey, Jan., 1880. 
* The mean diameter 20"-593 resulting from my observations, when reduced to 
distance unity, gives 10°-068; a value sensibly identical with that obtained by 
" 
