
270 Mr. Herscugr’s Obfervations 
_ It was obferved, O&. 17, 1783, that the equatorial. diarhe- 
ter of Mars was full greater ‘than the polar, notwithfandir ° 
the depredation of the defe& of light upon it.’ On calcilating 
the phafes, we find, that the longitudinal diameter was, that 
day, to the latitudinal one as 19711 to 20000, which therefore 
could not be an equal balance to oppofe the {ph Soci fone 
fo as to render it invifible. ‘ Loada 
But, Nov. 10, the proportion | of the joneluiling Seueth 
to the Jatitudinal one, from a ‘computation .of the phafe of 
Mars, mui{t have been as 18762 to 20000 ; and accordingly it 
was by obfervation found to be more than. fufficient to take off 
all appearance; of the polar flattening, ne leave a vilible excels 
in the axis above the equator. ’ * 
To obviate any doubts concerning a fallacy that might arife 
from the conyexity of the eye-glafs, or irregular fhape of the 
{mall {peculum, I need only refer, for the latter, to the expe= 
riments of the 7th and gth of Oober, 1783.: for thould the 
fhort diameter of my {mall: plane fpeculum have oceafioned, a 
comprefling of the polar diameter off Mars when, expofed: to ity, 
half a turn of the telefcope muft bring the other diameter of 
that f{peculum into the fame fituation, and a contrary effect 
would have followed.; With regard to the former, not only, 
the experiments made with the achromatic, but principally the 
obfervation with the 20-feet reflector, where Lufed a com- 
pound eye-piece magnifying only about 300 times, will fuffi- 
ciently exculpate the eye-glafles. It is alfo well known, that 
in a fingle Jens the diftortion of the images, if any fuch there 
ashige Soe 
jp en 
5 4) 
Wi, 
ays , 


pmC) as go (=qu~—vo) to qn 3 which is the required correction or adhe 
of the equatorial diameter eg of Mars. 
Or, pees mCi and vgm=cofine of the angle Pmp; it will be 
quam. Cy 
3 > “fhouid 
