266 Mr. Herscner’s ‘Objersiasione 
rahe 
1703; Nov. 10. gh. 30% The gibbofity of Mars is now fuch, : 
that the polar diameter is confiderably longer than the 
equatorial; but the deficiency not being exadtly from 
pole to pole, makes the difk of a crooked, irregular 
figure, and renders precifion 7 in this eftimation impof- 
fible; otherwife the phate of Mars would have made 
a pretty good micrometer upon the equatorial diame- 
ter, and it was with fuch a view I had dire¢ted my 
‘attention to this circumftance: appearances, how- 
ever, are vifibly in favour of the polar’ diameter’ S 
bein — the toi geft. 
We find that the quick alterations in the vifible difk of Mars,. 
during the time it is in the beft fituation for us to obferve it, 
are fuch, that if we were to ufeé many meafures which have 
been taken of its diameters, we fhould be obliged to have re- 
courfe to a computation of its phafes, in orde? to make proper’ 
allowance for them. Now, fince thefe changes are in a longi- 
tudinal dire€tion, and the poles of Mars are not perpendicular 
to the ecliptic, it would bring on a calculation of fmall quan- 
tities, which it is always beft not to run into where it cam be 
avoided. For this reafon, I fhall at once fettle the proportion 
of the equatorial to the polar diameter of this. planet, from the 
meafures which were taken on the very day of the oppofition. 
I prefer them alfo on another account, which is, that they were — 
made in a very fine, clear air, and were repeated with a very 
high power, atid with two different inftruments, of whofe 
faithful reprefentation of celeftial objects, the many obferva- 
tions on very clofe double ftars I have made with them have 
given me very evident proofs. 
