on the Planet Mars: 264 
_ As we are at prefent only in queft of the. proportion of: one 
diameter to the other, the meafures of the 20-feet reflector, 
though not piven in angular quantities, will equally fuffice for 
the purpofe.. By them we have the equatorial diameter to the 
polar as 1030 98, or as 1355 to 128g. I have turned the 
proportion into the latter numbers by way of comparing them 
the better with the meafures of the 7-feet refleQor. By. that 
inftrument the equator of Mars, Oct. 1. we find, was mea« 
fured three tinies; but from the remarks annexed to the dif- 
férent refults, I think the third meafure fhould be ufed. In- 
deed, on taking the difference of the two firft. which is 34’”, 
and dividing by three, we have the quotient, 112/”; then, 
allotting two-thirds to the firft, becanfe the remark fays pofi- 
tively ‘¢ narrow meafure,”’ it becornes. 22’ 342/’’, and taking 
one-third fromthe fecond,. which. is. expreffed doubtfully, 
‘rather too, full,’ it becomes 22’ 352’: this refleCtion on 
the two firtt meafures gives additional validity to the third, 
_ which: is 22/35’, of 1355/”. The polar diameter was meafured. 
twice; and asno reafon appears againft either of the obfervations,, 
I fhall take the mean of both, which is:21’’ 29’’, ot 1289/3 
fo that by thefe meafures the equatorial.diameter of Mars is to 
the polar as 1355.to. 1289. A. lefs perfect agreement between. 
the proportions of the-diameters arifing, from the meafures of 
the 20-feet refle€tor and thofe which we have juft now deduced 
from the 4-feet, would have been fafficient for our purpofe, as 
we might eafily Have excufed' one or two thoufandths of the 
whole quantity ; however, we have no caufe to be. difpleafed 
with this coincidence, though it fhould’ in part be owing to 
accident, and therefore fhall’ admit the above proportion, and 
proceed to a farther examination of it, 
Nn2 ts In 
