on the Planet Mars. * . 269 
~Juthall now’ alfo mention fome of the other. meafures, ‘but 
with'a view only to fhew that they are very confiftent with the 
above'determination.” From thofe of the 3oth of September, 
for inftance, we colle&t the proportion of the diameters of 
Mars as 1340 to. 1286; or, reduced to our. former numbers, 
1355 to 1300. Now, fince thefe meafures were taken the 
night before the oppofition, they muft'on that: account be as 
good as the formers; and, had thofe of the day of oppofition 
not been preferred, becaufe they were oftener repeated, and 
the fuperior power of the 7, and: great light of the 20-feet 
reflector, gave them additional weight, I fhould have taken them 
into the account; the very {mall difference, however, cannot 
but ftrengthen the refults of the former meafures. | | 
From the obfervations of the 25th of September we ae 
the proportion of the diameters as'1313 to 12753 and if the 
equatorial meafure be increafed in the ratio of 20000: to 19953, 
on account of the different heliocentric and geocentric longi+ 
tude, Mars not being at the full, it will give the ratio of 1316 
t0.1275;3 or, conforming to our former numbers, as 1355 to 
1 1312. I have not ‘been very ftrict in'the application of the 
rrection deduced from the ‘phafes of Mars, fince no other 
4 was intended to be made of thefe’numbers than merely to 
fhew, that they do not very our differ from thofe we have 
pened | before *. | ids 
AG | pose, Bite 
* Lf more firifinefs be required, let EC, fig. 22. be the ecliptic; PS its-poles; 
ps the poles of Mars, and eg its equator. Then, the angle pmcC being found, by 
calculation,. we fhall have Cz (radius) to em, (cofine of the difference between 
the heliocentric and geocentric longitude) as gv, (line of the angle gmv Or pmC). 
toov. Then, fince with Mars C¢ can never be very great, the fimall triangle 
qno may be taken for finilar to gum; therefore qi (radius) isto gu (fine of 
fm C} 
————— Sa 
