Be eae 
the proportion of the polar and equatorial dimeters of Jupiter, 
by comparifon of the denfity and rotation of the -earth with thofe 
of Jupiter. 
Ler T?= the fquare of the time of the fidereal rotation of the 
earth, and #= the fquare of that of Saturn, the root of which 
is fought: let A= the earth’s denfity, and ¢ that of Saturn; D= 
Saturn’s greater diameter, and d his leffer one. Then from Sir 
dxA Xt 
Ifuac Newton’s formula I deduce s=V TTF igh 5 a ~ 
x 23.56" 
Tue micrometers ordered for the Obfervatory being not yet 
arrived, I requefted a gentleman of known accuracy to take 
thefe meafures for me; he was fo obliging as to fend me a great 
number, agreeing remarkably together, from which I find Saturn’s 
diameters, reduced to his mean diftance, 18°,12 and 15,855. 
From hence, taking Sir Ifaac Newton’s ratio of the earth’s 
equatorial diameter and axis, and that of the earth and Saturn’s 
denfities, as by him computed, the formula will give for Saturn’s 
fidereal rotation 10H. 12‘ | 
Ir is a circumftance worth remarking, that the celebrated 
Huyghens, in his whimfical and ingenious work, intitled Cofmothe- 
oros, has the following paffage: “ Qyam habeant dierum longi- 
“ tudimem (Saturnicole fciz.) certo cognofci nequit; fed ex 
“ comitis intimi diftantia ac periodo, exque eorum comparatione 
“cum intimo Jovialium ; verifimile fit non longiores effe dies 
 illas 
