i2o Mr. herschel’s AJlronomical Qhfervations 
Mars to as great an exaClnefs as we (hall think neceffary for the 
purpofe of our comparative view. 
Had fueh observations as thefe been made two thoufand, or 
perhaps only fo many hundred years ago, we might now, by 
repeating them, mod; probably become acquainted with fome 
curious minute changes of the folar fyftem that have hitherto 
palled unnoticed. 
There is a certain circumstance which would almoll create a 
fufpicion that there has been fome retardation in the diurnal 
motion of the earth. The difference between the equatorial 
and polar diameters of the earth, by aCtual meafurement, has 
been found to be about 36 Englifh miles and 9 tenths ; but, by 
a calculation wherein the prefent rotation is made ufe of, it 
will only amount to about 33 miles and 8 tenths : from which 
it Should feem probable, that when the earth a’flumed the prefent 
form, the diurnal rotation was Somewhat quicker than it is at 
prefent, by which means the centrifugal force bore a greater 
proportion to the force of gravity to which it is contrary, and 
thus occalioned a higher elevation of the equatorial parts. But 
I would not lay much ffrefs upon this argument ; for, in the cal- 
culation, it has been fuppofed, that the earth is nearly of an 
equal denlity at the Surface and towards the center, which it 
Seems is not agreeable to fome late curious experiments and 
calculations that have been made under the conduit of the 
Aftronomer Royal upon the attraction of a mountain the 
refult of which ought now to be taken into consideration, and 
the calculation repeated. If all the data could be exaCtly de- 
pended upon, it would be practicable enough from the laws of 
* See Mr. hutton’s Account of the Calculations made from the Survey and 
Meafures taken at Schehallien, in order to afcerttin the mean Denfity of the 
Earth. Phil. Tranf 1778. 
3 gravity, 
