on the Rotation of the Planet s, See. ny 
vain to expert any help from another quarter ; for what mecha- 
nical movement on earth, or motion of the heavens, is there 
that can meafure out fuch equal portions of time as we require to 
compare the diurnal motion of the earth to ? However,, to pro- 
ceed, fince w r e have already great proofs that the diurnal motion 
of the earth is, if not perfectly equable, at leaf! more fo than 
any other motion we are acquainted with, it will not appear 
abfurd to fuppofe the diurnal rotation of the other planets to 
be fo like wife. This fuggeffed to me the thoughts of effi- 
mating the diurnal motion of one planet very exactly by that 
of another, making each the ftandard of the other. In this 
manner we may obtain a comparative view, by which future- 
aflronomers, if they fhall hereafter be inclined to purfue the 
fubjedt, may be enabled to make fome effi mate of the general 
equability of the rotatory motions of the planets. For if in 
length of time they fhould perceive fome fmall retardation in 
the diurnal motion of a planet occafioned by fome refinance of 
a very fubtle medium in which the heavenly bodies perhaps 
move *, or, on the other hand, if there fhould be found an 
acceleration from fome caufe or other, they might then aferibe 
the alteration either to the diurnal motion of the earth, or to 
the gyration of the other planet, according as circum fiances,, 
or obferved. phenomena, fhould make one or the other of thefe 
©pinions* moil probable.. 
Now, this method of comparing together different rotations 
of feveral- planets, fimple as it may appear, was not without 
fome. difficulties. In the firfl place it was evident, that the 
common account of their diurnal motions which makes that 
* Venus fpatio 23: horarum- gyrationem circa axem ab occidente in' orientenu 
perficit. Mars finailem rotationem, horis 24* min. 40.abiblv.if.. Macularum revo • 
kitionibus faepius obfervatis, Ds. cassini comper-uit periodum Jovis circa pro r 
mum. Axem.. efle horarurn 9, minuter urn. 56. keixl,. Aft. Led. V.. 
