on the Rotation of the Planets , &c. 1 1 7 
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, Cnee we have already great proofs that the diurnal motion 
of the earth is, if not perfectly equable, at leaft 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 likewife.. This fuggefted to me the thoughts of efti- 
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 
aftronomers, if they {hall hereafter be inclined to purfue the 
fubjedt, may be enabled to make fome eftimate of the general 
equability of the rotatory motions of the planets. For if in 
length of time they fhould perceive fome fmali retardation in 
the diurnal motion of a planet occaConed 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 circumftances, 
or obferved phenomena, fhould make one or the other of thefe 
©pinions moft probable*. 
Now, this method of comparing together different rotations 
of feveral planets, f mple as it may appear, was not without 
fome difficulties. In the Crft place it was evident, that the 
common account of their diurnal motions % which makes that 
* Venus fpatio y horarum gyrationem circa axem ab occidente in orientem- 
perficit. Mars limilem rotationem, horis 24 min. 40 abfolvif. Macularum revo- 
lutionibus taepius obfervatis, Ds. .cassini comperuit periodum Jovis circa pro » 
priam Axem effe horarum 9, minutorum 56. keill, Ait. Left* -V. 
of 
