bl PHILOSOPHICAL TRANSACTIONS. [ANNO 1781. 



already great proofs that the diurnal motion of the earth is, if not perfectly equa- 

 ble, at least more so than any other motion we are acquainted with, it will not 

 appear absurd to suppose the diurnal rotation of the other planets to be so like- 

 wise. This suggested the thoughts of estimating the diurnal motion of one 

 planet very exactly by that of another, making each the standard of the other. 

 In this manner we may obtain a comparative view, by which future astronomers, 

 if they shall hereafter be inclined to pursue the subject, may be enabled to make 

 some estimate of the general equability of the rotatory motions of the planets. 

 For if in length of time they should perceive some small retardation in the diurnal 

 motion of a planet, occasioned by some resistance of a very subtle medium in 

 which the heavenly bodies perhaps move ; or, on the other hand, if there should 

 be found an acceleration from some cause or other, they might then ascribe the 

 alteration either to the diurnal motion of the earth, or to the gyration of the 

 other planet, according as circumstances, or observed phenomena, should make 

 one or the other of these opinions most probable. 



Now, this method of comparing together different rotations of several planets, 

 simple as it may appear, was not without some difficulties. In the first place it 

 was evident, that the common account of their diurnal motions, (Keill, Ast. 

 Lect. 5) which makes that of Jupiter 9 h 56 m , of Mars 24 h 40 m , how true soever 

 it may be in a general way, was much too inaccurate for this critical purpose. 

 The gyration of Venus was still less to be depended on, being only noted to the 

 hour, without the minutes : it became therefore necessary to proceed to observa- 

 tions of a more determinate kind. From what had already been seen of the rota- 

 tion of the planets, Mr. H. concluded, that Mars on several accounts would be 

 the most eligible planet for his purpose: for the spots on Jupiter change so 

 often, that it is not easy, if at all possible, to ascertain the identity of the same 

 appearance, for any considerable length of time. Nor do the dark spots only 

 change their place, which may be supposed to be large black congeries of vapours 

 and clouds swimming in the atmosphere of Jupiter; but also the bright spots, 

 though they may adhere firmly to the body of Jupiter, may undergo some appa- 

 rent change of situation, by being differently covered or uncovered on one side 

 or the other, by alterations in the belts. For Mr. H. had observed the revolu- 

 tion of a very bright spot, not suspected of any change of situation, to be first, 

 by one set of observations, at the rate of 9 h 51 m 45 s .6; and afterwards, by an- 

 other set immediately following, at the rate of 9 h 50 m 48 s . 



As the principal belts on Jupiter are equatorial, and as we have certain con- 

 stant winds on our planet, especially near the equator, that regularly, for certain 

 periods, blow the same way, it is easily supposed, that they may form equatorial 

 belts by gathering together the vapours which swim in our atmosphere, and car- 

 rying them about in the same direction. This will, by analogy, account for all 



