348 PHILOSOPHICAL TRANSACTIONS. [aNNO 1794. 



not be amiss to record. April 6, 1780, I had a fine view of Jupiter, and saw, 

 as soon as I looked into the telescope, without having any previous notice 

 of it, the shadow of the 3d satellite, and the satellite itself, on the lower 

 part of the disc. See fig. 6. The shadow wqs so black and well defined, 

 that I attempted to measure it, and found its diameter by the micrometer 1''.562. 

 This measure of the shadow should be checked by the following observation. 

 March 15, 179'2, ll"" 54""; With the 20-feet reflector, and a power of 800, 



1 estimate the apparent diameter of the largest of Jupiter's satellites to be less 

 than ^ of the diameter of the Georgian planet, which I have just been viewing. 

 With 1200, it seems also to be less, in the same proportion. With 2400, I 

 can plainly perceive the disc of the satellite. With 4S00, the apparent diameter 

 of the largest of the satellites is less than i of that of the Georgian planet. 



The analogy alluded to in the first paragraph of this paper, refers to the nu- 

 merous parallel belts which we have noticed, in the above given observations, on 

 the discs of Jupiter and Saturn. That belts are immediately connected with the 

 rotation of the planets will hardly be denied, when those of Jupiter are so well 

 known always to lie in the direction of its equatorial motion. Since then it ap- 

 pears that the belts of Saturn are very numerous, like those of Jupiter, and are 

 also placed in the direction of the longest diameter of the planet, it may not be 

 without some reason that we infer the period of the rotation of the former to be 

 short, like that of the latter. 



The planet Mars, in all my observations, never presented itself with any 

 parallel belts, nor do we observe such phenomena on the disc of Venus. The 

 first is known to have a rotation much slower than Jupiter; and the latter, ac- 

 cording to the accounts of Cassini and Bianchini, is certainly not one that 

 moves quickly on its axis. However, I do not mean to enter into the strength 

 of an argument for a quick rotation of Saturn, that may be drawn from the 

 condition of its belts. The circumstance of a quintuple belt, is adduced here 

 with no other view, than merely to point out an analogy in the condition of the 



2 largest planets of our system; and thence to infer, that every conclusion on 

 the atmosphere and rotation of the one, drawn from the appearance of its belts, 

 will equally apply to the other. 



F'. Observations on the Fundamental Property of the Lever ; ivilh a Proof of 

 the Principle assumed by Archimedes, in his Demonstration, By the Rev. 

 S. Fince, A. M., F. R. S. p. 33. 



The want of a demonstration of the property of the lever, on clear and self- 

 evident principles, has justly been considered as a great desideratum in the science 

 of mechanics, as the most important parts of that branch of natiual philosophy 

 are founded on it. Archimedes was perhaps the first who attempted it. He 



