54 PHILOSOPHICAL TRANSACTIONS. [ANNO 1781. 



some late curious experiments and calculations that have heen made on the at- 

 traction of a mountain,* the result of which ought now to be taken into consi- 

 deration, and the calculation repeated. If all the data could be exactly depended 

 on, it would be practicable enough from the laws of gravity, and the present 

 rotation and given form of the earth, to find the centrifugal force required to 

 produce that form, and thence to show what must have been its diurnal motion 

 when it assumed the same. However, these are researches that in the present 

 situation Mr. H. neither had opportunity nor perhaps ability enough to investi- 

 gate properly; and which therefore he hoped some of our excellent mathema- 

 ticians will think worth while to look into. 



Mr. H. now relates his observations on Jupiter and Mars. The telescopes 

 used were of his own construction; and were, a 20-feet Newtonian reflector, a 

 10-feet reflector of the same form, and a 7-feet reflector, already mentioned in 

 his paper on the mountains of the moon. His time he gained by equal altitudes 

 taken with a brass quadrant of 2-feet radius, carrying a telescope which magnifies 

 about 40 times: for the correction of altitudes taken of the sun, he used De 

 Lalande's tables. He kept the time by two very good pieces; one having a deal 

 pendulum-rod, the other a compounded one of brass and iron, both having a 

 proper contrivance not to stop when winding up. The rate of going of his 

 clocks he determined by the transit of stars. 



Observations on Jupiter in the year 1778. — Feb. 24, clock l m 10 s too soon. 

 About 9 o'clock saw a bright belt on one part of Jupiter's disc. About 10 

 o'clock it was advanced as far as the centre. At 1 l h the white belt still more 

 advanced. At ll h 25 m it approached towards the edge of the disc; and at 12 h 

 was extended all over it. 



Feb. 25, 8 h , the same bright belt observed yesterday extended all over. At 

 8 h 45 m it was divided by a darkish spot, situated at some distance from the centre. 

 At Q h 5 m the small dark division was advanced a little farther than the centre. 

 At 9 h 23 m the spot visibly advanced a considerable deal farther. 



March 2, 8 h 2 m , the darkish spot, with some alteration in its shape, was in 

 the middle of the disc. 



March 3, 10 h 34 m , the bright belt on the south of the equator was in the 

 middle: that is, if a line be drawn perpendicular to the equatorial belt, and 

 through the centre, the end of the equatorial belt touches it. At 13 h 49 m the 

 darkish spot, in which there had been some alteration, seemed to be in the 

 centre. 



March 14, the clock altered to true equated time; but the rate of going not 



* See Mr. Hutton's Account of the Calculations made from the Survey and Measures taken at 

 Schihallien, in order to ascertain the mean Density of the Earth. Phil. Trans., 1 7 7 S, Abridg. vol. 



xiv. p. 420. — Orig. 



