VOL. XXXA'III.] PHILOSOPHICAL TRANSACTIONS. 653 



of the lowest point to which the spirits sunk at London in the winter 1731, 

 and the greatest height to which they rose in the following summer; and com- 

 paring the motion of the spirits in this thermometer, with the alterations in 

 another made with quicksilver, which he had for some years made use of; he 

 concluded that at London, the spirits in this thermometer would stand, one 

 year with another, about 6o divisions higher in sumn)er than in winter. 



By several years experience he likewise found, that his clocks, of the same 

 sort with Mr. Campbell's, when exposed as usual to the different degrees of 

 heat and cold of our climate, do not vary in their motion above '25 or 30 se- 

 conds in a day. 



From these observations and experiments, we may therefore reasonably con- 

 clude, that sufficient allowance will be made for the lengthening of the pen- 

 dulum by heat, if we suppose the clock on that account to go one second in a 

 day slower, when the spirits of this thermometer stand 2 divisions higlier, and 

 in the same proportion for other heights. 



Admitting then that the mean height of the thermometer, while the clock 

 was compardl with the stars at Jamaica, exceeded that at London between 15 

 and 20 divisions; if we allow 8 or Q seconds on that account, the remaining 

 difference must be entirely owing to the difference of tiie force of gravity in 

 the two places. 



On comparing the observations, it appears, that in one apparent revolution 

 of the stars, the clock went 2"" 6i' slower in Jamaica than at London; deduct- 

 ing therefore 8-l% on account of the greater heat in Jamaica, there remains a 

 difference of 1™ 58% which must necessarily arise from the diminution of gra- 

 vity, in the place nearest the equator. 



Mr. Bradley has allowed the clock to have lost somewhat more, on account 

 of the difference of heat, than the mean heights of the thermometer may seem 

 to require, on a supposition that the total heat of the days, compared with the 

 cold of the nights, bears a greater proportion in Jamaica, than in London; 

 but if that supposition be not admitted, then the clock in Jamaica must have 

 gone rather more than l' 58" in a day slower than in England. 



Mr. Campbell's observations were made at Black-river, in 18° n. lat. Now 

 if we suppose, with Sir Isaac Newton, that the difference in the going of the 

 clock, is owing to the greater elevation of the parts of the earth towards the 

 equator; it will follow from these observations, and what is delivered by him in 

 prop. 20 of the third book of his Principia, that the equatorial diameter is to 

 the polar as IQO to ISQ; the difference between them being 414- miles; which 

 is somewhat greater than what Sir Isaac Newton had computed from his theory, 

 on the supposition of a uniform density in all the parts of the earth. 



