606 I'HILOSOPHICAL TRANSACTIONS. [aNNO I762, 



Allowing therefore the clock to get 1" SpMn a mean solar day, on mean solar 

 time, in 23^ 56"" 4^ of mean solar time, or in a sidereal day, it will get only I'" 

 58'^ on mean solar time: but a clock adjusted to mean solar time loses 3"" 56* 

 by the stars in a sidereal day, therefore the clock loses 1™ 57?-^ of sidereal time, 

 in one revolution of the stars, which agrees exactly with what he found by the 

 transits 5, u, a, and C, Leonis. 



The same clock at Greenwich, with the pendulum adjusted to the same length, 

 lost ir on the stars in a sidereal day. Therefore the force of gravity at Green- 

 wich is to the force of gravity at St. Helena as the square of 23^ SQ™ 49* to the 

 square of 23*^ SS*" 2^^ ;: 10000000 : 9975405. The extent of the vibrations of 

 the pendulum here, as well as in England, is exactly 1° 45' on each side of the 

 perpendicular, according to the divided arch, which is at the bottom of the 

 pendulum. 



Mr. M. excuses himself from attempting to deduce any consequences at 

 present from the above observations, either with respect to the law which the 

 force of gravity observes in its changes in different latitudes, or with respect to 

 the figure of the earth, which it has been supposed might be determined from 

 experiments of this kind alone, independently of any others, the great Sir Isaac 

 Newton having himself set us the example. If the body of the earth were ho- 

 mogeneous throughout, not only the figure of the earth, but also the law of the 

 variations of gravity in different latitudes, would be given, and would be the 

 same as Sir Isaac Newton has described them. But if the earth be not homo- 

 geneous, and there seems great reason, from late experiments, to doubt if it be 

 so, we can form no certain conclusions concerning the figure of the earth, from 

 knowing the force of gravity in different latitudes ; as this force must depend not 

 only on the external figure, but also on the internal constitution and density of 

 the earth. Many more experiments, not only of the kind, but also of other 

 different kinds, may be necessary before we shall be able to infer any thing with 

 certainty concerning the internal constitution of the earth, or even to determine 

 its external figure. But every experiment is useful which tends to throw a light 

 over this intricate subject, and to show the perfect agreement of the laws of na- 

 ture with the actual constitution of things. 



LXf^II, Observations on some Gems Similar to the Tourmalin. By Air. Ben- 

 jamin JVilson. F.R.S. p. 443. 



Mr. W. here mentions that he had met with several gems of different sizes 

 and colours, that resemble the tourmalin in regard to electrical experiments. 

 The most beautiful of them were something like the ruby, others were more 

 pale, and there was one inclining to the orange colour. In point of hardness and 

 lustre they were nearly the same with the topaz. 



Six of these gems were cut brilliant fashion, and 3 retained their natural 



