134 Examination of St. Pierre's Hypothecs 



lens for five minutes, that the thermometer might aflume tKe 



temperature of its filuation. The refult was as follows : 



No. 3.-4 Here, in one minute, the 

 The lens covered o' 64 > thermometer received 



Open - 1 176 J iia degrees of heat, 



w hich came with the coloured part of the folar fpccl:rum,and 

 '.vcre refracted to a focus; fo that, if the coloured rays them-' 

 iclvesare not of a heat-making nature, they arc atleait accom- 

 panied with rays that have a power of heating bodies, and are 

 i'ubject to certain laws of refraction, which cannot differ 

 much from thole anecting light. 



[To be continued.] 



VII. An Examination of St. Pierre's Hypothtjis r,j]vcling 

 '/be Caufe of the Tides, which, in oppojition to the received 

 r Jheory, attributes them to fuppofed periodical F,//u/ions of 

 7 be Polar lees. By Samukl Woods, Efq. Read before 

 the AJkeJiari Society November 5, 1799. 



T, 



HE tides are two periodical motions actuating the ocean 

 (called the flux and reflux, or ebb and flow), which fucceed 

 each other alternately at an interval of about fix hours; the 

 period of a flux and reflux being, upon an average, 12 hours 

 .24 minutes, the double of which, 24 hours 48 minutes, cor- 

 refponds to that of a lunar day, or the time elapfing between 

 f.he moon's paffing a meridian and coming to it again. Thefe 

 alternate elevations and depreflions of the ocean fo exactly 

 correfpond with the courfe of the fun and moon, as to time 

 and quantity, that the influence of thole luminaries has in 

 all ages been confidcrcd as the caufe of their production ; 

 but it was referved for modern times to afcertain the princi- 

 ple of their laws, and to calculate, with precifion, the effect* 

 produced by the different fituation? of the fun and moon, and 

 the proportions of their power. This principle js no other 

 than gravitation. It is evident that, if the earth were entirely 

 fluid and qujefcent, its particles, by their mutual gravity, 

 would form the whole mats into a perfect fphere : now, if 

 any power be fuppofed to a£t on all the particles of this fphere 

 8 -wall 



