422 Curfory View offome of the late Difcoveries in Science. 

 focus by a lens were made to fall upon them. Prevoft is of 

 opinion that the rays of the fun penetrating the tin-plate are 

 thus combined with a matter more denfe, and form therein 

 a very cxpanfible fluid ; but being lefs fubtle, it becomes fuf- 

 ceptible of acting by impulfion on very large mafles, and 

 Hliiing with impetuofity from the plate, but more rapidly 

 from the focus, pufhes it backwards, and purfues its way 

 through the water, which acls as a conductor to this fluid. 



The author conjectures, from thefe experiments, that it is 

 poflible to deterrmine the gravity of light. It is well known 

 that it takes about eight feconds to come from the fun to the 

 earth, that is to fay, to traverfe a fpace of about thirty-three 

 millions of leagues. According to fome fuppofitions, he finds 

 that the light which in a fecond of time falls upon the fur- 

 face of a fquare league, containing 2,283 toifes on each fide, 

 would weigh about a dram (gros) and a quarter. It may be 

 readily perceived that all thefe calculations are very hypothe- 

 tical ; and we may add, that from fome experiments made 

 by Profeflbr Wilfon of Glafgow*, with a view to a different 

 objeA, there is reafon for calling in queftion the efteft pro- 

 duced upon the thin plates, or tin foil, as being at all pro- 

 duced by the impulfe of light. Heat only feems to be here 

 concerned. The water at firft being: of an uniform tempera- 

 ture, has all its parts in a ftate of equilibrium and reft. 

 When the tin foil is heated, it is evident that a very active 

 eaufe is introduced tending to deftroy that equilibrium. Heat 

 being communicated by the tin to the water in contact with 

 it, that portion of the water endeavouring by its expanfion to 

 move from under the tin, mull tend to move the tin from its 

 place, 



[To be continued.] 



* 5ee the Profcflbr's experiments on fmall lighted wicks floating on oil, 

 info ted in the Edinburgh Tranfaftions, Vol. IV. See alfo Count Ruin- 

 ford"* Experiment;, on Heat. 



XIX. Ccm* 



