"LIFE of Dr HUTTON. Gj 



After the period of the two pubUcations juft mentioned, Dr 

 HuTTON made feveral excurfions Into diflerent parts of Scotland, 

 with a view of comparing certain refults of his theory of the 

 earth with acftual obfervation. His account of granite, viz. that it 

 is a fubftance which, having been reduced into fufion by fubter- 

 raneous heat, has been forcibly injecfled among the ftrata alrea- 

 dy confolidated, was fo diflerent from that of other mlneralo- 

 gifts, that it feemed particularly to require farther examination. 

 He concluded, that if this account was juft, foine confirmation 

 of it muft appear at thofe places where the granite and the (trata 

 are in contadl, or where the former emerges from beneath the 

 latter. In fuch fituations, one might expecfl veins of the (lone 

 which had been in fufion to penetrate into the ftone which 

 had been folid ; and fome in:iperfe6l defcriptions of granule 

 A'eins gave reafon to imagine that this phenomenon was adlu- 

 ally to be obferved. Dr Hutton was anxious that an injlantia 

 crucis might fubjedt his theory to the fevereft teft. 



One 



latent, it will become colder than before. Thus alfo, when a quantity of heat 

 afcends by any means whatever, from one ftratum of air to a fuperior flratum, a 

 part of it becomes latent, fo that an equilibrium of heat can never be eftablilhed 

 among the flrata ; but thofe which are lefs, mult always remain colder than thofe 

 that are more, comprefled. This was Dr HurroN's explanation, and it contains 

 no hypothetical principle whatfoever. 



To one who confiders meteorology with attention, the want of an accurate hygro- 

 meter can never fail to be a fubjeft of regret. The way of fupplying this deficien- 

 cy which Dr Hutton praiStifed was by moiflening the ball of a thermometer, and 

 obferving the degree of cold produced by the evaporation of the moifture. The 

 degree of cold, ctcteris paribus, will be proportional to the drynefs of the air, and 

 aflPords, of courfe, a meafure of that drynefs. The fame contrivance, but without 

 any communication whatfoever, occurred afterwards to Mr Leslie, and bein<T 

 purfued through a feries of very accurate and curious experiments, has produced 

 an inftrument which promlfes to anfwer all the purpofes of photometry, as well 

 as hygrometry, and fo to make a very important addition to our phyfical appara- 

 tus.. 



