ON THE NATURE OF HEAT. 19.Q 



The author again reverts to the application of thefe fa<$ls Effects of the 

 , , ,. ,-^, , , ., . 1 • 1 r oil on the fkins 



to animal bodies. Whether the oily coating which ^^^^g^^ ^f nottentot%, 



apply to their (kins in cold climates, may not add to their &c. and of the 

 comfort by reflefling frigoritic rays; whether the Hottentots, T^^'^"^''^^^.^"!'^' 

 flill more difguftingly befraeared, may not derive advantages negroes, 

 fimilar to thofe derived by negroes from their black colour ? 

 are queftions that promlfe to lead to refults of prai^lical value. 

 Jie then proceeds to explain more fully the manner in which 

 ^pegroes may be fappofed to refill the adion of a burning fun, 

 .^n oil exudes from the fliin of thefe people when expofed 

 naked to the fun; which oil refleds the fun's calorific rays. 

 Heat more intenfe produces fweat, which not only aids the 

 former procefs, but generates cold by evaporation. But when 

 the fun is fet, the oil retires from the furface, and the Ikin 

 becomes well adapted to admit frigoritic rays from the neigh- 

 bouring bodies. 



Exp. 29. The thermofcope was perceptibly afiedled by the Exp. 29. Veffel 



radiation of cold bodies: It was defirable to know whether °^ '^f'^ "J^f. 



rapidly cooled by 



..^his efFedl would be (hewn in a grolTer way, by accelerating radiation from 

 the cooling of a hot body. For this purpofe, two conical '^'^' 

 veflels of ihin flieet brafs, each four inches diameter at the 

 bafe, and four inches high, ending above in a cylindrical 

 peck, were feparately enclofed in a cylinder of thin pafle- 

 board covered with gilt paper, and then the veifels were 

 covered up with rabbit-flvins having the hair on them, in luch 

 a manner that in> part of thefe veffels, except their flat bot- 

 toms, was expoled naked to the air. The bottoms were co- 



1^, yered with gold-beater's Oiin painted black with Indian ink, 

 in order to render them as fenfible as poffible to calorific 



r .^nd frigorific rays. 



The two veifels thus prepared were fufpended, with their 

 bottoms downwards from the arms of a fiand, and under each 

 was placed a pewter platter blackened on the infide by fmoke 

 from a candle. The platters ihemfeives were fupported on 

 iliallow earthen difties which refted upon wooden ftands; each 

 pewter platter having a perforated cover of tiiick paper, in the 

 center of which was a hole fix inches in diameter. The diftance 

 from the floor of the room to the fmoked furface of each platter 

 was 40 inches, and the interval between the furface of each 

 conical velTel and its correfpondent platter immediately beneath, 

 was four inches. One of the platters remained at the tempera- 

 ture 



