On the Nature of tieat.^ 317 



tion which mav be fonr yards above the water, it may be 

 feen from the other at an elevation of fiftecri yards above the 

 water; and confequentlv, the interval of time between the 

 obfervation of the flafli and report of a gun from either velfel 

 mav be obferved at the oilier at this diftance, which time, 

 accordino to the laws of found, will be about 55 j feconds : 

 and, if there be no current, each will obferve the fame in- 

 terral of time between the flalh and the report of the (hot 

 from the other ; but if thev float on a current which runs at 

 the rate of feven knots art hour, for inftance, one veffel ad- 

 vances 570 feet towards the point whence the found iflued 

 during the palfaoe of the found, and will confcquently hear 

 it half a fccond fooner than if flie remained at reft; whilft 

 the other, by receding from the former vefl'el, lengthens the 

 fpace through which the found is to pafs by 570 feet, and 

 therefore will liear it half a fecond later: therefore there will 

 be an entire fecond of time difference between the obierva-' 

 ttons of the two (liips : and in fuch cafe it may be inferred 

 that the fhip which obferved the itiorler interval floats on 

 a current which runs (oirarJs the other ftiip, and vice 

 •verfa : fo that two fliips, making this experiment in fevcral 

 points, will dcteft the exiftence of a current, and the true 

 direilion of it; and, if it be not too much to expcft from 

 the accuracy <jf future obfervers, that the fubdivifions of a 

 fecond fliould be diitinguiihed, the r,^/^ of the current may 

 alfo be found from the diftance between the Ihips and the 

 time of the found's paftage from one to the other, provided 

 this portion of time can be mealured with fufficient accuracy. 

 [To be continued.] 



LI y . On the Nature of Hi at. By a Correfpondent. 



It has long been a queftion whether the fcnfation we exi>e- 

 ricnce, called heat, arifes from a iubllnnce of a didintl nature 

 per fe, or whether it is generated from an inlenie vibratory 

 motion of the infenftbli; pirticlos of bodies. In reafoning 

 on this fubjeol, our arguments can only be founded on the 

 effects that are produced by this unknown caufe ; for it iecms 

 more than j)robiible that the degree of accuracy of experiment 

 ncceflary to render our coudulions certain with regard to its 

 inirinfic nature, is incompatible with the prefent means ot 

 our knowli-dge. The following obfervations are intended 

 therefore, bv dire(!-ting the attention to a conlidcration of 

 ibme of the thief phceiioinena of heat, to (Iiua' that wc can 



feek 



