[ 149 ] 



out the year. Here then the diurnal and even monthly variations 

 of temperature lofc their influence, and the annual changes alone 

 become perceivable. 



Hence it appears that the diurnal xmriations of temperature 

 manifeft their eftedls at the furface of the earth, where they are 

 beft to be obferved. That the monthly changes alone, are 

 perceivable at depths cf thirty or forty feet ; in Vv^hich fituations- 

 we have the power of comparing correfpondent increments or 

 decrements of heat in different months with each other, or 

 even the medium temperature of different . years. It further 

 appears that at the depth of eighty feet and upward, even the 

 monthly variations lofe their influence, while the annual changes 

 alone, become perceivable : At fuch depths, therefore, the accefs; 

 or diminution of heat, in fucceffive periods of years, may moft 

 accurately be obferved ; and attentive experiments, thus made 

 in different climates, may not only throw light on the variations 

 of temperature in fuch particular climates, but may at length 

 tend to folve the problem concerning the general temperature of 

 the earth itfelf, which has in vain been agitated for fo many 

 ages. 



The beft method of determining the degree of heat, at con^ 

 {iderable depths, might naturally feem to offer itfelf in mines j. 

 but experience proves thefe to be extremely fallacious, fince the 

 decompofition of mineral fubflances often produces cafual varieties- 

 ■which may very much miflead us. Thus, to give a fingle 

 inftance ; the ftreams which run from three of the principal levels 

 cf the Ballycaftle coal pits give a mean degree of heat equal to 5 i °- 



oi 



