^6^ Prof. Biscbof on the Temperature of 



of the variations of temperature during the year, diminishes as 

 the height increases ; so that in higher latitudes the greatest dif- 

 ference of temperature at a certain elevation is not greater than 

 under the equator. The more finely divided the atmosplieric 

 waters are during their infiltration, and the less the degree of 

 conductibility of heat of the soil, the sooner will springs of a con- 

 stant temperature be formed, as will be shewn more fully in 

 Chap. VII. 



CtiAP. VI. — Can the Mean Temperature of a place he determined 

 from the Temperature of Springs ? and is the Mean Tempera- 

 ture of the Soil the same as that of the Air f 



Roebuck seems to have been the first who called attention to 

 the accordance of the mean temperature of the air with that of 

 the soil. He found, by three years* observation, that the former 

 is for London 5isi°.16, for Edinburgh 47°.68, and adds that the 

 mean temperature of the springs in London is 51°.01, and in 

 Edinburgh 46^.985. * 



At the time when Wahlenberg made his observations, the hy- 

 pothesis of an increase of temperature towards the centre of the 

 earth was not yet so generally adopted by philosophers as it is 

 at present. It was, consequently, not taken into consideration, 

 that the mean temperature of the soil can only be learnt from 

 springs which do not sink below the limit of the influence of the 

 external temperature. At a later period, it was considered plau- 

 sible to assume, that only the mineral springs rose from a more 

 or less considerable depth, and that all pure water springs had 

 their origin near the surface. But even if this be admitted, it 

 will be found by no means an easy matter to ascertain whether 

 a certain spring be a mineral or a pure water spring; for there 

 exists no general mark of distinction. Neither can the contents 

 of fixed, nor of gaseous substances, and still less the tempera- 

 ture, serve as the distinguishing character of a mineral spring. 

 There are springs, indeed common well waters, which are to- 

 lerably rich in fixed substances, and, in that respect, surpass 

 others which no one hesitates to call mineral springs, because 

 they contain gaseous substances in considerable quantities, which 



• Transactions, 1775, p. 469. 



