ON ITS EXTERIOR. THERMAL SPRINGS. 185 



mutually illustrative of each other, and some allusion 

 to such connecting links as can be recognised between 

 them should not be omitted where generalisation of 

 view is desired. 



The division of springs into warm and cold, which is 

 in such general use, and in ordinary parlance appears so 

 natural, will be seen to rest only on very uncertain 

 foundations, if it is wished to reduce it to definite nu- 

 merical expression. If the temperature of springs is 

 to be compared with that of the human body (the in- 

 ternal heat of which Brechet and Becquerel found with 

 thermo-electric apparatus to be from 98-1 to 98'6Fahr.) 

 the degree of the thermometer scale at which a liquid 

 when placed in contact with parts of the body would be 

 called cold, warm, or hot, will differ considerably ac- 

 cording to individual feelings. No absolute degree of 

 temperature can be determined on, as that above or 

 below which a spring shall always be termed warm or 

 cold. The proposal to call, in each climatic zone, a 

 spring, whose mean annual temperature does not ex- 

 ceed the mean annual temperature of the air in the 

 same zone, a cold spring, offers, at least, scientific 

 accuracy in the comparison of definite numbers. It has 

 also the advantage of leading to considerations respecting 

 the different origin of springs; since the agreement 

 with the mean annual atmospheric temperature is to be 

 looked for in "invariable" springs directly, and in 

 " variable " ones (as Wahlenberg and the elder Erman 

 have shown) in the mean of their summer and winter 

 temperatures. But according to this criterion, in one 

 zone a spring may be termed " warm," whose tempera- 

 ture is scarcely a seventh or an eighth part of that of 

 another which being situated in the equatorial regions 



