THERMAL SPRINGS. 197 



of late years, by the successful employment of chemistry, in 

 the geognostic investigation of the formation and metamorph- 



springs have been made the subject of investigation which undergo an 

 alteration in their temperature according to the seasons. 



"The variable springs fall into two natural groups : 



"1. Purely meteorological springs: that is to say, those whose aver- 

 age is demonstrably not elevated by the heat of the earth. In these 

 springs the amount of variation of the average from the aerial average 

 is dependent upon the distribution of the annual amount of rain through 

 the twelve months. These springs are on the average colder than the 

 air when the proportion of rain for the four cold months, from Decem- 

 ber to March, amounts to more than 333- per cent. ; they are on the 

 average warmer than the air when the proportion of rain for the four 

 warm months, from July to October, amounts to more than 33^- per 

 cent. The negative or positive difference of the spring average from 

 the air average is larger in proportion to the excess of rain in the 

 above-mentioned cold or warm thirds of the year. Those springs in 

 which the difference of the average from that of the air is in accord- 

 ance with the law, that is to say, the largest possible by reason of the 

 distribution of rain in the year, are called purely meteorological springs 

 of undistorted average ; but those in which the amount of difference of 

 the average from the air average is diminished by the disturbing ac- 

 tion of the atmospheric heat during the seasons which are free from 

 rain are called purely meteorological springs of approximate average. 

 The approximation of the average to the aerial average is caused 

 either by the inclosure, especially by a channel at the lower extremity 

 of which the temperature of the spring was observed, or it is the con- 

 sequence of a superficial course and the poverty of the feeders of the 

 spring. In each year the amount of difference of the average from 

 the aerial average is similar in all purely meteorological springs, but 

 it is smaller in the approximate than in the undistorted springs, and in- 

 deed is smaller in proportion as the disturbing action of the atmospher- 

 ic heat is greater. Of the springs of Marienberg four belong to the 

 group of purely meteorological springs ; of these four one is undis- 

 torted in its average, the three others are approximated in various de- 

 grees. In the first year of observation the portion of rain of the cold 

 third predominated, and all four springs were on the average colder 

 than the air. In the four following years of observation the rain of 

 the warm third predominated, and in these all the four springs had a 

 higher average temperature than the air; and the positive variation 

 of the average of the spring from that of the air was higher, the greater 

 the excess of rain in the warm third of one of the four years. 



" The view put forward in the year 1825 by Leopold von Buch, that 

 the amount of variation of the average of springs from that of the air 

 must depend upon the distribution of rain in the seasons of the year, 

 has been shown to be perfectly correct by Hallmann, at least for his 

 place of observation, Marienberg, in the Rhenish Graywacke mount- 

 ains. The purely meteorological springs of undistorted average alone 

 have any value for scientific climatology ; these springs are to be 

 sought for every where, and to be distinguished on the one hand from 

 the purely meteorological springs with an approximate average, and 

 on the other from the meteorologico-geological springs. 



"2. Meteorologico-geological springs: that is to say, those of which 



