156 Prof. TMschoff on the Temperature of 



four to six feet the maxima and minima follow about a month 

 later than in the air. In the shafts in which I am making thermo- 

 metrical observations at various depths, the maxima and minima 

 at a depth of 24 feet follow more than two months after those 

 of the air. The observations in the Erzgebirge also shew, in 

 general, that the maxima and minima take place later, the deeper 

 the points of observation lie ; however, there is no perfect regu- 

 larity in their succession, which is, no doubt, caused by the dis- 

 tributing influence of the external temperature. The springs 

 also, which shew the smallest variations in their temperature, 

 reach their maxima and minima of temperature about three 

 months after those of the air. But as the depths from which 

 they rise are unknown, no positive conclusion can be drawn from 

 them, as to the length of time necessary for the temperature of 

 the air to penetrate to a certain depth. Thermometrical obser- 

 vations, extending to the greatest depths, to which the external 

 influences penetrate, will, at a future time, furnish us with accu- 

 rate data, from which to determine the time necessary for the 

 spreading of heat into the interior of the earth. Till then we 

 can always assume, that about three months must elapse before 

 the temperature at the surface can reach a depth of about 60 

 feet. But this depth does not only depend on the conductibility 

 of heat of the strata of rocks and earth ; but also on the differ- 

 ence of temperature. For the greater the change of temperature 

 which a point of the earth suffers, the more quickly will this 

 change of temperature be communicated to the interior. The 

 greater the reduction of temperature, therefore ( to apply this 

 to the example in question), which the bottom of a lake at the 

 moment of its creation, or a valley which becomes filled up with 

 a glacier, suffers ; the more rapidly will the reduction of tem- 

 perature, all other circumstances being the same, travel into the 

 interior. This is perfectly analogous to the effect of latitude on 

 the depth, to which the external temperature penetrates into the 

 earth. In high latitudes, where the yearly variations of tem- 

 perature are greater than in low ones, the external temperature 

 also penetrates more rapidly than in low latitudes ; because the 

 point where the temperature first becomes constant lies deeper 

 in the former than in the latter. But that which is effected by 



