864 Sir J. Herschel on some Phenomena on Glaciers, SfC. 



I am not aware of any observations on the internal temperature 

 of glaciers — they are of course difficult from their usual rifty state ; 

 but the point may not be unworthy the attention of the scientific 

 traveller. May not this be the cause of those natural formations of 

 ice which have been observed in caverns, in Teneriffe, and on some 

 elevated points of the Jura chain, below the level of perpetual snow ? 

 It is obviously no matter whether the interior mass in the above 

 reasoning be ice or rock. It is enough, that its surface, during the 

 whole or great part of the year, should be covered with ice to bring 

 down the mean annual temperature of its interior materially below 

 the temperature due to its elevation, and which it would have were 

 it not so covered. Conceive now a mountain whose summit is in 

 this predicament, viz. constantly maintained at a mean temperature 

 below that due to its elevation. This intense cold will not break off' 

 at the level of the line of perpetual snow, which is determined by 

 the mean temperature of the atmosphere due to elevation, but will 

 be propagated downwards in the interior of its mass. Hence, if at 

 a short distance below the line of perpetual snow, where the mean 

 diurnal temperature of the exposed part, taken at a few feet or a few 

 yards deep in the soil or rock, is a little above freezing, we drive an 

 adit, or take advantage of a natural fissure to obtain the internal 

 temperature at a much greater depth from the surface ; we ought to 

 find it below 32°, and ice ought constantly to form in such cavities. 

 But even when the summit of a hill is not covered with ice, and 

 when therefore this particular principle does not apply, it is easy to 

 see, on the same general grounds, that something of the same kind 

 may obtain. It is obvious, that whenever a change of temperature on 

 the surface of a solid takes place, a wave of heat or cold, as the case 

 may be, will be propagated through its substance ; and if the changes 

 be regularly periodic, the waves will be also. Moreover it is clear 

 that the longer the periods of the external fluctuations are supposed, 

 the greater will be the interval of the waves, so as to make the 

 time taken for the propagated heat to run over them precisely equal 

 to the period of fluctuation. Now the rapidity with which succes- 

 sive waves of heat and cold destroy each other, is inversely as the 

 intervals, and thus the fluctuations of temperature depending on 

 long periods of external change will be propagated to greater depths 

 than those arising from shorter periods, nearly in the ratio of the 

 lengths of the periods. Thus the depths at which the annual fluc- 

 tuations of temperature cease to be sensible, will be between 300 and 

 400 times greater than those at which the diurnal ones are neutral- 

 ized. Now it may happen, from the slowness of propagation through 

 so considerable a depth, that the winter wave of cold (consisting of 

 many diurnal waves of alternate, greater and less intensity) may not 

 travel down to the adit or cavern till the hottest period of the next 

 summer, or of many summers ; in short, that if at any given time 

 the interior of the mountain were sounded by thermometers down its 

 whole axis, these instruments would exhibit alternate deviations + 

 and — from the mean temperature of the air. 



