GRAVISTATIC HEAT 301 



quote the testimony of Alfred Eussell Wallace (Is Mars 

 Habitable, p. 40) : 



In order that the problem may be understood and its im- 

 portance appreciated, it is necessary to explain the now generally 

 accepted principles as to the causes which determine the tempera- 

 tures on our earth, and, presumably, on all other planets whose 

 conditions are not wholly unlike ours. The fact of the internal 

 heat of the earth which becomes very perceptible even at the 

 moderate depths reached in mines and deep borings, and in the 

 deepest mines becomes a positive inconvenience, leads many 

 people to suppose that the surface-temperatures of the earth are 

 partly due to this cause. But it is now generally admitted that 

 this is not the case, the reason being that all rocks and soils, in 

 their natural compacted state, are exceedingly bad conductors of 

 heat. 



A striking illustration of this is the fact, that a stream of 

 lava often continues to be red-hot at a few feet depth for years 

 after the surface is consolidated, and is hardly any warmer than 

 that of the surrounding land. A stilll more remarkable case is 

 that of a glacier on the south-east side of the highest cone of 

 Etna underneath a lava stream with an intervening bed of vol- 

 canic sand only ten feet thick This was visited by Sir Charles 

 Lyell in 1828, and a second time thirty years later, when he made 

 a very careful examination of the strata, and was quite satisfied 

 that the sand and the lava stream together had actually preserved 

 this mass of ice, which neither the heat of the lava above it at 

 its first outflow, nor the continued heat rising from the great 

 volcano below it, had been able to melt or perceptibly to diminish 

 in thirty years. Another fact that points in the same direction is 

 the existence over the whole floor of the deepest oceans of ice- 

 cold water, which, originating in the polar seas, owing to its 

 greater density sinks and creeps slowly along the ocean bottom 

 to the depths of the Atlantic and Pacific, and is not perceptibly 

 warmed by the internal heat of the earth. 



In boring tunnels through the Alps several independ- 

 ent preliminary surveys are made with infinite care, so 

 that the work may be prosecuted from both ends at once 

 with the certainty of accurately meeting in the middle. 

 What would the chances be of meeting thus fortuitously? 

 Now, we have just such a parallel instance in the case of 

 the earth's heat, for, just about five feet under the sur- 

 face the temperature is uniform the year round and 

 exactly the mean of that of the atmosphere above. How 

 comes it that scientists have never thought worth while 



