THE INTERIOR OF THE EARTH. 301 



In this class of researches thermometers d deversement are used, 

 the reservoirs of which overflow as the temperature rises ; the mercury- 

 remaining in the ball shows the maximum attained. Waif erdin's regis- 

 tering thermometer and the geothermometer of Magnus are constructed 

 on this principle. Thermometers d minima, of a different construc- 

 tion, are used to determine the temperature of the ocean-depths, which 

 are generally colder than the water at the surface. The many sound- 

 ings made by the English scientific expeditions established beyond a 

 doubt the fact that the temperature at the bottom of the sea is often 

 but little above zero. This would be explained by supposing the 

 colder water to be carried to the bottom by its specific gravity, the 

 water warmed and dilated by the sun's heat remaining at the surface. 

 The bed of the ocean at large, where the normal temperature is not 

 affected by warm currents, such as the Gulf Stream, may be said to be 

 covered with water at the freezing-point. The water at the bottom 

 of fresh-water lakes is less cold because the maximum density of fresh 

 water is 4. It results from this that the portions possessing this 

 temperature are carried to the bottom, while the colder or warmer por- 

 tions rise to the surface. Thus, that portion of the earth's shell that 

 is covered by water remains at a relatively low temperature, in conse- 

 quence of the stratification resulting from the varying densities of the 

 liquid, but, if it were possible to carry on in the bed of the sea such 

 investigations as have been made on land, an increase of temperature, 

 such as has been proved to exist in the frozen soil of Siberia, would 

 doubtless be found. 



The increase in heat as we descend is generally admitted to aver- 

 age 1 in 30 metres. If this rate is constant it is clear that, at a depth 

 of 2,700 metres, the temperature must equal that of boiling water ; 

 and that, at a depth of 50 kilometres, the heat must exceed 1,600, a 

 point at which iron and the greater part of the rocks would melt. 

 This is the principal ground for the argument of those who maintain 

 that the earth's crust is not more than 40 to 50 kilometres thick or, 

 relatively to its size, of the thickness of an egg-shell compared with 

 the egg. Certain it is that the increase of heat with the depth, con- 

 firmed by so many observers, perforce gives a warrant to the idea of 

 a subterranean fire possessing an inconceivable degree of heat ; but 

 the question is, At what depth from the surface does this fire exist ? 



The thermometric observations thus far made are insufficient to 

 decide this question. Among the mines that have reached a great 

 depth may be mentioned those of Kitzbilhel in the Tyrol (900 metres) ; 

 Kutteuberg in Bohemia (1,200 metres) ; Mouille-Louge (920 metres); 

 and Speremberg (1,260 metres). Why may not borings be made at 

 the bottom of some of these very deep mines, by means of which the 

 bowels of the earth can be still further penetrated ? 



It is also desirable that the natural cavities in the earth should 

 be utilized for scientific investigation. The accounts contained in the 



