The Evolution of the Sciences 



terranean magnetism, and to determine the 

 surfaces of which the curves on our magnetic 

 charts are merely the surface traces. This will 

 be of great assistance to stratigraphy. But 

 no one can tell at present whether this means of 

 investigation will enable us to study the metallic 

 masses, whose existence we suspect beneath 

 the siliceous crust which forms the globe's 

 surface. 



If the magnetic needle does not, as yet, 

 supply useful indications regarding the deeper 

 regions of the globe, the thermometer instead 

 affords us information of the highest interest. 

 The surface temperature of the ground varies 

 according to place, season, day and hour, and 

 depends closely on the heat received from the 

 sun, and on that radiated by the earth into the 

 atmosphere, but the temperature varies only in a 

 shallow, superficial layer of ground. Even the 

 most extreme temperatures in the year do not 

 affect it to a greater depth than seventy feet. 

 Below this film, sensible to external influences, 

 the temperature remains rigorously constant at 

 every point of the globe. Thus a thermometer 



kept for over a century in the cellars of the Paris 



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