beneath the surface of the Earth. 15 



from the surface, sources of heat amply sufficient to raise the air and 

 water to the temperature they are found to possess at great depths. 

 These objections have been severally and satisfactorily answered, and 

 the last and most ingenious of them has been ably refuted by Mr. 

 Fox,* who has shown, that in the Cornish mines the ascending has 

 generally a higher temperature than the descending currents. The 

 difference varied from 9° to 17°, showing that, instead of imparting 

 heat, the currents of air actually carry off a large quantity of heat 

 from the interior of the mines. 



It has been objected also to the doctrine of a central fire, that we 

 perceive no traces of increase of temperature in the ocean at great 

 depths. Now, in the consideration of this point, two principles are 

 involved, ^yirsi", that to which Mr. Fox has adverted, that the strata at 

 the bottom of the ocean, were they composed of solid rock the most 

 favorable for transmitting heat, would yet propagate it much more 

 slowly than the water which covers it, and thus all accumulation must 

 necessarily be prevented ; second, the principle of the maximum 

 density of water. 



All the observations hitherto made on masses of fresh water show, 

 that at great depthsf the temperature differs only a very few degrees 

 from the point of maximum density. At greater depths it will prob- 

 ably be found to be very near that point. The latest experiments 

 make the maximum density of pure water at about 38.75°, while those 

 of Hallstrom make it, 39.38°. If the water be impure the point of 

 maximum density will fall more or less, according to the nature and 

 amount of the foreign bodies it may hold in solution. The experi- 

 ments of Ermann show that the point of greatest density sinks very 

 rapidly as we add any saline ingredient. 



Now, as the heaviest parts of any fluid will always find their way 

 to the bottom, the deeper we descend in a mass of water, either salt 

 or fresh, we must find it the colder until we come to the limit of 

 greatest density. In fresh water lakes of great depth the tempera- 

 ture of the water will decrease as we descend till we reach the limit 

 of 39° Fah. when the temperature will undergo no farther change. 

 In salt water the point of maximum density increases with its saltness 

 towards the poles, J so that the depth at which the temperature be- 



* Phil. Mag. and An. Feb. 1830. 



t At 1000 feet Saussure found the lake of Geneva to have a temperature of 42° F. 



i See this Number, page 11, 



