FOR THE ADVANCEMENT OF SCIENCE, 1857. 501 



stratum of coal seventeen miles in thickness : and if the Sun's 

 capacity for heat be assumed equal to that of water, and the heat 

 be supposed to be drawn uniformly from its entire mass, its tem- 

 perature would thereby undergo a diminution of 2'4 Fahr. 

 annually. 



On the other hand, there is a vast store of force in our System 

 capable of conversion into heat. If, as is indicated by the small 

 density of the Sun, and by other circumstances, that body has not 

 yet reached the condition of incompressibility, we have, in the 

 future approximation of its parts, a fund of heat probably quite 

 large enough to supply the wants of the human family to the end 

 of its sojourn here. It has been calculated that an amount of con- 

 densation, which would diminish the diameter of the Sun by only 

 the ten-thousandth part, would suffice to restore the heat emitted 

 in 2000 years. 



Again, on our own earth, vis viva is destroyed by friction in 

 the ebb and flow of every tide, and must therefore reappear as heat. 

 The amount of this must be considerable, and should not be over- 

 looked in any estimation of the physical changes of our globe. 

 According to the computations of Bessel, 25,000 cubic miles of 

 water flow, in every six hours, from one quarter of the earth to 

 another. The store of mechanical force is thus diminished, and 

 the temperature of our globe augmented, by every tide. We do 

 not possess the data which would enable us to calculate the magni- 

 tude of these effects. All that we know with certainty is, that the 

 resultant effect of all the thermal agencies, to which the earth is 

 exposed, has undergone no perceptible change within the historic 

 period. We owe this fine deduction to Arago. In order that the 

 date palm should ripen its fruit, the mean temperature of the place 

 must exceed 70 Fahr. ; and, on the other hand, the vine cannot be 

 cultivated successfully when the mean temperature is 72 or up- 

 wards. Hence, the mean temperature of any place, at which these 

 two plants flourished and bore fruit, must lie between these narrow 

 limits, i. e. could not differ from 71 Fahr. by more than a single 

 degree. Now, from the Bible we learn that both plants were 

 simultaneously cultivated in the central valleys of Palestine, in the 

 time of Moses ; and its then temperature is thus definitively deter- 

 mined. It is the same at the present time ; so that the mean tem- 

 perature of this portion of the globe has not sensibly altered in the 

 course of thirty-three centuries. 



