ox THE IXTERACTIOX OF XATUEAL FORCES. 189 



the mean time tbe ocean did not become frozen, that one 

 side of the earth would be constantly turned towards the 

 sun, and enjoy a perpetual day, whereas the opposite side 

 would be involved in eternal night. Such a position we 

 observe in our moon with regard to the earth, and also in 

 the case of the satellites as regards their planets ; it is, 

 perhaps, due to the action of the mighty ebb and flow to 

 which these bodies, in the time of their fiery fluid con- 

 dition, were subjected. 



I would not have brought forward these conclusions, 

 which again plunge us in the most distant future, if they 

 were not unavoidable. Physico-mechanical laws are, as 

 it were, the telescopes of our spiritual eye, which can 

 penetrate into the deepest night of time, past and to 

 come. 



Another essential question as regards the future of our 

 planetary system has reference to its future temperature 

 and illumination. As the internal heat of the earth has 

 but little influence on the temperature of the surface, 

 the heat of the sun is the only thing which essentially 

 affects the question. The quantity of heat falling from 

 the sun during a given time upon a given portion of the 

 earth's surface may be measured, and from this it can be 

 calculated how much heat in a given time is sent out 

 from the entire sun. Such measurements have been 

 made by the French physicist Pouillet, and it has been 

 found that the sun gives out a quantity of heat per hour 

 equal to that which a layer of the densest coal 10 feet 

 thick would give out by its combustion ; and hence in a 

 year a quantity equal to the combustion of a layer of 

 17 miles. If this heat were drawn uniformly from the 

 entire mass of the sun, its temperature would only be 

 diminished thereby 1^ of a degree Centigrade per year, 

 assuming its capacity for heat to be equal to that of water. 

 These results can give us an idea of the magnitude of the 



