THE HEAT OF THE SUN. 5 



actual variations in the amount of heat radiated from the 

 sun. In other words, there is not the slightest reason for 

 the belief that the sun has been during geological times 

 either appreciably hotter or appreciably colder than at the 

 present moment. 



One of the indications of the existence of sunbeams 

 throughout geological times is afforded by the eyes of 

 certain fossil animals. It may, for instance, be specially 

 noted that the great fish-like reptile, the Ichthyosaurus, 

 which lived during a remote period of this earth's history, 

 possessed an eye apparently unequalled in size and com- 

 plexity by the eye of any other animal living or extinct. 



There is no reason for being surprised that the sun 

 should have attracted the earth by the force of gravi- 

 tation a million years ago with just the same intensity 

 as it does at present. The gravitation between two 

 masses does not necessarily involve anything in the 

 nature of expenditure. Two cannon balls, for instance, 

 placed at a certain distance apart attract each other with 

 a certain force, and that force remains the same after any 

 lapse of time, provided the masses of the cannon balls 

 and the distance between them continue unchanged. The 

 case is, however, totally different, if it be radiation of 

 heat and not attraction of gravitation that is the pheno- 

 menon in question. For example, let us regard one of 

 the cannon balls as a red-hot body dispensing heat around 

 so as to warm the objects in its vicinity. Here, evidently, 

 the question of time enters as an important element. The 

 hot body will, in general, be able to impart heat to a cold 

 one, so long as the one retains an excess of temperature 

 over the other. It is, however, in the nature of a heated 



