ON GEOLOGICAL TIME. 47 



mechanical energy at all. Yet what an amount of 

 mechanical energy is emitted from the sun every 

 year ! If we calculate the exact mechanical value 

 of the heat he emits in 81 days we find it 

 equivalent to the whole motion of the earth in her 

 orbit round the sun. The motion of the earth in 

 her orbit round the sun has a certain mechanical 

 value ; a certain quantity of steam power would be 

 required, acting for a certain time, to set a body 

 as great as the earth into motion with the same 

 velocity. That same amount of steam power 

 employed for the same time in rubbing two stones 

 together would generate an enormous quantity of 

 heat, as much heat as the sun emits in 81 days. 

 But suppose the earth's motion were destroyed, 

 what would become of the earth ? Suppose it 

 were to be suddenly, by an obstacle, stopped in its 

 motion round the sun ? It would suddenly give 

 out 8 1 times as much heat as the sun gives out in 

 a day, and would begin falling towards the sun, 

 and would acquire on the way such a velocity that, 

 in the collision, a blaze of light and heat would be 

 produced in the course of a few minutes equal to 



