ORIGIN AND AGE OF THE SUNS HEAT. 285 



other with the required velocity wholly independent 

 of gravitation. 



o 



We know nothing whatever regarding the absolute 

 motion of bodies in Space ; and, beyond the limited 

 sphere of our observation, we know nothing even of 

 their relative motions. There may be bodies moving 

 in relation to our system with inconceivable velocity. 

 For anything that we know to the contrary, were one 

 of these bodies to strike our earth the shock might be 

 sufficient to generate an amount of heat that would 

 dissipate the earth into vapour, though the striking- 

 body might not be heavier than a cannon-ball. There 

 is, however, nothing very extraordinary in the velocity 

 which we have found would be required to generate the 

 50 millions years' heat in the case of the two supposed 

 bodies. A comet having an orbit extending to the 

 path of the planet Neptune, approaching so near the 

 sun as to almost graze his surface in passing, would 

 have a velocity of about 390 miles per second, which is 

 within 86 miles of that required. 



It must be borne in mind, however, that the 476 

 miles per second is the velocity at the moment of colli- 

 sion ; but more than one-half of this would be derived 

 from the mutual attraction of the two bodies in their 

 approach to each other. Suppose, for simplicity of 

 calculation, each body to be equal in volume to the 

 sun, and of course one-half the density, the amount of 

 velocity which they would acquire by their mutual 

 attraction would be 274 miles per second. Conse- 

 quently we have to assume an original or projected 

 velocity of only 202 miles per second. And if we 

 assume the original velocity to have been 1700 miles 

 per second, an amount of heat would be generated in 

 a single moment which would suffice for no less than 

 800,000,000 years. And when we take into considera- 



