374 Mr. J. Croll on Geological Time, and the probable 



amount of heat as would thus be generated; and when they 

 condensed on cooling, they would form one spherical mass 

 like the sun. It is perfectly true that two such bodies could 

 never attain the required amount of velocity by their mutual 

 gravitation towards each other. But there is no necessity 

 whatever for supposing that their velocities were derived from 

 their mutual attraction alone. They might have been approach- 

 ing towards each other with the required velocity wholly inde- 

 pendent of gravitation. 



We know nothing whatever regarding the absolute motion of 

 bodies in space. And beyond the limited sphere of our obser- 

 vation, we know nothing regarding even their relative motions. 

 There may be bodies moving in relation to our system with in- 

 conceivable velocity. For anything that we know to the con-, 

 trary, were one of these bodies to strike our earth, the shock 

 might be sufficient to generate an amount of heat that would dis- 

 sipate 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 re- 

 quired in the two supposed bodies to generate the 50,000,000 

 years' heat. 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 the re- 

 quired velocity. 



But in the original heating and expansion of the sun into 

 a gaseous mass, an amount of work must have been per- 

 formed against gravitation equal to that which has been per- 

 formed by gravitation during his cooling and condensation, a 

 quantity which we have found amounts to about 20,000,000 

 years' heat. The total amount of energy originally communi- 

 cated by the concussion must have been equal to 70,000,000 

 years' sun-heat. A velocity of 563 miles per second would give 

 this amount. All, however, that is intended by this hypothesis 

 is merely to show how easy it is to explain how the sun may 

 originally have become an incandescent gas ^filling the entire 

 space occupied by the planetary system. 



But is it the case that geology really requires such enormous 

 periods as is generally supposed ? At present, geological esti- 

 mates of time are little else than mere conjectures. Geological 

 science has hitherto afforded no trustworthy means of estimating 

 the positive length of geological epochs. Geological phenomena 

 tell us most emphatically that these periods must be long ; but 

 how long, these phenomena have, as yet, failed to inform us. 

 Geological phenomena represent time to the mind under a most 

 striking and imposing form. They present to the eye, as it were, 



