THE PROBABLE AGE OF THE WORLD. 66j 



next prospectus ? " " A figure of two," says Mr. Tigg, " and as many 

 naughts after it as the printer can get into the same line." 



It is hard for imagination to compass the meaning of a million, 

 and, when that number is multiplied by hundreds, the effort is alto- 

 gether beyond us. But we need not dwell on this consideration ; we 

 turn at once to the practical comments made by physical science on 

 these and such-like opinions. The first is founded on the secular cool- 

 ing of the earth. 



If a red-hot ball be taken from a furnace, it begins at once to part 

 with heat at a certain definite rate. As it becomes colder it cools 

 more and more slowly. From the known laws of heat it is quite 

 possible roughly to appi-oximate to the period during which the earth 

 has been habitable for animals and plants such as we now find upon 

 it. Whenever a body is hotter at one part than at another, the ten- 

 dency of heat is to flow from the hotter body to the colder. As the 

 earth's crust is warmer as we go farther down, there must be a steady 

 increase of heat from the surface to the centre, and the earth is even 

 now losing heat at a perfectly measurable rate ; therefore it is pos- 

 sible to calculate what was the distribution of heat a hundred thou- 

 sand or a thousand thousand years ago, supposing the present natural 

 laws to have been then in existence. According to these data, about 

 ten millions of years ago the surface of the earth had just consoli- 

 dated, or was just about to consolidate; and in the course of com- 

 paratively few thousand years after that time the surface had become 

 so moderately warm as to be fitted for the existence of life such as 

 we know it. If we attempt to trace the state of affairs back for a 

 hundred millions, instead of ten millions of years, we should find that 

 the earth (if it then existed at all) must have been liquid, and at a 

 high white heat, so as to be utterly incompatible with the existence 

 of life of any kind with which we are acquainted. 1 



The next argument, namely, that founded on the earth's retarda- 

 tion by the tidal wave, is more recondite, and the theory that there is 

 such a retardation at all is quite of recent date. Theoretical reasons 

 connected with mechanics caused it to be adopted, and its establish- 

 ment depends on the most refined astronomical investigation. 



It is one of the peculiarities of time-measurement that, from the 

 nature of things, no two periods of time can be compared directly 

 one with another. The standards by which we measure time are less 

 and less precise as we recede farther into the past. To-day we have 

 as the standard unit of duration the interval between tw T o successive 

 transits of a star over the cross-wires of a fixed observatory-telescope. 

 This measure has been considered until lately as absolutely fixed and 

 invariable. And so it is for all practical purposes ; the sidereal time 

 of any heavenly body passing the meridian on a given day in 1880 



1 " The ' Doctrine of Uniformity ' in Geology briefly refuted." " Proceedings of the 

 Royal Society, Edinburgh, December, 1865." 



