THE DISCOVERER 89 



Of these, only the briefest summary is here possible. 

 As to the first, Lord Kelvin (then Sir William 

 Thomson) assumed that the matter of which the 

 globe is made up is uniform throughout, and, there- 

 fore, that the rate at which it has parted with its heat 

 is uniform. After hesitating between the statement 

 that " the consolidation cannot have taken place less 

 than twenty million years ago, nor more than four 

 hundred million years ago," Lord Kelvin agreed that 

 " some such period of time as one hundred million 

 years ago " might be taken as a safely approximate 

 estimate. As to the second, when the earth and 

 moon were very near each other, the rotation was 

 enormously quickened, and, as the moon retreated, 

 the earth's rotation slowed, involving the gradual 

 lengthening of the day. The retardation is due to 

 the friction of the tides, which, under the pull of sun, 

 and, in far greater degree, of moon, act as a brake 

 upon the globe, increasing the day by about twenty- 

 two seconds of time in every century. The period 

 of the earth's habitability was reached when the day 

 was approximately what it is now, permitting of the 

 alternations of light and darkness, heat and cold, and 

 other conditions now prevailing. Lord Kelvin esti- 

 mates " that the centrifugal force at the time of solid- 

 ification cannot have been more than three per cent. 

 greater than it is at present, and, therefore, having 



