ON GEOLOGICAL TIME. 67 



i inch). The mass of a globe of water equal in 

 bulk to the earth is -97 x io 21 tons. 1 And, the 

 moon's mass being about B \j- of the earth's, the 

 attraction of the moon on a ton at the earth's 



distance is x 7- -^ or of a ton force, if, for 



80 6o 2 290,000 



brevity, we call a ton force the ordinary terrestrial 

 weight of a ton that is to say, the amount of the 

 earth's attraction on a ton at its surface. Hence 

 the whole force of the moon on a globe of water 



'Q7 X I O^ 



equal in bulk to the earth is ^ ' or 3-3 x IO 15 



290,000 



tons force. If, then, the tidal disturbance were 

 exactly what we have supposed, or if it were 

 (however irregular) such as to have the same re- 

 sultant effect, the retarding influence of the moon's 

 attraction would be that of 3*3 x io 15 tons force 

 acting in the plane of the equator and in a line 

 passing the centre at -.^ of a foot distance. Or it 

 would be the same as a simple frictional resistance 

 (as of a friction-brake) consisting of 33 x io 15 

 tons force acting tangentially against the motion 

 of a pivot or axle of about ^ inch diameter. To 

 estimate the retardation produced by this, we shall 

 suppose the square of the earth's radius of gyration, 



1 In stating large masses, if English measures are used at all, the 

 ton is convenient, because it is 1000 kilogrammes nearly enough for 

 many practical purposes and rough estimates. It is !Oi6'O47 kilo- 

 grammes ; so that a ton diminished by about I - 6 per cent, would 

 be just icoo kilogrammes. 



F 2 



