NEWTON S PRINCIPIA. 295 



than the earth in the ratio of 11 to 9. The true diameter 

 of the moon is to the true diameter of the earth (according 

 to Newton) as 1 to 3-65. Hence the mass of the moon 

 is gjj-L. (or nearly ^th) that of the earth. It is now known 

 that the true mass is about ^th that of the earth. 



If the moon s body were fluid like our sea, the force of 

 the earth would raise tides in it. The tide in the moon 

 caused by our earth is to the tide in our sea caused by the 

 earth as the mass of the earth to the mass of the moon. 

 This ratio Newton imagined to be 1 to 39-788. Hence 

 he concluded the lunar tide was 93 feet. Upon this ac 

 count the figure of the moon would be a spheroid whose 

 greatest diameter produced would pass through the centre 

 of the earth and exceed the diameters perpendicular by 186 

 feet. It might be supposed that the tides in the moon 

 must be overwhelming. But as the moon always turns the 

 same face to us, the tide is stationary, and would therefore 

 merely affect the permanent surface of the lunar ocean. 

 But no sea has ever been detected in the moon. If the 

 moon had been originally fluid, or had been created in a 

 form which would have no tendency to break up, her 

 longest diameter must point to the earth. Hence the same 

 face of the moon would always appear to us, and the body 

 could not rest in any other position, but would always re 

 turn by a slow vibratory motion. 



[See Note IX.] 



u 4 



