NAVIGATION. 35 



inner parts steel (less expansible), and thus carry 

 the attached weights inwards. The whole vibrating 

 mass, composed of axle, diameter, rims, and at- 

 tached weights, has thus less moment of inertia, 

 and so, with the less elastic stiffness of the hair- 

 spring, the balance-wheel vibrates with the same 

 quickness. 



This mode of compensating for temperature was 

 invented about one hundred years ago by Thomas 

 Earnshaw, to whom is also due the excellent form 

 of escapement now universally used in the marine 

 chronometer. 



28. The first chronometer used for determining 

 the longitude was invented by John Harrison, and 

 completed by him in a life-work of fifty years. 

 The origin of this first marine chronometer pre- 

 sents a most interesting chapter in the history of 

 inventions. Sir Isaac Newton pointed out the 

 great importance of an accurate chronometer at 

 sea, for determining the longitude. On the I ith of 

 June, 1714, the House of Commons appointed a 

 Committee, of whom he was one, to consider the 

 question of encouragement for the invention cf 



D 2 



