AIDS OF THE NEWTONIAN PERIOD. 277 



metals, as iron and copper, which expand in a 

 different degree, in such a way that their effects 

 compensate each other. Graham afterwards used 

 quicksilver for the same purpose. The Escapement 

 too, (which connects the force which impels the 

 clock with the pendulum which regulates it,) and 

 other parts of the machinery, had the most refined 

 mechanical skill and ingenuity of the best artists 

 constantly bestowed upon them. The astronomer 

 of the present day, constantly testing the going of 

 such a clock by the motions of the fixed stars, has 

 a scale of time as stable and as minutely exact as 

 the scales on which he measures distance. 



The construction of good Watches, that is, port- 

 able or marine clocks, was important on another 

 account, namely, because they might be used in 

 determining the longitude of places. Hence the im- 

 provement of this little machine became an object 

 of national interest, and was included in the reward 

 of 20,000. which we have already noticed as offered 

 by the English parliament for the discovery of the 

 longitude. Harrison', originally a carpenter, turned 

 his mind to this subject with success. After thirty 

 years of labour, in which he was encouraged by 

 many eminent persons, he produced, in 1758, a 

 time-keeper, which was sent on a voyage to Jamaica 

 for trial. After 161 days, the errour of the watch 

 was only one minute five seconds, and the artist 

 received from the nation 5000/. At a later period 5 , 



2 Mont. iv. 554. 3 Ib. iv. 560. 



