AIDS OF THE NEWTONIAN PERIOD. 473 



a measure of time more accurate than the sun itself. Hence astroo. 

 omers were soon led to obtain the right ascension of a star, not direct- 

 ly, by measuring a Distance in the heavens, but indirectly, by observ- 

 ing the Moment of its Transit. This observation is now made with a 

 degree of accuracy which might, at first sight, appear beyond the limits 

 of human sense, being noted to a tenth of a second of time : but we 

 may explain this, by remarking that though the number of the second 

 at which the transit happens is given by the clock, and is reckoned 

 according to the course of time, the subdivision of the second of time 

 into smaller fractions is performed by the eye, by seeing the space 

 described by the heavenly body in a whole second, and hence esti- 

 mating a smaller time, according to the space which its description 

 occupies. 



But in order to make clocks so accurate as to justify this degree of 

 precision, their construction was improved by various persons in suc- 

 cession. Picard soon found that Huyghens' clocks were affected in 

 their going by temperature, for heat caused expansion of the metallic 

 pendulum. This cause of error was remedied by combining different 

 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 then. 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 meas- 

 ures distance. 



The construction of good Watches, that is, portable or marine 

 clocks, was important on another account, namely, because they might 

 be used in determining the longitude of places. Hence the improve- 

 ment 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, 8 originally a carpenter, turned his mind to this subject with 

 success. After thirty years of labor, 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 erroi 



2 Mont. iv. 554. 



