INSTRUMENTS OF PRECISION 



balance-wheel was not original with Huygens, how- 

 ever, as it had been first conceived by Robert Hooke; 

 but Huygens's application made practical Hooke's 

 idea. Tn England the importance of securing accurate 

 watches or marine clocks was so fully appreciated 

 that a reward of 20,000 sterling was offered by Par- 

 liament as a stimulus to the inventor of such a time- 

 piece. The immediate incentive for this offer was the 

 obvious fact that with such an instrument the de- 

 termination of the longitude of places would be much 

 simplified. Encouraged by these offers, a certain car- 

 penter named Harrison turned his attention to the 

 subject of watch-making, and, after many years of labor, 

 in 1758 produced a spring time-keeper which, during a 

 sea- voyage occupying one hundred and sixty-one days, 

 varied only one minute and five seconds. This gained 

 for Harrison a reward of 5000 sterling at once, and a 

 little later 10,000 more, from Parliament. 



While inventors were busy with the problem of 

 accurate chronometers, however, another instrument 

 for taking longitude at sea had been invented. This 

 was the reflecting quadrant, or sextant, as the im- 

 proved instrument is now called, invented by John 

 Hadley in 1731, and independently by Thomas God- 

 frey, a poor glazier of Philadelphia, in 1 730. Godfrey's 

 invention, which was constructed on the same principle 

 as that of the Hadley instrument, was not generally 

 recognized until two years after Hadley's discovery, 

 although the instrument was finished and actually in 

 use on a sea-voyage some months before Hadley re- 

 ported his invention. The principle of the sextant, 

 however, seems to have been known to Newton, who 



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