THE CONQUEST OF TIME AND SPACE 



subsequently devised by Mr. Graham, through the use 

 of a well of mercury in connection with the pendulum, 

 so arranged that the expansion of the mercury upward 

 in its tube would compensate the lengthening of the 

 pendulum itself under effect of heat, and vice versa; 

 but the Harrison pendulum, variously modified in 

 design, remains in use as a highly satisfactory solution 

 of the problem. 



Harrison early conceived the idea that it might be 

 possible to apply the same principle to the balance- 

 wheel of the watch. This problem presented very great 

 practical difficulties, but by persistent effort these were 

 finally overcome, and a balance-wheel produced, which, 

 owing to the unequal expansion and contraction of 

 its two component metals under changing temperature, 

 altered its shape and so maintained its rate of oscilla- 

 tion almost — though never quite — regardless of chang- 

 ing conditions of temperature. 



In 1 761 Harrison produced a watch which was tested 

 on a British ship in a trip to the West Indies in that 

 and the succeeding year, and which proved to be a 

 time-keeper of hitherto unexampled accuracy. The 

 inventor had calculated that the watch, when carried 

 into the tropics, would vary its speed by one second 

 per day with each average rise of ten degrees of tem- 

 perature. Making allowance for this predicted alter- 

 ation, it was found that the watch was far within the 

 limits of variation allowed by the conditions of the test 

 above outlined. It had varied indeed only five seconds 

 during the journey across the ocean. On the return 

 trip the watch was kept in a place near the stem of the 



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