THE CONQUEST OF THE ZONES 



efforts of a large number of experimenters were directed 

 towards this end in the course of the eighteenth century. 

 These efforts were stimulated by the hope of earning 

 a prize of twenty thousand pounds offered by the British 

 Government for a watch sufl&ciently accurate to deter- 

 mine the location of a ship with maximum error of 

 half a degree, or thirty nautical miles, corresponding 

 to two minutes of time, in the course of a transatlantic 

 voyage. It affords a striking illustration of the relative 

 backwardness of nautical science, and of the diflScuI- 

 ties to be overcome, to reflect that no means then avail- 

 able enabled the navigator at the termination of a 

 transatlantic voyage to be sure of his location within 

 the distance of thirty nautical miles by any means 

 of astronomical or other observation known to the 

 science of the time. 



The problem was finally solved by an ingenious 

 British carpenter named John Harrison, who devoted 

 his life to the undertaking, and who came finally to be 

 the most successful of watchmakers. Harrison first 

 achieved distinction by inventing the compensating 

 pendulum — sl pendulum made of two metals having a 

 different rate of expansion under the influence of heat, 

 so adjusted that change in one was compensated by a 

 different rate of change in the other. Up to the time 

 of this discovery, even the best of pendulum clocks had 

 failed of an ideal degree of accuracy owing to the 

 liability to change of length of the pendulum — ^and so, 

 of course, to corresponding change in the rate of its 

 oscillation— with every alteration of temperature. An- 

 other means of effecting the desired compensation was 



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