THE CONQUEST OF TIME AND SPACE 



caused by one running too fast or the other too slowly. 

 But with a third chronometer to check the comparison, 

 it is equally obvious that a dependable clue will be given 

 as to the exact time. 



It is to be understood of course that the variation 

 of any of the chronometers will be but slight if they are 

 good instruments. Moreover the tendency to vary 

 in one direction or the other of each individual instru- 

 ment will be known from previous tests. Such tests 

 are constantly made at the Royal Observatory in Eng- 

 land and elsewhere, and the best chronometers bear 

 certificates as to their accuracy and as to their rate of 

 variation. It may be added that a chronometer or 

 other timepiece is technically said to be a perfect 

 instrument, not when it has no variation at all — since 

 this has proved an unattainable ideal — but when its 

 variation is slight, is always in one direction, and is 

 perfectly or almost perfectly uniform. 



FINDING THE TIME WITHOUT A CHRONOMETER 



In the reference made above to the testing of Harri- 

 son's watch, it was stated that that instrument varied 

 by only a certain number of seconds in the course of the 

 westerly voyage across the Atlantic, and that its varia- 

 tion was somewhat greater on the return voyage. This 

 implies, clearly, that some method was available to 

 test the watch in the West Indies, without waiting for 

 the return to England. At first thought this may 

 cause no surprise, since the local time can of course be 

 known anywhere through meridian observations; but 



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