340 THE PRINCIPLES OF SCIENCE. 



ing the principle of repetition in perfection, was employed 

 with wonderful skill by Sir George Airy, in his experi- 

 ments on the Density of the Earth at the Harton Colliery ; 

 the pendulums above and below being compared with 

 clocks, which again were compared with each other by 

 electric signals. So exceedingly accurate was this method 

 of observation, as carried out by Sir George Airy, that he 

 was able to measure a total difference in the vibrations at 

 the top and bottom of the shaft, amounting to only 2*24 

 seconds in the twenty-four hours, with an error of less 

 than one hundredth part of a second, or one part in 

 8,640,000 of the whole day x . 



The principle of repetition has been elegantly applied 

 in observing the motion of waves in water. If the canal 

 in which the experiments are made be short, say twenty 

 feet long, the waves will pass through it so rapidly that 

 an observation of one length, as practised by Walker, will 

 be subject to much terminal error, even when the observer 

 is very skilful. But it is a result of the undulatory theory 

 that a wave is quite unaltered, and loses no time by com- 

 plete reflection, so that it may be allowed to travel back- 

 wards and forwards in the same canal, and its motion, say 

 through sixty lengths, or 1 200 feet, may be observed with 

 the same accuracy as in a canal 1200 feet long, with the 

 advantage of greater uniformity in the condition of the 

 canal and water y. It is always desirable, if possible, to 

 bring an experiment into a small compass, so as to be well 

 under command, and yet we may often by repetition 

 enjoy at the same time the advantage of extensive obser- 

 vation. 



One reason of the great accuracy of weighing with a 

 good balance is the fact, that weights placed in the same 



* 'Philosophical Transactions/ (1856) vol. 146, Part i. p. 297. 

 y Airy, < On Tides and Waves,' Encyclopaedia Metropolitan, p. 345. 

 Scott Russell, ' British Association Report/ 1837, p. 432. 



