178 Performance of Some Timekeepers. 



I suppose that it is owing to want of attention to this that 1 

 frequently find expensive English watches, furnished with a 

 fusee, showing a greater variation of rate during the twenty- 

 four hours than a Waltham watch without fusee and without 

 stopwork. Very likely the Swiss going barrel watches 

 would go as well, but I have not had the opportunity of 

 testing high class Swiss watches. Those that were officially 

 tested at the Observatory during the Exhibition trials were 

 only timed once in twenty-four hours. In these circum- 

 stances this variation would not show itself. As a matter 

 of fact, I have in my possession a Waltham watch purchased 

 in Melbourne four years ago. It has such a long mainspring 

 and runs so easily that it will go forty-eight hours without 

 stopping. I have on three or four occasions forgotten to 

 wind it, so that at the end of the two days I have found it 

 moving very sluggishly, barely escaping, yet the greatest 

 difference of rate I have found from this cause has been only 

 nine seconds a day. It is sometimes less. The last occasion 

 was on the 8th of the present month, when it was not 

 wound. The two previous rates had been 5*7 sees, and 5 5 

 sees, losing, and between the 6th and the 8th the rate was 

 1*7 sees, losing. 



Supposing the watch to be now put together in good con- 

 dition and the balance perfectly poised, we will consider the 

 causes that will make it change its rate. First and foremost 

 will be change of temperature ; and unfortunately its effects 

 will be all in one direction, an increase of temperature will 

 weaken the elasticity of the balance spring, increase its 

 length, and also enlarge the diameter of the balance, each of 

 which makes the watch go slower, so that the whole effect 

 will be the sum of these partial ones. A decrease of tempera- 

 ture will act in a contrary direction, and cause the watch to 

 gain. By far the principal cause of variation is the alteration 

 of the elasticity of the spring; according to experiments made 

 by Berthoud, and confirmed by Mr Dent, the effect of this 

 amounts to more than four times that of the enlargement of 

 the balance, and that the total effect will be to make an 

 ordinary watch lose 63 seconds a day for an increase of 10° 

 Fahr.; whereas an ordinary clock with an iron rod to the 

 pendulum would only lose 3 seconds a day for the same 

 increment of heat. To obviate this difficulty, the first 

 chronometers were supplied with an apparatus that moved 

 the curb pins, and thus shortened the effective part of the 

 balance spring as the temperature increased; as this plan 



