ON TIME MEASURERS. 341 



This glass spring is the invention and handiwork of the late 

 Frederick Dent, and the chronometer before us represents the only 

 perfect specimen now in existence. 



The elasticity and strength of this spring are so extraordinary that 

 until you had examined it you could scarcely credit its being glass, but 

 as seeing is believing, and we shall be happy to exhibit a portion of 

 another one to any person who may be interested in it after the 

 demonstrations. 



But meanwhile, 'for illustration, we will tell you two experiments, the 

 second one being quite unintentional. In the first a chronometer was 

 taken having an ordinary steel spring, the balance of which was oscil- 

 lating 1 80 from rest a glass spring was substituted, everything else 

 remaining exactly the same, the oscillation of the balance rose to 200. 



The unintentional experiment was that the chronometer was 

 knocked off a table, and though both points of the staff were broken 

 "Tn^^balance'spring sustained no injury. 



With regard to the specimen before us you will see that with the 

 exception of having rather more coils, it is shaped like a steel spring. 

 The oscillation of the balance is 180 from rest. The glass spring was 

 under trial at the Royal Observatory for a period of nearly three years, 

 and the following results were attained. The figures are the average 

 daily rates calculated from periods of a month, the mean temperature 

 being given opposite each. There is one peculiarity about this per- 

 formance to which we must briefly call attention. 



This rate shows that the chronometer had always a tendency to 

 accelerate or to advance upon its rate, and with regard to this it may 

 be interesting here to state that steel springs exhibit the same tendency 

 just in proportion to their hardness. And that a soft steel or gold 

 spring "accelerates" in the reverse direction, i.e., loses upon its rate. 



Now let us return and inquire what is the behaviour of balance 

 springs of different materials under a given change of temperature. 



Suppose that we take four chronometers, the first with a balance 

 spring of gold, the second with a balance spring of steel, the third with 

 a balance spring of palladium, and the fourth with a balance spring of 

 glass ; and in place of the ordinary balance we substitute upon each a 

 balance composed of a glass disc, so as practically to eliminate any 

 error due to the expansion of the balance. Then, if we regulate each 



