668 THE POPULAR SCIENCE MONTHLY. 



tion to be made to learn its causes, and what can be done to arrest it 

 and restore the trade. 



Perhaps we ought yet to pause, before taking up the balance itself, 

 to take note of a form of escapement employed in chronometers (using 

 the word in its strict sense as applied to clocks which may be moved, 

 but must not be jarred or tossed about, and are to remain in nearly the 

 same position). In this escapement the tooth of the scape-wheel acts 

 directly upon the balance, and after so acting is caught upon a shoul- 

 der, from which it is only released by the return of the balance. Bet- 

 ter results have been obtained with this than with the detached lever 

 for chronometers, but it is not so good for watches, as it will not bear 

 violent and sudden tossing about into all positions. 



To measure time accurately with a balance-wheel, three sources of 

 variation must be overcome — that is, the balance must be adjusted to 

 heat and cold, to position, and to " isochronism," as it is termed — that 

 is, it must in some way counteract its own vai-iation, due to tempera- 

 ture as well as that of the hair-spring ; its bearings must be so cut or 

 shaped that changes of position will not occasion unequal friction ; it 

 must be made to beat uniformly, in spite of the variations in power 

 which result from doing away with the fusee. 



The method of compensating a balance is to-day everywhere the 

 same. A compensated balance consists of a bar of steel, to the oppo- 

 site ends of which are attached semicircular bows of brass and steel 

 soldered together, the brass outside, and into these bows are screwed 

 w^hat are termed " set-screws." It will be seen that, in case of a change 

 for the warmer, the brass outer rim will expand more than the inner 

 steel one, in each of the arms, and that this will throw the extremi- 

 ties of the arms in toward the center, thus compensating, if the pro- 

 portions are right, for the general expansion of the balance ; and vice 

 versa in case of a change for the colder. Of course, it is a very nice 

 piece of experimentation which ascertains these proportions. After 

 the approximate proportions are secured, the exact ones are obtained 

 by means of the screws in the rim. Compensation in a balance-wheel 

 is of far more importance than in a pendulum, for the variation of the 

 rate of the time-piece, if not compensated, is far greater. And this is 

 due not only to the expansion and contraction of the balance, but also 

 to the variation in the power of the hair-spring under various tempera- 

 tures, as above remarked. 



I may pause to note that here is one of the tests of a watch that any 

 one may resort to. There are many imitation compensating balances 

 which look very like the genuine, save that they have not the cut, or 

 have only a notch at the extremity of the bow, so that the bow is not 

 free, and, of course, there is no compensation. 



After a watch has been adjusted to heat and cold, it must be 

 adjusted to " position " — that is, so that its rate will not be altered by 

 changes of position. This is a nice piece of work. It is accomplished 



