THE ACCURATE MEASUREMENT OF TIME. 66^ 



temperature quite so promptly as the slim rod of tlie pendulum. But 

 this is not found to be a serious matter. 



Of late, zinc has been coming into use in the construction of com- 

 pensating pendulums. The best tower-clocks now all have zinc pendu- 

 lums. The principle and the application are the same as in the " grid- 

 iron," but the higher sensitiveness of the zinc simplifies the construc- 

 tion to such an extent that, only one return-rod of zinc being called for 

 instead of two, as in the case of the brass, it can be made in the form 

 of concentric tubes, and thus appear as a simple rod. 



Such are the jDrincipal devices for securing compensation. There 

 are many other ways of reaching the same end, and patents have been 

 issued for ingenious designs, no one of which, however, has come into 

 general use. 



But, even with a pendulum compensated with brass, zinc, or mer- 

 cury, we have not yet conquered the problem of measuring time with 

 great accuracy, for, aside from the practical difficulties of obtaining 

 homogeneous material and getting the proportions of the metals exact, 

 there is yet to be taken into account another cause of variation 

 which one could only be convinced by demonstration is capable of 

 having any appreciable effect. It is that of atmospheric pressure. It 

 has long been known that clocks vary somewhat with the barometer ; 

 and, of course, we see that they must, when we remember that a heav- 

 ier or denser atmosphere tends to decrease the gravity of all objects — 

 to bear them up, as it were — and hence, when the barometric pressure 

 is greater, a clock will run slower. To counteract this, a magnet is re- 

 sorted to, which is made to approach or withdraw from the pendulum 

 by means of the rise and fall of the barometer. This is placed, of 

 course, below the pendulum, and if nicely adjusted will keep the weight 

 of the pendulum uniform. 



If all the above disturbing elements are met by counteracting ex- 

 pedients, we now have a clock which will theoretically run without 

 variation, provided it is once brought into the proper beat. A thumb- 

 screw at the bottom of the bob will accomplish this approximately, 

 but, to do it with the greatest accuracy, it is necessary to have a little 

 cup suspended on the pendulum-rod near its upper end, into which one 

 can drop some small weights, as shot, quickening the variation there- 

 by, since it is a virtual raising of the center of oscillation. Fine astro- 

 nomical clocks usually have this cup, and the best turret-clocks also. 



It is at this point interesting to inquire how closely a clock, con- 

 structed in observance of all these principles, can be made to run. 

 Generally speaking, it may be replied that it is a pretty good regu- 

 lator which can be depended upon for a variation of less than a second 

 a day, through all weathers, despite what is claimed for them by their 

 owners. A distinction must be made between a uniform variation 

 and keeping a mean time. I have a common eight-day spring-clock 

 which does not seem to vary a half-minute in a month. This is, per- 



