MARINE CHRONOMETER. 



48. Marine chronometers are usually suspended in a box on 

 gimbals, like those which support the ship's compass. The 

 balance-wheel usually vibrates in half seconds, being a much 

 slower rate than that of common watches. They are of immense 

 utility in navigation, and especially in long voyages. See Tract 

 on " Latitudes and Longitudes," Museum, vol. i. p. 97. 



49. In observatories where stationary timepieces can be used, 

 the clock moved by weights and regulated by a pendulum is 

 invariably adopted. The pendulum, in such cases, is always 

 so constructed that its rate of vibration shall not be affected by 

 variations of temperature. This is accomplished usually by con- 

 structing it of two different kinds of metal, which are differently 

 affected by heat, one being more expansible than the other. They 

 are so arranged that the expansion of one shall elevate the centre 

 of gravity, while that of the other lowers it, and the two effects 

 are made to compensate each other, so that, however the tempera- 

 ture may change, the rate of vibration will remain the same. 



50. In clocks adapted for domestic and public use, it is found 

 desirable that they should give notice of the time, not only to the 

 eye, but to the ear ; and for that purpose a bell is attached to 

 them, which tolls at given intervals, the number of strokes being 

 equal to that of the units in the number expressing the hour. 

 This appendage is called the STUIKING TRAIN". 



The striking train, though connected with that which moves 

 the hands, is quite independent of it, having its own moving and 

 regulating power, and its own system of wheels by which the 

 effect of the moving power is submitted to the regulator, and 

 transmitted to the tongue of the bell. 



Unlike the train which moves the hands, the striking train is 

 not in continual motion. Its motion is always suspended, except 

 at the particular moment at which the clock strikes. The mecha- 

 nism partakes of the character of an alarum, being stopped by a 

 certain catch until the hands point to some certain hour, and then 

 being set free by the withdrawal of the catch. It remains free, 

 however, only so long as is necessary for the tongue or hammer to 

 make the necessary number of strokes upon the bell, after which the, 

 catch again engages itself in the striking mechanism, and stops it. 



Some clocks only strike the hour. Others mark the half hours, 

 and others the quarters, by a single stroke. 



The general principle of the striking mechanism will be rendered 

 intelligible by fig. 31, which represents it in the case of a common 

 clock moved by a weight. 



The weight suspended from the cord E moves the train in the 

 same manner as in the case of the train which moves the hands. 

 The motion of the first wheel, c, is transmitted to the last wheel, I, 



45 



