84 ' CHRONOMETERS. 



was that first reduced the portable spring clock 

 into a size of a watch, though he is also supposed to 

 have been a German. Watches were common in 

 France before 1544. 



The usual distinction of modern chronometers 

 is into clocks and watches; the latter being very 

 small and portable. When watches are constructed 

 with extraordinary care for astronomical or nautical 

 purposes, they are called time-keepers. 



Clocks sometimes are made to strike the hour, 

 and sometimes they do not; when watches are 

 made to strike, they are called repeating watches. 



In chronometers the moving power is of two 

 kinds; either a weight or a spring: the latter only 

 can be employed in portable instruments. 



When a weight is the moving power, it is sus- 

 pended by a cord, which passes several times round 

 a cylinder, which is made to turn, as the weight 

 descends by the action of gravity. Were there no 

 resistance from the wheels, the weight would fall, 

 as all other bodies, with an accelerated motion; 

 but the friction of the teeth and the resistance of 

 the air check this acceleration, which arrives at 

 last at its maximum. But it is necessary, for the 

 exact measurement of time, that all sources of in- 

 equality in the motion of the machinery should be 

 removed, and this is effected by a very ingenious 

 contrivance; but which cannot be understood 

 without reference to a figure. 



Plate 5. fig. 1. represents the profile of a clock, 

 in which P is the weight that is suspended by a 

 rope that winds about the cylinder or barrel C, 

 which is fixed upon the axis a a : the pivots b b go 

 into holes made in the plates T S, T S, in which 

 they turn freely. These plates are made of brass 

 or iron, and are connected together by four pillars 



