148 LECTURE XVII. 



on a perfect cylinder. But in some clocks, and in all watches, the moving 

 power is a spring. One of the first clock springs is said to have been an 

 old sword blade ; a clock with such a spring was lately preserved at Brus- 

 sels : the spring which is at present used, is a thin elastic plate of steel, 

 coiled into a spiral form. Every spring exerts the more force as it is more 

 bent ; in order to correct this inequality, the chain or cord by which it acts 

 on the work is wound on a spiral fusee ; so that in proportion as the force 

 is lessened, it is applied to a larger cylinder or a longer lever. The gene- 

 ral outline of the fusee must be nearly such that its thickness at any part 

 may diminish in the same proportion as it becomes more distant from the 

 point at which the force would cease altogether, the curve being that which 

 is denominated a hyperbola ; but the workmen have in general no other 

 rule than an habitual estimation.* (Plate XV. Fig. 199.) 



Notwithstanding all possible precautions in the immediate application of 

 the weight or spring, the irregular action of the teeth of the wheels, the in- 

 creasing tenacity of the oil usually employed, and other accidental dis- 

 turbances, make it still desirable to procure a further equalisation of the 

 force ; which is sometimes obtained in clocks by raising the loaded arm of 

 a lever to a given height whence it may descend ; and in watches, by 

 bending a spring into a given position from which it may return, so as to 

 limit with great precision the propelling force employed in each vibration. 

 The necessity of applying oil is sometimes in great measure removed by 

 jewelling the holes in which the axes or verges run; a perforation being 

 made in a plate of ruby, and a diamond applied upon this, in contact with 

 the end of the axis ; the hardness and high polish of these stones tending 

 very considerably to diminish the friction. 



There are also different methods of continuing the action of the force 

 while the clock or watch is wound up : a spring is interposed between the 

 fusee and the wheel impelled by it, a little inferior in force to the original 

 weight or spring, so as to remain always bent, until, when the pressure of 

 the main spring is removed, it begins to act upon a fixed point on one side, 

 and upon the wheel of the fusee on the other, so that it propels the work for 

 a short time with a force nearly equal to that of the main spring. Some- 

 times also the spring is wound up by causing a small wheel to revolve 

 round the centre of the fusee, having its teeth engaged on one side in those 

 of a wheel which makes a part of the fusee, and on the other side with the 

 internal teeth of a hoop connected with the work, so that the same pressure 

 which winds up the spring tends also to turn the hoop round, and to con- 

 tinue the motion. (Plate XVI. Fig. 200.) 



The scapement, by which the sustaining force is communicated to the 

 pendulum or balance, demands a greater exertion of skill and accuracy 

 than any other part of a timekeeper. Sometimes the alternate motion of 

 the pendulum has been produced by the action of a crank, but this con- 

 struction subjects it too much to the irregularities of the wheel work, and 

 is liable to several other objections. A crank cannot properly be called a 

 scapement, for according to the etymology of the term, the pendulum must 



* Lahire on the Figure of Fusees, Hist, et Mem. de Paris, ix. 102. Varignon 

 on do. ibid. 1702, p. 192, H. 122. 



