PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 403 



devices however sink into insignificance when compared with the modern, 

 mechanism for measuring- time. 



Tlie principal parts of the clock and watch are — 



1st. The motive power, called in this case the maintaining power, which 

 is either a weight or spring. 



2d. Either the pendulum or the balance wheel. 



3d. The escapement. 



4th. The wheel work connected with the three hands. 



5th. The retaining power used during the process of winding up. These 

 parts are essential to every time-piece ; to these may be added — 



6th. The striking apparatus, which is motion at fixed periods, and the 

 repeating apparatus which is put in motion at will and gives any moment 

 the hours and quarters that have past. 



*Ith. The alarm attachment which can be fixed in advance to give an 

 alarm at any appointed time. 



Leaving it to otliers present to discuss these parts in detail your atten- 

 tion is now directed for a moment to the pendulum. To Gallileo, we are 

 indebted for the discovery and practical application of its peculiar property 

 called isochronism, or the disposition to vibrate in different arcs in very 

 nearly the same time. It was said he was led to the investigation by 

 watching the swinging of a chandelier, hung by a long chain from the roof 

 of a church in Florence. But the celebrated Dutch philosopher Huyghens, 

 first demonstrated mathematically that if a weight sustained by a string or 

 rod describe a cycloid, its vibrations, however large or small, would be 

 performed in the same time. Attempts were made to give this motion to 

 the pendulum of a clock, but they were not successful, and it was soon 

 found that if the pendulum was allowed to describe a very small arc the 

 error would be quite small and would be partly remedied by the spring 

 which usually sustains the pendulum. It will be remembered that the 

 time of its vibration is proportional to the square root of its length. A 

 two seconds pendulum being four times as long as a second pendulum. 

 This relation is expressed by the formula 



t being the time in seconds ; tt, the ratio of the circumference to the diameter, 

 or 3.14159 to 1 ; Z, the length of the pendulum, and g the force of gravity 

 in the latitude where the pendulum is to vibrate. The regulation of the length 

 of the pendulum is by a nut and screw, on the rod below the weight or bob, 

 and upon which it rests. It was early discovered that clocks gained time 

 in cold weather and lost time in hot weather, owing to the contraction and 

 expansion of the pendulum rod. Many devices have been made to remedy 

 this defect ; one is a weight of lead whose length greatly exceeds its diame- 

 ter sustained by a wooden rod at the bottom so that while the wood ex- 

 panded and made the weights slightly lower, the lead expanded several 

 times more in proportion and thus kept the centre of gravity at its proper 

 position. Wood is affected less by temperature and more by moisture, 

 which is a serious objection. The two best pendulums are known as the 

 mercury weio-ht and the compensation bars or gridiron rod. The principle 

 of compensation is the same in both; while the rod is moved downward the 



