TRANSACTIONS OF SECTION A. 455 



and trouble incident to the preparation and development of sensitive papers are 

 avoided. 



The system can also he applied to the regulation of clocks driven by revolving 

 pendulum or other continuous motion. The index hand may he moved by an 

 oscillating pendulum, and the independent hand moved by a revolving pendulum. 

 If the revolving pendulum train goes too slow, contact is made and the current can 

 be made to diminish friction on a revolving disc ; and if the train goes too fast, 

 the friction can be increased by the current until the two movements become 

 isochronous. In fact, any angular or rectilineal movement can be repeated with 

 any required force without imposing any sensible friction on the originating 

 apparatus. 



12. On an Isochronic Pendulum. By Denny Lane. 



Every dynamic effect can be expressed by three factors — weight, space, and 

 time. If successive events can be made exactly similar, they must occupy the 

 same time, and time can be measured by counting the number of such events. 



The oscillations of a pendulum are the events that are most generally employed 

 for this purpose, but the successive events are not exactly similar, in consequence of 

 variations of five kinds : — 



1. Variation in temperature causing change in length of pendulum. 



2. Variation in density of the air in which pendulum moves. 

 8. Ditto in viscidity of the air. 



4. Change in arc of oscillation. 



5. Change in impulse given in order to maintain arc. 



It has been the habit hitherto to introduce compensations in order to counter- 

 balance these variations, or some of them. I propose, instead of compensating for 

 variations after they have arisen, to prevent the variations from occurring, or at 

 least to confine them within limits so narrow that they shall not affect the time of 

 the oscillation. 



1st. To prevent variations of temperature the pendulum is enclosed in a case 

 which is kept at a constant temperature, the case being surrounded by a fluid kept 

 always at the same heat by a thermostat. This thermostat is a vessel filled with 

 an expansible liquid, like alcohol, which by variation in volume raises or depresses 

 mercury contained in a bent tube. The upper surface of the mercury in one leg 

 of the bent tube acts as a valve to increase or diminish the supply of coal- 

 gas to a burner, and maintains at a constant temperature the fluid sur- 

 rounding the pendulum case, and therefore the pendulum itself. Thus variation in 

 heat is prevented. 



2nd. The pendulum case is hermetically sealed, the sides being rigid, and thus 

 any change in the pressure of the air contained is prevented. 



3rd. The air remaining at the same temperature and pressure, any variation in 

 its viscidity is prevented. 



4th. All the retarding forces being equal, the arc must be equal if the impulse 

 given to maintain the arc remains constant. 



5th. The impulse is given to the pendulum by the intermittent action of an 

 electro-magnet, which is active as the pendulum descends, but loses its magnetism 

 as the pendulum rises. This impulse should remain the same if the current which 

 produces it remains the same, and this can be effected in three different ways. 

 Every electric current depending directly on the electro-motive force, and inversely 

 on the resistance, if the electro-motive force and the resistance remain the same or 

 retain the same ratio, the current is constant. 



(a) It is proposed to maintain the current the same by a vibrating regulator, 

 consisting of an adjustable armature, attached to a balance, and placed above an 

 electro-magnet included in the main circuit. As the current passes it attracts the 

 armature, which, as soon as the current ceases, is again drawn away by the action 

 of a counterpoise on the other end of balance. The intermittent current, therefore, 

 keeps the balance in vibration until the current falls below a certain fixed point, 

 when the vibration ceases. So long as the vibration lasts, a small funnel attached 



