470 Sir W. Thomson on a New Astronomical Clock. [June 10, 



cent, faster than the central shaft. This outer shaft, by means of friction 

 produced by the pressure of proper springs, carries the nut collar round 

 along with it, except when the escapement-tooth is stopped by either of the 

 pallets attached to the pendulum. A stiff cross piece (like the head of a T), 

 projecting each way from the top of the tubular shaft, carries, hanging 

 down from it, the governing masses of a centrifugal friction governor. 

 These masses are drawn towards the axis by springs, the inner ends of 

 which are acted on by the nut collar, so that the higher or the lower the 

 latter is in its range, the springs pull the masses inwards with less or more 

 force. A fixed metal ring coaxial with the main shaft holds the governing 

 masses in when their centrifugal forces exceed the forces of the springs, 

 and resists the motion by forces of friction increasing approximately in 

 simple proportion to the excess of the speed above that which just balances 

 the forces of the springs. As long as the escapement-tooth is unresisted, 

 the nut collar is carried round with the quicker motion of the outer tubular 

 shaft, and so it screws upwards, diminishing the force of the springs. 

 Once every semiperiod of the pendulum it is held back by either pallet, 

 and the nut collar screws down as much as it rose during the preceding 

 interval of freedom when the action is regular ; and the central or main 

 escapement-shaft turns in the same period as the tooth, being the period of 

 the pendulum. If through increase or diminution of the driving-power, or 

 diminution or increase of the coefficient of friction between the govern- 

 ing masses and the ring on which they press, the shaft tends to turn faster 

 or slower, the nut collar works its way down or up the screw, until the 

 governor is again regulated, and gives the same speed in the altered circum- 

 stances. It is easy to arrange that a large amount of regulating power 

 shall be implied in a single turn of the nut collar relatively to the central 

 shaft, and yet that the periodic application and removal of about of this 

 amount in the half period of the pendulum shall cause but a very small 

 periodic variation in the speed. The latter important condition is secured 

 by the great moment of inertia of the governing masses themselves round 

 the main shaft. I hope, after a few months' trial, to be able to present a 

 satisfactory report of the performance of the clock now completed accord- 

 ing to the principles explained above. As many of the details of execution 

 may become modified after practical trial, it is unnecessary that I should 

 describe them minutely at present. Its general appearance, and the arrange- 

 ment of its characteristic parts, may be understood from the photograph 

 now laid before the Society. 



VII. " On the Effect of Changes of Temperature on the Specific 

 Inductive Capacity of Dielectrics/'' By Sir W. Thomson, LL.D., 

 F.li.S. 



[The publication of the text of this paper is postponed.] 



