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February 7, 1853. 



An addition was read to a paper by Professor De Moi-gan on 

 the Symbols of Logic, the Theory of Syllogism, &c. 



A paper was read by Mr. Denison on some Recent Improvements 

 in Clock Escapements. 



The object of this paper was to explain the construction of a new 

 remontoire or gravity escapement invented by the author, which has 

 now been in action for some time on the pendulum of the great 

 clock for the houses of parliament, and is in course of application to 

 others, both turret clocks and astronomical. 



But by way of introduction to this, which may be called the three- 

 legged gravity escapement (from the form of the scape- wheel), Mr. 

 Denison gave a description of another, which would similarly be 

 called the three-legged dead escapement, and had been previously in- 

 vented by him for the purpose of giving the impulse to the pendu- 

 lum with far less friction than usual. He found that it required only 

 jth of the force which a common dead escapement had required to 

 make the pendulum swing the same arc. And therefore, as com- 

 pared with a gravity escapement in which there is no sensible fric- 

 tion on the pendulum, there must be still more than fths of the 

 force in a common dead escapement wasted, in first producing fric- 

 tion on the pendulum, and then overcoming it by an increased im- 

 pulse. The time of the pendulum would be much more disturbed 

 than it is by the inevitable variations of this large amount of friction, 

 as well as that of the clock train, but for a fortunate tendency of the 

 different errors, which are caused by these variations of force and 

 friction, to correct each other. 



But the amount of this self-correction is uncertain, and some- 

 times one set of errors preponderates and sometimes the other ; and 

 so a dead escapement clock sometimes gains and sometimes loses 

 simultaneously with either an increase or a decrease of the arc of 

 vibration. And, consequently, none of the contrivances for iso- 

 chronizing a pendulum for different arcs can secure isochronism of 

 the clock ; and no further material improvement in clocks can be 

 expected, but from the solution of what has long been known as the 

 great problem of clock-making, viz. the invention of a simple escape- 

 ment which will give a constant impulse to the pendulum without 

 any sensible friction. 



Mr. Denison showed that his new gravity escapement satisfies all 

 the requisite conditions, mechanical, mathematical, and oeconomical. 

 Its principal features are, that the scape- wheel has only three pins, 

 not far from the centre, which lift the pallets or gravity-arms, and 

 three long teeth which are locked by stops on the arms. The velo- 

 city of the scape-wheel, which usually produces tripping, if the force 

 of the train is increased beyond what is just enough to lift the arms, 

 is moderated by a fan-fly set on the axis of the scape-wheel. The 

 arms are necessarily longer in this than in any other gravity escape- 

 ment, and this also gives a greater depth of locking within a given 

 angle, and therefore a still further security against tripping. And if 

 an arm is by accident lifted a little too high, the tooth does not 



