Cambridge Philosophical Society. 463 



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 y 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 

 escape, and the arm falls down again to its proper height until the 

 pendulum carries it off, the pressure of the long teeth on the stops 

 not being enough to hold it up. For these reasons also there is no 

 difficulty in satisfying the mathematical condition investigated by 

 Mr. Denison in a paper read before the Society in 1848, viz. that y 

 (the angle at which the pendulum leaves one arm and takes up the 



other) should =-^, or at any rate not be less than — (a being the 



extreme arc of vibration). The escapement requires no oil in the 

 parts affecting the pendulum ; and it contains no delicate work, and 

 is very easy to make ; and as a highly finished train will be no 

 longer necessary, astronomical clocks may be made on this plan much 

 cheaper, as well as better, than heretofore. 



In turret clocks an escapement of this kind supersedes the 

 necessity for a remontoire in the train to equalize the force on the 

 scape-wheel, and also of long and heavy pendulums, which are expen- 

 sive when compensated, and are sometimes difficult to fix. It will 

 also allow cast-iron wheels to be used throughout the clock (which 

 Mr. Dent has now used for several years in connexion with Mr. 

 Denison's spring remontoire for the train), as the friction of the train 

 can no longer affect the pendulum. 



Feb. 21. — Professor Challis gave a Lecture on Halos, Parhelia, 

 land Paraselense. 



March 7. — Professor Stokes gave an account of some further 

 researches relating to the Change of Refrangibility of Light. 



