CLOCK. 



iise on Clock and Watch Making," pub- 

 lished in 1766, who has also asserted that 

 the cycloidal principle would not be of 

 the benefit imagined, " as the inequality 

 of the vibrations of the pendulum moving 

 in a circular arc correct those caused by 

 the alteration of its weight from the va- 

 riations of atmospherical gravity, so as 

 mutually to balance each other, while in 

 those moving in cycloidal curves there 

 is no principle to counteract the varia- 

 tions of gravity." It must, however, be 

 noticed, that Mr. Cummings is evidently 

 not correct in his statement, that the loss 

 of specific gravity in the pendulum, 

 caused by an increase in the weight of 

 the atmosphere, would equally tend to 

 prolong its vibrations, as the increased 

 resistance caused to its motion by the 

 same means would tend to diminish 

 them, as he has by no means proved the 

 equality of those opposite effects. Mr. 

 Cummings also mistakes the loss of rela- 

 tive gravity for the loss of real gravity ; 

 the momentum of a body in motion is ge- 

 nerally considered to be the same in dif- 

 ferent mediums, except so far as the ad- 

 ditional resistance from a denser medium 

 retards it, and so far from Mr. Cum- 

 ming's opinion in opposition to this be- 

 ing as evident as he supposes, it is well 

 known that no proof has ever been ad- 

 vanced to support it. 



Many very curious and useful theo- 

 rems have been discovered relative to 

 the pendulum, most of which originated 

 with Huygens, among these one of the 

 most noted is, that " the times wherein 

 pendulums of different lengths perform 

 their vibrations, are to one another in the 

 same proportion with the square roots of 

 the lengths of the pendulums." 



The length of a pendulum vibrating 

 Inches. 



in a second is 



9781 Halley. 

 9.801 Newton. 



And from these data, and the above 

 theorem, the lengths of pendulums to vi- 

 brate any other required time may be 

 determined. 



The next improvement of consequence 

 on clocks, after the pendulum, was the es- 

 capement performed with anchorpallets, 

 which Bcrthoud states to have been the 

 invention of Clement, a London clock- 

 maker, in the year 1680. The escape- 

 ment used by Huygens, and still continu- 

 ed .n many chamber clocks and all the 

 wooden clocks, is that made by two flat 



pallets attached to an horizontal arbor, 

 acting at opposite sides of the upper part 

 of a horizontal crown wheel ; the anchor 

 pallets, on the contrary, act on a vertical 

 swing wheel, and move in the plane of 

 the wheel. The chief advantage of the 

 anchor pallets is, that they will permit 

 the escape to take place with a small 

 angle of vibration, so as to prevent the 

 maintaining power from acting on the 

 pallets a long time by a direct push, as 

 was the case with the crown wheel es- 

 capement. 



Dr. Hooke also claimed the invention 

 of the anchor escapement, which he as- 

 serted that he exhibited to the Royal So- 

 ciety in a clock of his construction in 

 1666. 



At the same time with the anchor es- 

 capement, the mode of suspending the 

 pendulum from a clock by apiece of watch 

 spring was introduced. 



The anchor escapement causes a re- 

 coil in the swing wheel, from the same 

 face of the pallet striking the tootle of 

 the wheel in its descent, which is after- 

 wards impelled by the same tooth in its 

 ascent ; this occasions the clock, in 

 which it is used, to go faster, when the 

 maintaining power is increased, or when 

 the weight of the pendulum ball is dimi- 

 nished. 



The advantage gained by the anchor 

 escapement shewn above may be consi- 

 dered in reality an approximation to a de- 

 tached escapement ; a farther step was 

 made towards this improvement about 

 the year 1715, by the celebrated George 

 Graham, in the contrivance of the dead- 

 beat escapement, which is principally 

 distinguished *from the anchor escape- 

 ment by having no recoil. This is ef- 

 fected by increasing the depth of the 

 pallets in the line towards the centre of 

 the swing wheel, and so forming the 

 teeth of that wheel, that the pallet in ac- 

 tion, in its descent, does not touch the 

 teeth at all, but lies between them, and 

 the tooth that impels it only comes in 

 contact with its inclined plane at the in- 

 stant previous to its ascent, when the op- 

 posite pallet becomes free. To avoid the 

 wearing out of the parts most in action, 

 and the influence of friction, the best 

 clocks of this construction have swing 

 wheels of hardened steel, with pallets of 

 ruby or agate. 



The detached escapement completed 

 the improvement of th is part of clock- 

 work. Its object is, to make the pendu- 

 lum perform the greatest part of each vi- 

 bration entirely free from contact, or con- 

 nection, with any part of the train. To 



