HOROLOGY. 



and hangs down behind the pallet-wheel (the last 

 of the train of wheel-work), 

 which revolves in the direc- 

 tion of BC, under the action 

 of the weight ; B and C are 

 the pallets. When the pendu- 

 lum swings to the left, AC 

 rises, and a tooth escapes 

 from C, while another falls 

 on the outside of B, and, 

 owing to the form of the 

 pallet B, this latter recoils 

 during the remainder of the swing. The same 

 thing occurs on the pendulum's return ; the arm 

 AB rises, a tooth escapes from B, and another 

 falls on the inside of C, and is pushed backwards 

 by it during the remainder of the swing. The 

 revolution of D is thus regularly retarded, one 

 tooth being allowed to escape for every two oscilla- 

 tions that is, every two seconds and as the wheel 

 contains 30 teeth, it performs one revolution per 

 minute (the seconds hand is fixed on the extremity 

 of the axle of this wheel). During a portion of 

 each contact between the pallets and teeth, the 

 onward pressure of the wheel gives an additional 

 impetus to the pendulum, so as to counteract the 

 retarding effects of the resistance of the air and 

 friction, which would otherwise bring it to a stand. 

 The only defect of this escapement is the recoil, 

 and various modifications have been devised to 

 obviate this. The first and 

 most successful was made by 

 George Graham, an Englisn 

 watchmaker in the beginning 

 of the 1 8th century, and his 

 improved form is called the 

 dead'scapement or dead-beat 

 escapement. Here the outer 

 surface of B and inner of C 

 are arcs of circles, whose 

 centre is A, and a little con- 

 sideration will shew that there can be no recoil 

 This escapement is adopted in time-keepers when 

 great accuracy is required. Other inventions, as 

 the detached escapement, the pin-wheel escapement 

 in various forms, and the gravity escapement 

 (described below), though very efficient, have not 

 come into general use. 



In the great clock in the new Houses of Parlia- 

 ment at Westminster, the pendulum is upwards of 

 13 feet long, to beat 2 seconds, and its bob weighs 

 6 cwts. The motion is kept up by a remontoir or 

 gravity escapement. On each side of the pendu- 

 lum-rod a small metallic hammer is hung upon a 

 peg. ' The swinging of the pendulum first draws 

 out a little bolt, that stopped the turning of a 

 wheel ; the wheel then goes round, under the in- 

 fluence of the weight, lifting one of the little 

 hammers as it does so, until it is caught by 

 another bolt The hammer-head next falls by its 

 own gravity, and strikes the pendulum-rod just as 

 it is in the act of descending, communicating the 

 force of its blow to quicken the movement ; the 

 s;ime thing is afterwards repeated on the opposite 

 side of the vibration, and then again on the same 

 side ; so going on alternately.' The push thus 

 given is evidently unvarying. The wheel has 

 three stops and cogs on it, and goes once round in 

 three beats of the pendulum, or in six seconds. 

 With this contrivance ' it is found that all the 

 teeth of the several wheels may be rough, just as 



turned out from the casting, and the clock will 

 nevertheless keep better time than it would have 

 done with the most perfectly finished teeth under 

 other arrangements.' 



The gradual perfection of the clock required 

 also improvements in the pendulum. No simple 

 pendulum, however, can be depended on for an 

 accurate time-keeper, for the isochronism of vibra- 

 tion of the pendulum depends on its being always 

 the same length ; now a cord contracts or ex- 

 pands with changes in the moisture of the atmos- 

 phere, and a rod with cold or heat To overcome 

 these defects in the pendulum, compensating 

 pendulums were invented, of which Graham's 

 mercurial compensation pendulum, invented in 

 1715, and Harrison's gridiron pendulum, in 1726, 

 are the two principal forms. 



Compensation Pendulums. 



In the mercurial pendulum represented in the 

 fig., the rod A, and the framework 

 CB, are of steel. Inside the framework 

 is placed a cylindrical glass jar, nearly 

 full of mercury, which can be raised or 

 depressed by turning a nut at B. By 

 increase of temperature, the steel por- 

 tion AB is lengthened by an amount 

 proportional to its length, its coefficient 

 of linear dilatation, and the change of 

 temperature, conjointly and thus the 

 jar of mercury is removed from the axis 

 of suspension. But neglecting the ex- 

 pansion of the glass, which is very 

 small, the mercury rises in the jar by 

 an amount proportional to its bulk, its coefficient 

 of cubical dilatation, and the change of tempera- 

 ture, conjointly. Now, by increasing or diminish- 

 ing the quantity of mercury, it is obvious that 

 we may so adjust the instrument that the length 

 of the equivalent simple pendulum shall be un- 

 altered by the change of temperature. The screw 

 at B has nothing to do with the compensation; 

 its use is to adjust the length of the pendulum so 

 that it shall vibrate in one second. 



The construction of the gridiron pendulum will 

 be easily understood from the cut. The black 

 bars are steel, the shaded ones are brass, copper, 

 or some substance whose coefficient of linear 

 dilatation is more than double that of steel. It is 

 obvious from the figure that the horizontal bars 

 are merely connectors, and that their expansion 

 has nothing to do with the vibration of the pendu- 

 lum, so they may be made of any substance. It 

 is easily seen that an increase of temperature 

 lowers the bob by expanding the steel 

 rods, whose effective length consists 

 of the sum of the lengths of Aa, BC, 

 and the steel bar to which the bob is 

 attached ; while it raises the bob 

 by expanding the brass bars, whose 

 effective length is that of one of them 

 only ; the other, as well as the steel 

 rod be, being added to the instrument 

 for the sake of symmetry, strength, 

 and stiffness only. If the effective 

 lengths of steel and brass be inverse- 

 ly as their respective dilatation co- 

 efficients, the position of the bob 

 is unaltered by temperature ; and 

 therefore the pendulum will vibrate in the same 



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