CLOCK-WORK. 



275 



of circles struck to different radii, decreasing in a 

 certain proportion each step, the length of each be- 

 ing one-twelfth part of the circumference of the circle 

 on which it is struck. These, circular arcs form so 

 many slips, constituting the snail, against which the 

 arm b of the rack is pressed by the spring w, which 

 is opposed by the hawk's-bill g, a click acting on the 

 teeth of the rack ; bk is the warning-piece, being a 

 three-armed detant, one arm of which is bent at the 

 end, and passes through the plate SS, in order to 

 catch a pin lixed in the arm of the wheel I (fig. 2). 

 The other arm b takes a direction so as to meet a 

 pin on the wheel 0. In fig. 3, the parts are repre- 

 sented as in motion, and the motion would continue 

 were it not that, at each stroke of the hammer, the 

 gathering pallet r, lifts the rack one tooth each turn 

 the hawk's-bill retaining the rack until a pin in 

 the end of the rack is brought in the way of the ga- 

 thering pallet lever, and thus stops the motion of the 

 wheels. At the end of every hour the pin in the 

 wheel O touches the end b, moving it towards the 

 spring, thus lowering the end k to the circle of motion 

 of the pin in the wheel t (fig. 2). The end of the 

 hawk's-bill is at the same time lowered, by the end 

 of the short tail, in consequence of which, the other 

 end g is raised so as to clear the head of the rack S, 

 when the rack is thrown back by the spring w, until 

 the end of the arm A is pressed against the snail. 

 The wheels are set in motion by the weight, when, 

 by the falling back of the rack, the pin in it clears 

 the gathering pallets ; but a few minutes before the 

 striking of the bell, the whole is stopped by the pin 

 in the wheel t, falling against the end k. The motion 

 of the wheels during tiiis action produces that noise 

 called the warning of the clock. When the hammer 

 is about to strike, at the end of the hour, the end of 

 the arm b, of the wheel O, slips over its pin, and it is 

 raised against the end k by a small spring. The 

 hammer/? is raised by the pin-wheel i,and the bell is 

 struck. The gathering pallet takes up a tooth of the 

 rack each turn, the hawk's-bill retaining it until the 

 pin of the rack comes under the gathering pallet, 

 and checks the motion of the striking department 

 until the next hour. The number of teeth that the 

 rack falls back will depend upon the number of 

 strokes .made by the hammer, and, from the form of 

 the snail, the rack falls back differently every hour, 

 the hammer making one additional stroke each hour, 

 from one to twelve. If, by any cord or other com- 

 munication, the arm b should be moved l>etween any 

 two hours, then will the striking part be put into 

 motion, and the arm A remaining in the step of the 

 snail, the last hour will be struck, which is called re- 

 peating. 



From this description it is easy to see that a clock 

 may be made to go for any length of time without 

 winding up, by increasing the number of teeth in the 

 wheels, or, what comes to the same end, diminishing 

 the number of hours in the pinions. The same may 

 also be effected either by lengthening the cord 

 to which the weight is attached, or by increasing 

 the number of wheels and pinions. The moving 

 power in clocks with short pendulums, called 

 time-pieces, is frequently not a weight, as is above 

 described, but a spring, such as that employed in 

 watches, for a description of which apparatus, see 

 JVatch-work. Many other appendages and peculi- 

 arities in the construction of escapements and other 

 parts of clocks, might have been described, but such 

 minute detail would be totally inconsistent with the 

 nature of a Popular Encyclopedia. We cannot, how- 

 ever, conclude this article without a more particular 

 description of the pendulum, on which depends the 

 regularity of the clock's motion. A heavy body/?, 

 (fig. 4,) attached to the end of a cord or slender rod 



PC, capable of moving round the centre C, forms the 

 common pendulum. The body or bob P, will, if un- 

 disturbed, remain in the lower point A of the arch 

 PE, but if drawn to one side, as shown in the figure, 

 and then let go, it will, by the action of gravity, have 

 a tendency to fall to the centre of the earth, in the 

 direction of PL, but because of the rod or cord PC, 

 it describes the arc PA, being part of a circle of 

 which C is the centre. When the bob has reached 

 the lowest point A, it has acquired such velocity as 

 to carry it on to the point E, from which it descends 

 and rises again towards A. These alternate motions 

 backwards and forwards continue ; but by reason of 

 friction, and the resistance of the air, the length of 

 the arcs described by the bob will continually de- 

 crease, until the action of gravity causes the pendu- 

 lum to cease its motion altogether. We have al- 

 ready seen how the stopping of the pendulum is pre- 

 vented, from a new impulse being given at every 

 vibration by the action of the teeth of the swing- 

 wheel upon the pallets. It may be demonstrated 

 that if two pendulums describe similar arcs, the 

 times of their vibrations are as the square roots of the 

 lengths of the pendulums, and also that the lengths 

 of pendulums are as the squares of the number of 

 their vibrations in equal times, or as the squares of 

 the times of vibration. Wherefore, since the length 

 of a seconds' pendulum at London, has been found 

 to be 39-1386 inches (which will answer sufficiently 

 well for all places in Britain), it follows, from the 

 foregoing statement, that the length of a half se- 

 conds' pendulum will be about 9-8, and a quarter 

 seconds' about 2-45 inches. The bob may be dis- 

 pensed with, and a simple rod BG (fig. 5) employed, 

 whose length is greater, by one third, than the length 

 of the pendulum with the bob. 



We have before alluded to the effect of gravity, 

 in causing a difference in the time of vibration of the 

 same pendulum in different latitudes, a circumstance 

 which will be fully considered in the article Pendu- 

 lum (q. v.) ; but there is another circumstance affect- 

 ing the time of vibration of a pendulum which we 

 must here consider, we mean the effect of heat and 

 cold, in lengthening and shortening the pendulum ; 

 so that the time of the going of a clock is influenced by 

 variations of temperature. Tin's circumstance for a 

 longtime rendered the clock a very unsafe guide to the 

 navigator, in determining the longitude (q. v.), and 

 accordingly several contrivances have been made to 

 remedy this defect. Many of these are exceedingly 

 ingenious, but our limits will only permit us to notice 

 three. These pendulums are called compensation pen- 

 dulums, because they contain witnin themselves 

 means of compensating for variations in length 

 caused by the differences of temperature. The first 

 we shall notice is the mercurial pendulum of Graham, 

 invented about 1721, which is exceedingly simple, 

 and serves well to illustrate the principle upon 

 which compensation pendulums are constructed. 

 Graham's pendulum consists of a steel rod, at the 

 end of which is fixed a glass jar containing mercury ; 

 so that when the rod expands by heat, the jar is 

 lowered, while at the same time the heat expands 

 the mercury, and thus the centre of oscillation is 

 raised, and the one expansion counteracting and 

 compensating for the other, the length of the pendu- 

 lum remains unchanged. This contrivance, though 

 simple and ingenious, is in little use, being exceeding- 

 ly difficult of adjustment. The gridiron pendulum of 

 Harrison consists of live, seven, nine, or any odd 

 number of rods of different metals which effect com- 

 pensation in a manner that will be understood by re- 

 ference to the pendulum represented in fig. G. The 

 two outer rods A B, C D, are of steel, fastened 

 by means of pins to the cross pieces AC, BD. 

 s2 



