PENDULUM. 



pansion it must extend the length of the 

 column upward, and consequently raise 

 the centre of oscillation ; so that by this 

 means its distance from the point of sus- 

 pension will be shortened, and therefore 

 the pendulum on this account will vi- 

 brate quicker ; wherefore* if the circum- 

 stances of the tube and mercury are skil- 

 fully adjusted, the time of the clock 

 might, by this means, for a long course 

 of time, continue the same, without any 

 sensible gain or loss. This was the in- 

 vention of Mr. Graham, in the year 1721, 

 who made a clock of tiiis sort, and com- 

 pared it with one of the best of the com- 

 mon sort for three years together, and 

 found the errors of the former but about 

 one-eighth part of the latter. 



Mr. Graham also made a pendulum 

 consisting oi three bars, one of steel be- 

 tween two of brass, and the steel bar act- 

 ed upon a lever, so as to raise the pendu- 

 lum, when lengthened by heat, and to let 

 Jt down, when shortened by cold ; but he 

 found this clock liable to sudden starts 

 and jerks in its motion. 



The ingenious Mr. Ellicott, in the 

 Transactions of the Royal Society, de- 

 scribes a pendulum of his invention, 

 composed of brass andiron, with the me- 

 thod of applying it, so as to avoid the 

 many jerks to which the machine might 

 be liable. 



But besides the irregultmties arising 

 from heat and cold, pendulum clocks are 

 liable to others from friction and foulness; 

 to obviate which, Mr. Harrison has seve- 

 ral excellent contrivances, whereby his 

 clocks are almost entirely free from fric- 

 tion, and never need to be cleaned. See 



LOXGITUDE. 



The gridiron pendulum is a contrivance 

 for the same purpose. Instead of one rod, 

 this pendulum is composed of any conve- 

 nient odd number of rods, as five, seven, 

 or nine ; being so connected, that the ef- 

 fect of one set of them counteracts that 

 of the other set; and therefore, if they 

 are properly adjusted to each other, the 

 centres of suspension and oscillation will 

 always be equidistant. Fig. 11 represents 

 a gridiron pendulum composed of nine 

 rods, steel and brass alternately. The 

 two outer rods, A B, C D, which are of 

 steel, are fastened to the cross pieces 

 AC, B D by means of pins. The next 

 two rods, E F, (i H, are of brass, and are 

 fastened to the lower bar B D, and to the 

 second upper bar E G. The two follow- 

 ing rods are of steel, and are fastened to 

 the cross bars EG and IK. The t\yo rods 



A r OL. V. 



adjacent to the central rod being of brass, 

 are fastened to the cross pieces 1 K. and 

 L M ; and the central rod, to which the 

 ball of the pendulum is attached, is sus- 

 pended from the cross piece L M, and 

 passes freely through a perforation in 

 each of the cross bars I K, B D. From 

 this disposition of the rods, it is evident 

 that, by the expansion of the extreme 

 rods, the cross piece B D, and the two 

 rods attached to it, will descend : but 

 since these rods are expanded by the 

 same heat, the cross piece E G will con- 

 sequently be raised, and therefore also 

 the two next rods; but because these 

 rods are also expanded, the cross bar 

 I K will descend : and by the expansion 

 of the two next rods, the piece L M will 

 be raised a quantity sufficient to counter- 

 act the expansion of the central rod. 

 Whence it is obvious, that the e fleet of 

 the steel rods is to increase the length 

 of the pendulum in hot weather, and to 

 diminish it in cold weather, and that the 

 brass rods have a contrary effect upon 

 the pendulum. The effect of the brass 

 rods must, however, be equivalent not 

 only to that of the steel rods, but also 

 to the part above the frame and 

 spring, which connects it with the cock, 

 and to that part between the lower 

 part of the frame and the centre of the 

 ball. 



Another excellent contrivance for the 

 same purpose is described in a French 

 author on clock-making. It was used in 

 the north of England by an ingenious 

 artist about fifty years ago. This inven- 

 tion is as follows : a bar of the same me- 

 tal with the rod of the pendulum, and of 

 the same dimensions, is placed against 

 the back-part of the clock-case : from the 

 top of this a part projects, to which the 

 upperpart of the pendulum is connected 

 by two fine pliable chains or silken 

 strings, which just below puss between 

 two plates of brass, whose lower edges 

 will always terminate the length of the 

 pendulum at the upper end. These 

 plates are supported on a pedestal fixed 

 to the back of the case. The bar rests 

 upon an immoveable base at the lower 

 part of the case; and is inserted into a 

 groove, by which means it is always re- 

 tained in the same position. From this 

 construction, it is evident that the exten- 

 sion or contraction of this bar, and of the 

 rod of the pendulum, will be equal, and 

 in contrary directions. For suppose the 

 rod of the pendulum to be expanded any 

 given quantity by heat; then, r.s the 



