158 



HOROLOGY. 



Compensa- 

 tion 

 Pendulums. 



_Mr Thomas 

 Reid's com- 

 pensation 

 pendulum 

 with a glass 

 tube- 

 PLATK 

 CCCVII. 

 I'ig. 7. 



Fig. 6. 



what greater, it may answer better. Tfcre must be a 

 core of brass fitted to each end of the glass tube, ground 

 gently into the glass, and fixed in by some of the lime 

 cements. To make the fixing more secure, a hole might 

 be bored through the tube and core, about half an inch 

 i'rom each end of the tube, and a copper pin put through 

 them. The core in the lower end must have a small 

 cylindrical piece, or wire formed from it of .3 or .4 of an 

 inch in diameter, stretching 1.5 inch beyond the end of 

 the glass tube, and tapped for the nut inside of the 

 zinc 'tube to work upon. The regulating nut should 

 be under the pendulum ball, yet a little free of it, as 

 it would be difficult to get at it, were it placed near the 

 lower end of the rod. The core at the upper end has 

 also a part of the brass, a little way above the end of 

 the tube, for the purpose of fixing in the pendulum 

 spring. The zinc tube goes up to the centre of the ball, 

 which rests on the end of it ; the lower end, having 

 the tapped nut in it, bears on the regulating screw. The 

 nut may be either soldered into the tube or not, pro- 

 vided it is fast there. As both the glass rod and zinc 

 tube are round, and go through the ball, it will be re- 

 quisite to have something to keep the ball to its proper 

 swinging position. For this purpose, let a brass tube 

 of an inch and a half in length, be fitted strongly spring 

 tight on the glass rod, and put on above the tipper edge 

 of the ball. To the lower end of the brass tube or soc- 

 ket is fixed a traverse piece, into which are fixed two 

 steel rods, a quarter of an inch in diameter, and 6 inches 

 long. These go within the ball, in the same manner as 

 the lower ends of the outside steel rods in the gridiron 

 pendulum. The distance between the holes in the tra- 

 verse should be 3 inches, that is, each rod should be 

 distant outside 1.5 inch from the centre of the glass 

 tube. When the traverse with the spring tube is once 

 set, so that the ball may have its proper position, it can- 

 not be easily altered. The length taken for the zinc 

 tube is rather in excess for the compensation of the glass 

 rod, and should it be found so, the tube can readily be 

 shortened. As the glass rod is not very fit for the pen- 

 dulum fork to work on, the following apparatus is pro- 

 posed, and has been found in other cases to answer the 

 end as well as could be wished. See Plate CCCVII. 

 Fig 8. 



AA is a hollow cylinder of brass 1.5 inch long, which 

 fits the glass rod rather more than spring tight. It is 

 made a little thin near the pnds, and at the middle it is 

 left thick, having the appearance of a brass ring a, a, a, a, 

 formed on the cylinder, into which are fixed two pivots 

 p, p, a piece of brass not very thick, .3 of an inch broad, 

 being bent up nearly in a bow form, as at B, B, hav- 

 ing a small hole at each end, b, b, to receive the pivots 

 p, p. One of the ends must be screwed on, in order to 

 get the pivots more conveniently into their holes. At 

 the end or middle of the bow, is a solid or round knob 

 of brass C, in which is a hole tapped to receive the 

 screw D, the head of which is nerrelled on the edge, 

 and sunk on the outside, to receive the round flat piece 

 of brass E, which snaps easily in like a barrel cover, 

 and is not left so tight, but that it may be easily turned 

 round in its place, without any risk of coming out, and 

 supposed to have no shake outwards. Into the piece 

 E is fixed a piece of brass, having a hole in it to re- 

 ceive the pin of the crutch. Two views of this piece 

 are seen at F F. When these pieces are all combi- 

 ned to act in their places, it can easily be seen, that 

 by turning the nerrelled head of the screw D, holding 

 the piece F in its front view position, the crutch pin 

 will be made to move out or in, according as the screw 



D is turned, by which the clock will be set in beat, to Compena. 

 a degree of nicety which is not easily obtained by bend- tic " 

 ing the fork or crutch shank. 



There are other modes of putting a clock in beat, but * """Y"" 

 they generally consist of an apparatus for that purpose, 

 carried by the crutch or fork, which is a load on the cock 

 pivot. The one which has been described, has the ad- 

 vantage of being supported by the pendulum rod. 



The glass tube having a length of 54 inches, will, Method of 

 when expanded by heat, carry down with it the zinc compensa- 

 tube, whose lower end rests on that of the glass tube. The tlon ' 

 centre of the ball of the pendulum resting on the up- 

 per end of the zinc tube, will expand upwards from the 

 same cause which lengthens the glass tube, and by this 

 means carry the ball of the pendulum up, and keep the 

 centre of oscillation always to the same distance from the 

 point of suspension. The length of the glass tube rod, 

 and its expansive ratio, will be found to be in just pro- 

 portion to the length of the zinc tube and its expansive 

 ratio. 



Troughton's Tubular Pendulum. 



Mr Edward Troughton's tubular pendulum, is a Trough- 

 very neat and ingenious one, and is in every respect ton ' s tubu - 

 worthy of that celebrated artist, to whom science is so J ar P endu ' 

 much indebted for the great perfection to which he has v 

 brought the division of astronomical instruments. 



" Fig. 9 Plate CCCVII. says Mr Troughton, drawn PLATE 

 to a scale of one-eighth of the real dimensions, exhibits cccvil, 

 the shape of the whole instrument, in which the parts Fig. 9. 

 of action being completely concealed from view, it ap- 

 pears, excepting the usual suspension spring, to be made 

 of solid brass. This figure gives a front view of the 

 pendulum. This form of the bob is used more on ac- 

 count of its being easy to make, and sightly, than from 

 any other considerations j it is made of one piece of 

 brass, about 7 inches diameter, 2.5 thick at the centre, 

 and weighs about 1 5 Ibs. avoirdupoise : the front and 

 back surfaces are spherical, with a thick edge or cy- 

 lindrical part between them. The afpparent rod is a 

 tube of brass, reaching from the bob nearly to the top. 

 This contains another tube and five wires in its belly, 

 so disposed as to produce altogether (like the nine-bar 

 gridiron of Harrison) three expansions of steel down- 

 wards, and two of brass upwards ; whose lengths being 

 inversely proportioned to their dilatation, when pro- 

 perly combined, destroy the whole effect that either 

 metal would have singly. The small visible part of 

 the rod near the top, is a brass tube, whose use is to 

 cover the upper end of the middle wire, which is here 

 single, and otherwise unsupported. 



Reckoning from the top, the first action is downwards, 

 and consists of the spring, a short wire 0.2 diameter, 

 and a long wire 0. 1 diameter ; these all of steel, firmly 

 connected, reach down within an inch of the centre of 

 the bob, and occupy the middle line of the whole appa- 

 ratus. To the lower end of the middle branch is fas- 

 tened the lower end of the interior brass tube, 0.6 in 

 diameter, which terminates a little short of the top of 

 the exterior tube, and produces the first dilatation up- 

 wards. From the top of the interior tube depend two 

 wires 0. 1 diameter, whose situation is in a line at right 

 angles to the swing of the pendulum, and reach some- 

 what lower than the attached tube itself, which they pass 

 through without touching, and effect the second expan- 

 sion downwards. The second action upwards is gained 

 by the exterior tube, whose internal diameter just al- 

 lows the interior tube to pass freely through it : its bot- 

 tom is connected with the lower ends of the last descri- 



