HOROLOGY. 



157 



bv the screws /, *, rest on the long arms m, n of the le- 

 vers ; and provided the ends of the screws press on the 

 levers at a proper distance from the centres o, o, the 

 ball will be always kept at the same distance from the 

 point of suspension, notwithstanding any alteration 

 the rod of the pendulum may experience from heat or 

 cold. What this distance ought to be may very nearly 

 be determined, if the difference of the expansion between 

 brass and steel is known ; for the proportion which the 

 shorter arms of the levers ought to bear to the lunet 

 ones, will always be as the excess of the expansion 

 is to the whole expansion of the steel Instead of the 

 brass heads of the screws being placed near the outer 

 edge of the ball, they may be more advantageously 

 placed within the ball, at the distance of about an inch 

 and a quarter from the edge, as shewn in Fig. 3. 



See I't-.il. Tram. 1 7 5 1 , n. 479. and a pamphlet which 

 Mr EUicott published in London in 1753. For an ac- 

 count of the improvement made upon it by Mr A. C'um- 

 ming, see his Element! fCtoct and Watchmaking, p. 107. 



V".:w I 



Mr Smeeton's compensation pendulum consists of a 

 glass rod A B, half an inch or more in diameter, and 45.5 

 inches long. To the upper end is fixed the pendulum 

 earing; to the lower a tciww t and regulating nut M. A 

 brass tube or ring , Fig. 5. of an inch or so in length, ia 

 pot to move easily on the lower end of the rod, having a 

 fillet at each end, one hiamia on the regulating nut, the 

 the sine, iron, and lead tubes, which 

 ilura ball. The zinc 



i in length, ;th of an inch 

 Jy on the gists rod, the lower end rest- 

 ing on the apper fillet ef the brass ring. On the zinc 

 lobe is put another of iron, 18 inches long, and Jth of 

 an inch thick, eaaily moveable on the sine tube, with 

 a kind of bottom to it, in which is a hole for the glaei 

 rod to go freely through. The bottom is uppermost, and 

 rests on the upper td of the line tube. The lower east 

 ef the iron tube has a fillet en its outside, on which rests 

 a leaden tube of IS inchea long, and ^th of an inch 

 thick, and which goes easily over on the iron tube. The 

 eutaide diameter of the lesrlm tube will be nearly two 

 inchea. Although this pendulum will not be thought 

 elegant in apiiaatama, yet it ia said to have anew trad 

 the purpose of rempiiiseiiiiii very well A 

 the rod is shewn in Fig. A. 



As glass does not suffer much 

 tion from beat or cold, it will be 



pm i The glass rod AB of Una 



append to lengthen in a 



' by a zinc tube of twelve inches and a quarter 



on the lower end of the 

 glass rod, would be carried down by the 

 ef the gjaei rod, bat the 

 thii, >11 make the 

 will carry up the 

 has in thai case Ha 

 tube 

 wards. 



its 



Rtift 



keeping it pretty warm, to prevent cracking 1 or breaking. Compui 



After this operation, a hole is pierced straight through tion 



the bar, from end to end, and opened up by means of Pendu1 ' 



a clean cutting broach, \mtil it is .450 of an inch, or so, * '"" Y "" 



in diameter. The outside may be turned down till it is 



.7 of an inch, or less. The length should be 25. St inch- 



es, the same as the zinc rods were taken at. The steel 



rods must be a quarter of an inch in diameter; the length, 



from pin to pin, in the upper and lower traverses of the 



two outside steel rods cd, ef, '27 inches, five or six inches 



more being prolonged to go within the ball. In the mid- 



dle of the lower traverse m n, is pinned a steel rod ? h, 



somewhat more than a quarter of an inch in diameter, 



and nine inches long, which comes through the centre 



of the ball, which is fitted up in the same way in every 



respect as was described for the gridiron pendulum. The 



steel centre rod a b, goes up inside of the zinc tube, from 



the pin which is in the lower end of it, which is in a tra- 



verse a very little above the lower one, to the upper end 



of the pendulum spring, 36.75 inches ; from the pin in 



the lower end of the centre rod, to the centre of the ball, 



4.75 inches. A hole in the upper traverse op allows 



the centre rod to pass freely through. The lower end of 



the tube rests on the traverse gr, in which the centre 



rod is pinned. The up|>cr traverse bears on the upper 



end, both traverses having a part turned from them, 



about one-tenth of an inch in height, and of such a 



diameter as to, go into the ends of the tube, for the pur- 



pose of keeping it to its proper place. The distance 



from the centre of the holes in the upper and lower tra- 



verse*, about I. 'Jo inch, which will be enough to maker 



the two outside steel rods stand clear of the zinc tube. 



A thin piece of brass, with three holes in it for the 



outside steel rods and tube, might be put half way be- 



tween the ends of the tube, to prevent any bending, or 



tremulous motion, a thing, however, not likely to take 



place. It would be proper to have a few holes in the 



tube, for the purpose of admitting air more freely to 



the crutrt- r<nl. 



The centre steel rod a 6, when lengthened by heat, M thod of 

 will make the lower end B of the zinc tube, (which is coopsua- 

 supported by the lower end of the steel rod ab,) de- tioo. 

 scend with it, but the same cause which lengthens the 

 steel rod a 6 downwards will expand the zinc tube AB 

 pw Bills, and this will carry up the two outside steel 

 rods with which the ball of the pendulum is connected ; 

 ards, as well as that of the centre 

 the upward expansion of the 

 length of the steel rods and of the zinc 

 tube, has been shewn to he in proportion to their cx> 



their expansion downwar 

 lod, b romprneated by 

 cine tube. The lenth o 



Mr 



i* . ,..-,< 



Mr Thomas Reid's cnmeeiiaedun pendulum i* com- 

 aosed of a sine tube A B, and three steel rodi, h, 

 In order to obtain a proper tube, the zinc must be very 

 gri'tly fused into a bar about an inch square, and 24.!i5 

 mcbe. long, and the mould into which it is poured 

 should be upright, or nearly so. Let this be very care 

 fully hianiii I to half an inch per foot, meanwhile 



It is about fifteen years since we contrived and made 

 this kind of compensation pendulum, which seems to do 

 *rry well. The following is of another kind, but we 

 never had it put in execution, although there is no 

 doubt but it would serve the purpose extremely well, 

 notwithstanding the risk arising from the briuleness 

 oftbegbai. 



I'rovide a white glass robe, whose outside diameter is r 

 Jths of an inch, its inside diameter Jths, and it* length Rnd'. 

 54 inches: Such a tube may be luppoted e<]ual in 

 strength to a solid glass rod, and will be considerably 

 lighter. Make a zinc tulx- from a, square bar. 1mm- 

 nerrd, Ac. in the same way as has just now been di- ' 

 retted, its length ling 16.3 inche*. sud iu Inside dia- 

 meter Jths of an inch, or as much more a i will allow 

 it to move !>< ly up and down on the outride of the 

 glass tube. If the thickness of the 7, me tu!>e is jth of 

 an inch, it ought to answer very well ; if it is soroe- 



4 



uh 



