HOROLOGY. 



tible of these changes, the operations of counteracting 

 the effects of them might be too sudden. 



It it of great importance, that a pendulum rod should, 

 with the smallest quantity of matter, be as stiff and in- 

 flexible as possible ; and although it was proposed to 

 have thicker ban and rods, let us suppose the same 

 quantity of matter is taken as before, but under a cir- 

 cular in place of a rectangular surface. It must in this 

 form require more time to get heated and cooled, which 

 is the end we now attempt to gain. The sides of the 

 parallelogram were .384 and .136 of an inch, the sum 

 of which, being doubled, will give 1.040 inch for the 

 circumference of the bar. To find the diameter of a 

 Irical rod containing the same quantity of matter 

 as the parallelogram, say, aa 355 is to 113, so is 1.040 

 to the diameter of the rod required, which will be found 

 to be .33104 of an inch. 



Let the improved jar frame, or stirrup, therefore, be 

 composed of two round steel rods, .331 ofan inch in dia- 

 meter, and 8.85 inches in length, from shoulder to shoul- 

 der, at each of which pivots are formed. Those for the up- 

 per ends should be a quarter ofan inch long, and of the 

 same diameter, tapped so as to screw firm into the upper 

 cross piece. The distance from the centre* of the tap- 

 ped holes, in the traverse or * piece, require* to be 

 2.8 inches, in order to give freedom for the jar between 

 the rods, the length of the cross piece over all should 

 be 3.3 inches, and its breadth at the place where the rod* 

 are screwed in .450 ofan inch. At the middle a circu- 

 lar part is formed, .5 of an inch in diameter, in the cen- 

 tre of which a hole i* tapped, by which the i 

 screw may raise or lower the jar, without 

 the place of the index point The croas piece may 

 either be bras* or steel, though the Utter is imhap* 

 preferable. The pivots on the lower end of the rod* 

 may be of the same diameter a* those on the upper end, 

 bat a link longer, and tapped a little way in on the 

 and*; when well fitted into the logs of the bra** sole, on 

 which the jar rests, and rank a little way down into 

 it, nuts are screwed on to the ends of the pivots, and 

 sunk in the lower tide of the brass sole. Care must be 

 taken to have the distance between the centra* of the 

 hole* in the sole the same as in the croas piece, so aa 

 to make the rod* stand parallel to each other. To go 

 oataide of the frame now described, another i* made, 

 to which attached the pendulum rod, the regulating 

 and lower sole, in which i. feed the amell * 

 which cannot rh*ng ha place, after being once 

 made for it The upper eras* piece for this may be of 

 the same thicknem a. the former, and i ' 

 than 4 inchea, 

 and their central 



oa nearly, and are wide rnangh to allow a round steel 

 rod of .25 ofan inch in diaaaeter to pas* freely through; 

 the length of these rods, from the lower aide of the 

 croas poo*, to the apper side of the lower aole, i* near, on each side cX 

 ly 10.6 inches. The lower sole may be a bra** wheel, lea* in height 

 d into fan- arm*, the centre part being large gree. Hence 

 gh to have a hale tapped, M aa to fa the eteel in. rules for ma* 



not, 



screwed into the upper cross-piece, it binds to it the romjn>- 

 lower end of the pendulum rod, which is formed into ' 

 a shape something like a compressed A, having lugs or 

 soles, through which the screws for fixing it passe*. 

 The regulating screw has an untapped or plain part, 

 which turns freely in a hole in the middle of this upper 

 cross piece, formed in the same way as the upper cro-s 

 piece for the jar frame; the nut or head of the regu- 

 lating screw is shaped so as to lie under the hollow of 

 the \, at the lower end of the pendulum rod, and on 

 the upper side of the cress piece ; the lower ends of 

 the roils of the outside frame are gently tapered, and 

 fitted into holes in the brass wheel, through the edge 

 of which are put pins to fix them and the wheel to- 

 gether ; this outside frame has no part in the compen- 

 sation. The brass cover for the jar, has the lugs hol- 

 lowed out a very little, so as to come in on the rods of 

 the jar frame ; the ends of the upper traverse, of which. 

 as well as the lugs of the jar sole, are hollowed, and 

 take in with a part of the rods of the outside frame, 

 along which the jar frame is moved up and down. 

 The height of the glass jar, outside, is about eight 

 inches; and it* weight and other dimensions nearly 

 the same as those stated for the former pendulum'; 

 and the diameter of the rod and A part is .331 of an 

 inch. A view of the improved mercurial pendulum is PLATC 

 given in Plate CCC VI I. Fig. 1. vij. 



Philosophers' seem not to be agreed respecting the *> l - 

 eipans'nm of mercury relative to that of other metals; 

 aame making it 15 times, others above 16 times that 

 of iron. In regard to the column of mercury for a 

 mercurial pendulum, something depend* on its' diame- 

 ter as well as it* height Suppose the length of steel 

 to be 43 inches, and the column of mercury 7.5 inch. 

 c in height, and 8 inches in diameter, which were 

 the dimenaiona used in a pendulum brought nearly 

 to its state of compensation, we may then find how 

 many times the expansion of this column is contained 

 in the 43 inches. Say, as 43 inches is to 7*. the ex- 

 pansive ratio of steel, so is 7.5 inches of mercury, to 

 its expansion far compensating the steel. 



.... Log. 1.633466S 

 Log. 1.869S317 



3.S08700S 



Length of steel 43 inches 



Katie 74 . 



eai as the former, and in length not lea* 

 A hole U tapped near to each of the ends, 

 s distant from one another 3J6 inch. 



enoai 



de* in it. The 

 length of the 



.25ofan 



in 



wheel i* the seme aa the 

 which i* 4 inches ; and 

 or .little more. This sole, 

 fixed, serves also as a resting 

 be taken 

 is formed 

 the 



which the index 

 part for the pendulum, when 

 from it. place. 

 into a sort of 



part i a little more than a quarter of an inch long, and 

 a* much hi diameter ; the length of the pi'?" and tap- 

 ped part of the screw i* about .7 of an inch, and when 



VOt. II. rABTL 



, ao as 



ndulum, when it happen* to 

 The upper end* of the rod* i 

 doable shouldered screw; th 



I {eight of column of mercury 7.5 inche* Log. 0.8750615 

 Hatw 484JM Log. 2.6276389 



3.5087008 



By this process, it appear* that the expansion of the 

 mercury is net quite 5.75 time* that of the steel. A 

 pandnhim, whoa* vibrations are three or four degree* 

 on each side of the point of rest, will require a column 

 lea* in height than that which vibrates only one de- 

 gree. Hence it is a very nice matter, to give precise 

 rule* for making a mercurial pendulum, that shall at 

 once be perfect in its compensation. 



If the steel rod a & and stirrup b c of a mercurial pen- 

 dulum i* lengthened by heat, the jar d< containing the . 

 aumiiy will, from this cause, be let lower down, and tjnn. 

 the centre of oscillation be carried farther from the point F >*> ' 

 of suspension a ; but the heat which lengthen* the rod 

 and stirrup at the same time expands the mercury up- 

 ward, and by this mean* the centre of oscillation is kept 

 always at the same distance from the point of suspension. 

 When the rod and stirrup are contracted or shortened 



