2o6 Account afa netu Pendulum. 



in this fituation, while the nut K is but loofely fcrewed on, the bars B B be pafled through 

 the perforations in the cylinder, the brafs and fteel pieces, and K, be then fcrewed up, it 

 will caufe the vertical hole O, in the one fteel piece, and L in the other, to approach nearer 

 together, and carry the bars B B along with them. But this can take place only to a very 

 fmall extent, becaufe the bars will be preffed againft the inner furfaces of the holes in the 

 brafs piece P-P, which very nearly fit thofe bars. The ufe of the milled head K is, there- 

 fore, to difengage the weight A A from the bars B B, in which fituation the former may 

 cither be taken pfF, or fiided up and down to any defired pofition, where it may immediately 

 be fixed by turning the nut. 



Fig. 2 (hews the whole pendulum, which vibrates half fccoads, in its full dimenfions, ex- 

 cepting as to its length, which is diminiflied by a frafture, reprefented in the middle of the 

 bars, to get it into the plate. The fyftem of bars confifts of a crofe-piece, G G, attached to 

 another piece, H H, to which the fprings of fufpenfion are fixed as ufual. From G G pro- 

 ceed downwards two fteel bars, which pafs through holes in the brafs ftage E E (reprefented alfo 

 in fig. 5), with which they have no eonne<Slion, and are firmly fecured in the ftage, D D„ 

 which is fimikr to E E. From the ftage E E^roceed a light pair of fteel bars, downwards 

 through the ftage D D, without connedlion with this laft, and thence through the cylinder 

 A A, where they appear at B B. Aad, laftly, from the centre of the lower ftage D. D, 

 proceeds upwards a bir of the fame dimenfions, but compofed of zinc and filver, forming a 

 compound, which expands more than twice as much by heat as fteel does. This bar paflcs, 

 through the ftage E E, till it almoft reaches G G. It is reprefented by the dotted outline. 

 The fta<Te E E may be ftided up and down, fo as to vary its diftance from D D ; but when 

 required to be fixed, the fcrew pin F is to be turned, which caufes a piece to pr.efs againft the- 

 central bar and binds it feft. 



It is fcarcely neceflary to explain this aftion. The pendulum A A hangs upon the fteel 

 bars B B, which arc fixed in the ftage E E. This ftage is fupported by the central rod or 

 pillar of zinc and filver which bears on the ftage D D, and this laft is fufpended from G G 

 by the bars C C. If, therefore^ the expanfion of the compound metal between the ftagea 

 E and D be equal to the whole expanfion of the fteel rods, and the reft of the apparatus 

 downwards, the centre of A A will remain at the fame diftance from th« axis of motion 

 whatever may be the temperature. But if the effeft of the fteel be greateft, it will l^e necet 

 Jkry to raife the ftage E and fix it higher, or in the contrary cafe it muft be fixed lower ;. 

 taking care to preferve the due fituation of A A with tefpedl to I, by means of its appara- 

 tus K. 



This may feem to be but a rough method of adjufting for temperature. But when it is 

 eonfidered that the whole daily difference in a fimple pendulum with a fteel rod amounts to. 

 fot feconds at a mean between fummer and winter *, it will be readily underftood that tha 



Philof. Jpurnal, {.58, 

 ' purpofe. 



