Cmnpenfatlons for Change of Tempeyntiire. ^lickfilver Peiuhdum. Gridiron. jt) 



in this aflumed cafe, the centre of ofcillation would afcend by heat. But, as the expanPion 

 of mercury is about fifteen times greater than that of iron, if the tube had been nearly filled, 

 the afcent of the mercurial column would be only one-fifteenth part lefs than if the iron 

 had not expanded at all. When the mercurial column is fliortened by pouring- out 

 fonie of the fluid, the efFe£l: of its expanfion will alfo be diminiflied. There is, confe- 

 quently, a ftate in which thefe two contrary expanfions fha'll counferacl each other, and 

 the centre of ofcillation continue invariable. When a clock thus fitted up gains by 

 heat, the quantity of mercury muft be diminiflied ; and the contrary if it lofes. I have 

 feen an improvement of this penduluin by Mr. Troughton of Fleet-ilreet, in which a bulb 

 and tube of glafs, refembling the common thermometer, was fubftituted inftead of the 

 Cmple tube of Graham. The variable furface of the mercury was brought into that part 

 of the rod where its changes of elevation were bcft calculated to counteraft the oppofitc 

 expanfion of the whole apparatus. 



John Hanilbn is admitted to be the firit who applied the oppofite expanfions of brafs 

 and Heel to correft each other, in the apparatus called the gridiron pendulum *. Sinco 

 the increafe by e.^panfion in brafs and Reel is as 113 to <58, and the expanfion in any 

 piete of metal is proportionate to its length, it will follow that thefe increments will be 

 equal in two rods of thefe metals refpetlively, when their lengths are inrerfely as thofe 

 numbers. That is to fay, if a rod of brafs 68 inches long be laid upon a rod of fteel 

 113 inches long, fo that one end of each may coincide, and thefe ends be fattened together j 

 then, if the oppofite end of the fteel be fixed to fome invariable fupport, the unconfincd 

 extremity of the brafs rod will alfo keep its pofition invariably in all temperatures. For 

 the expanfiorl of the fteel will in every cafe carry the whole bar of brafs as much in one 

 direftion as the expanfion of this lad bar would have carried its inner extremity in the 

 oppofite direftion. We may imagine the ball of a pendulum to be fixed at this invariable 

 point, and the centre of fufpenfion to be at the fingle end of the fteel bar. 



But it is inconvenient to have nearlyJialf the pendulum rod beneath the ball ; for which 

 reafon Mr. Harrifon cut the fteel bar into three parts, and tlie brafs into two. The firft 

 fteel bar hung downwards from the centre of fufpenfion. To its lower extremity was 

 attached the firft brafs bar leading upwards. To the upper extremity of this was attached 

 the fccond fteel bar, which in like manner fupported by its lower extremity the fecond 

 brafs bar leading upwards. A double combination, of the nature of the fimple pair of 

 bars firft defcribed, was thus formed, in which the predominant expanfion was upwards, 

 becaufe the brafs and fteel were nearly equal in length. But the addition of the third 

 fteel bar leading downwards from the upper end of the fecond brafs bar, anfwercd the 

 purpofc of making tiie defired compenfation. To its lower extremity then the ball of 

 the pendulum was attached. 



Pendulums of this conftruclion have been made, though it is certain that fuch a com- 

 bination of bars, inftcad of remaining parallel, would tend, by their own weight and that 

 of the ball, to open like the legs of a pair of compafles, and that the remedy of bracing to 

 keep them together would certainly impede their regular a£li<5n. Mr. Harrifon had rc- 

 courfc to another expedient. Inftcad of a fingle fteel bar, in the firft injlancc, he made ufe of 



• Elements of Clock and W.ucli VVi^rk, by AlcxanJcr Ciiinming, LoBdj,-, n't(>, p.igc 92. 



I 2 twn, 



