TrmcJj National Ihjlituie. • 1 8 jj ' 



thefame for: t<.vo bars as for one. This, indeed, the anihor 

 proved by taking two bars of a white temper of 80 iniHi- 

 metres (3,14960 inches) in length, a third of a milh- 

 riK'tre (o'Ol^lZ iucii) in thicknefs, and forty niillinictres 

 (l,fi74SQinch) in breadth. Experience (iiowcd that the ratio 

 of the momentum of the direttinc; force of one infulatcd bar 

 to that of the fame bar united, is as 183 to loD, or nearly, 

 as rnav be feen^ I he ratio of a to i. 



•But it mav be readily conceived that, in proportion as bars ' 

 have nioe thicknefs and lef^ breadth, as tlic-fiuid condeni'es ' 

 itf«?ifr in the edges and antries to which it is impelled in one •; 

 dircdlon '.vithoiit experiencinji reaction, it mutt then hnppea^ 

 that this rativj will \k^ diniiniflicd. In thi:s rei^cdly experience iv 

 gave a refult of great importane© to prad-Hce.' ' It is a ratia'^- 

 ahiiuflconiiant J. fo that, if a bar a miHiinctre (0-0^937 inch) 

 in ihi^'lcnefs bo joincJ t) another of the fau'eihtckneis; ifthefb 

 two bars be uiagn-etized to laturaiion icjv.irateiy, and if they 

 be then magnetized logether united, the ratio of the mo- ' 

 nieatiMn of tiieir direituis; forces in thefe two cafes will be ■ 

 vcrv nearly as 165 to loo. 



This ratio takes place aifo, as theory indicates, and as is 

 confirmed by-experience, whether the bars be hardened of a 

 white teiupcr, or wliether they be tempered to ino fame de- 

 gree of Iprinii, provided they are compared in the fume Hate* 

 of temj cr and hardnefs. 



it nnili. however be remarked, that either by the method ' 

 by which we magnetize, or fome other property of the mag- 

 netic fluid which' theory has not yet anprccialed, when a bar 

 very long compared with its otlier dinienliotis is magnetized, 

 the whole fluid proceeds towards its extremities in an extevit 

 fo much narrov\er as the breadth and thicknefs of the bar are 

 lefs, and as the degree of temper is greater. Between thefe 

 two parts there is an interval which gives fcarcely any fign of 

 ni;ignctifm; whence it happens, that by joining two very lono- 

 barn the preceiling law cannot liave place, bccaulc the aftiau 

 of the magneiifni of t!ie two bars united only caufcs thcmasf- 

 nelic fluid to ilow back into tlie parts which gave no fign of 

 magnet ifm when each needle was inlulated. 



It is only with needles then v, hich give figns of magnetifm 

 throughout their whole length that we ought to find an aor.'e- 

 ment between experience and tli»<>ry. It is always found,. 

 U)T example, that if the dircihtive force of an infulaied bar of/ ' 

 twjO hundred millimetres (7,87400 inches) in length', what- 

 ever may be its other dinu nfimis, is to the directive force of 

 the fame bar, whrn united with one equal to it, as 16-3 to 

 iQOj ail fliorter bars will give the fame rcfull, ^tad all bun-. 



dies 



