ggS On t/ie Plumb Line 



IV. 

 On the Plumb Line and Spirit Level. By M. Chez v.* 



XX. WEIGHT fufpended by a long {lender and flexible thread, and afled upon by 

 no other force than that of gravity, will ftretch the firing into a right line perpendiciilai' 

 to the horizon ; or which is the fame thing, the plane to which this line is perpendicular 

 is confidered as the horizon itfelf, or a parallel to that circle. 



Such a weight affords, therefore, a very fimplc and eafy method of determining the 

 level at all places. But this method is not without its inconveniencres, when the utmoft 

 precifion is required, and in levelling this precifion is often necelTary to be had. 



It is fcarcely poflible to ufe a thread, or wire, much finer than one fourth part of the 

 thicknefs of an hair. If to this be given a length of ten feet from the point of fufpen- 

 fion to the mark intended to be interfered by the wire for the purpofe of afcertaining 

 the fituation of any inllrument ; this inllrument will be inconvenient, on account of its 

 magnitude, and of the agitation which the leaft movement of the air will^produce, notwith- 

 ftanding the guard commonly made ufe of. If the wire befhort or thick, the precifion 

 will belefs, and even in the firft cafe it will not be very great unlefs themoft fcrupulous 

 attention of a fkilful obferver be applied to infure it. To judge of this degree of ac- 

 curacy the diameter of a hair was meafured, and it was found that thirty or thirty-five 

 were required to cover the fpace of one line of the Royal Paris foot ; that is to fay, the 

 thicknefs of an hair is about -^^ or rlly The fine or arc of one fecond to a radius of 

 ten feet is o, 0069816 of a line, or nearly 0,007, or the ^ part of the thicknefs of a 

 hair; a quantity, fo fmall, as eafily to elude common obfervation, and much lefs than 

 the derangement which the flighteft current of air can produce in the wire. 



The' fame force of gravity which ftretches the. thread by means of a weight, likewlfe 

 renders the furface of fluids horizontal. If any very fluid liquor, fuch as fpirit of 

 wine, be included in the cavity of a glafs tube, fo as not to occupy its whole capacity, 

 the empty fpace will be conftantly at the upper part of the tube;'tliat is to fay, at 

 one of the ends if its pofition be not horizontal, or in the middle if it be fo ; and by a 

 very flight inclination the bubble will be removed to a great diftance from the middle of 

 the tube. A tube of this kind is confidered as being very ufeful to afcertain the po- 

 fition of the hwizon, with great accuracy; and, for this purpofe.it is very fuceefsfuliy 

 ufed under the name of the fpirit level. 



Thefe levels are commonly made of glafs tubes in the flate they are obtained at the 

 glafs-houfe. Of thefe the flrarghteft and moH regular are felefted and exammed, by filling 



♦ On a former occafion, when fpeaking of thcfc Inftruments, (Fliilof. Journal I. 134.) I forbore to enter 

 npon the ntethods of grinding the Spirit Level; but having fince met v/ith the preCiat valijabk memoir I liave 

 thought it defiieaWe to treat the fubjeft more fully. — N. 



them 



