THERMOMETER. 



ttianner direfted in the fequel of this article. The hall of 

 the thermometer muft be placed at fuch a depth within the 

 pot, that the boiUng point may rife very little above the 

 cover ; and the furface of the water in the pot fhould be 

 at lead one or two inches below the bottom of the ball. 

 Care muft be taken to ftop up the hole in the cover through 

 which the tube is inferted, and to make the cover fit pretty 

 clofe, fo that no air fliall enter into the pot that way, and 

 that not much iteam may efcape. A piece of thin flat tin- 

 plate mull alfo be laid on the mouth of the chimney, fo as 

 to leave no more paflage than what is fufBcient to carry off 

 the fteam. 



If the artift pleafes, he may tie each corner of tliis plate 

 by a ftring to prongs fixed to the chimney, and Handing on 

 a level with the plate, as it will be thus always kept in its 

 place. 



Fig. 7. is a perfpcftive view of the chimney and tin- 

 plate ; A B C D is the plate, E the chimney, F/, G g, 

 M m, and N n, the prongs fallened to the chimney, to 

 which the four corners of the plate are to be tied by the 

 firings A F, B G, CM, and D N ; the ends F, G, M, 

 and N, of the prongs muft be on a level with the plate, 

 and the ftrings (hould not be ftretched tight. The cliimney 

 ought not to be lefs than half a fquare inch in area, and 

 not lefs than two or three inches in length. The cover 

 fhould be made to take on and off eafdy, and a ring 

 (>f woollen cloth may be placed under it, fo as to 

 lie between it and the top of the pot. The hole in the 

 cover may be ftopped up by a cork, with a hole bored 

 thi-ough it, big enough to receive the tube, and then cut 

 into two, parallel to the length of the hole. Another 

 method, more convenient in ufe, but not fo eafdy made, is 

 reprefei'iUd in Jig. 8. which exhibits a pcrfpcftive view of 

 the apparatus : A a is the cover, H the hole through which 

 the thermometer is pafled, Bis. flat piece of i.rafs fixed 

 upon the cover, and D ^/ E f a fliding piece of brafs, made 

 fo as either to cover the hole H, or to leave it uncovered, 

 as in the figure, and to be tightened in either pofition by 

 the fcrew ,1 fliding in the flit Mm; alfo in the edge D (/, 

 to enclofc . the tube of the thermometer : pieces of 

 woollen cloth fliould alfo be faftened to the edges B I and 

 D i/, and alfo to the bottom of the fliding-piece T)dlLe, 

 unlefs that piece and the cover are made fufficiently flat to 

 prevent the efcape of the fteam. In order to keep the 

 thermometer fufpended at the proper height, a clip may 

 be ufed like that reprefented in Jig. 9. which, by the 

 fcrew J-, muft be made to embrace the tube tightly, and 

 may reft on the cover. 



Another method, which is rather more convenient, when 

 the top of the tube of the thermometer is bent into a right 

 angle, in the manner often praftifed at prefent for the fake 

 of more conveniently fixing it to the fcale, is reprefented in 

 fig. 10. : GgT'f is a plate of brafs ftanding perpendicu- 

 larly on the cover, and L/ N?j a piece of brsls bent at the 

 bottom into the form of a loop, with a notch in it, fo as to 

 receive the tube of the thermometer, and to fuffer the bent 

 part to reft on the bottom of the loop ; this piece muft 

 flide in a flit Ki, in the plate L /Nn, and be tightened at 

 any height by the fcrew T. 



Moreover, it is beft to make the water boil pretty briflily, 

 as otherwife the thermometer is apt to be a great while 

 before it acquires its full heat, efpecially if the veffel is very 

 deep ; and the obferver fhould wait at leaft one or two 

 minutes after the thermometer appears to be ftationary, 

 before he concludes that it has acquired its full height. 



Another way of adjufting the boiling point is to try it in 

 a veflel of the fame kind as the former, only with the water 



J2 



rifing a little way, viz. from one to three or four inehes 

 above the ball, taking cai-e that the boihng point fliall rife 

 very little above the cover. In this method there is no need 

 to cover the cliimney with the tin-plate, and there is lefs 

 need to make the cover fit clofe, uuIefs to prevent the 

 operator from being incommoded with the fteam. The 

 height of the barometer in this method is 29-0 inches. 



It will be convenient to have two or three pots of dif- 

 ferent depths for adjufting thermometers of different 

 lengths. A third way of adjufting the boiling point is to 

 wrap feveral folds of linen rags or flannel round the tube of 

 the thermometer, and to try it in an open veffel, taking care 

 to pour boiling water on the rags, in order to keep the 

 quickfilver in the tube as nearly of the lieat of boihng water 

 as pofTible. In this method the barometer ftiould be at 

 29.8 inches ; the water fliould boil faft, and the thermometer 

 fliould be held upright, with its ball two or three inches 

 under water, and in that part of the veffel where the current 

 of water afcends. 



Whichever of tliefe methods of adjufting the boiling 

 point is ufed, it is not neceffary to wait till the barometer is 

 at the proper height, provided the operator will take care 

 to correft the obferved height according to the following 

 table. 



In ufing this table, feek the height of the barometer in 

 the column anfwering to the method of adjufting the boil- 

 ing point, the correfponding number in the third column 

 fliews how much the point of 212° muft be placed above or 

 below the obferved point : e. gr. fuppofe the boiling point 

 to be adjufted in them when the barometer is at 29 inches, 

 and that the interval between the boiling and freezing points 

 is I I inches ; the nearefl number to 29 in the left-hand 

 column is 29.03, and the correfponding number in the table 

 is 7 higher, and therefore the mark of 212° muft be placed 

 higher than the obferved point by ^^g'^^ths of the interval 



II X 7 

 between boiling and freezing, i. e. by , or .077 



of an inch. This method of correfting the boiling point is 

 not ftriftly juft, unlefs the tube is of an equal bore in all its 

 parts ; but the tube is feldom fo unequal as to caufe any 

 fenfible error, where the wliole correftiou isfo fmall. The 

 trouble of making the correftion will be abridged by a 

 diagonal fcale, fuch as is reprefented in_yfj. 10. 



Although it is of no great confequence what kind of 

 water is ufed in adjufting the boiling point, fo that it is not 



fait. 



