184 



cesses adopted for the two kinds of instruments being different, I 

 shall describe them separately. 



I. Mercurial Thermometers. — These were divided into degrees of 

 Fahrenheit's scale in ther following manner : — The tube was first 

 calibrated in the way already described in my former report on the 

 construction of thermometers (dated March 22) ; marks being made 

 on the tube at each extremity of the calibrated space. The bulb was 

 then made and the mercury introduced by the glass-blower, the di- 

 mensions of the bulb and the quantity of fluid being made as nearly 

 as possible to correspond with the scale proposed to be made. The 

 thermometer was then placed in melting ice and the freezing-point 

 approximately set off with an ink mark ; a similar mark being also 

 made for a temperature of between 95° and 100°. A short arbitrary 

 scale of four or five divisions was then divided at each of those points. 

 The thermometer was then again placed in ice and the freezing- 

 point determined accurately with reference to the lower short scale ; 

 and comparisons with two standard thermometers in water deter- 

 mined the value of the upper short scale. Let OP be the calibrated 

 portion of the tube, O being the point of commencement, F the 



OF HP 



freezing-point as determined by means of the short arbitrary scale, 

 and H the higher point of the scale similarly obtained. Let the 

 distances OF, FH, HP be measured by the screw of the dividing en- 

 gine. Let Rp Rg, R3, &c., R^, &c., R^, &c. be the lengths, in revo- 

 lutions of the dividing screw, of the calibrating column of mercury 

 for each successive step in its progress along the tube during the 

 process of calibration ; R^ being the length of the step in which the 

 point F occurs, and R^ that in which H occurs. The values of 

 Rp Rg, &c. have been registered in the process of calibration ; OF 

 and OH have been obtained independently ; the second measure- 

 ment of OP, when compared with the sum of all the R's, will show 

 with what exactness* the column of mercury has been passed through 

 its own length in its progress along the tube. Let r^. be the number 

 of revolutions between the first end of the step / and the point F, 

 and similarly for the step h. We have then 



OF =Ri -H R2 + R3 + &c. -F R^_^ -f r, 

 and OH=Ri + R24- .... + 

 whence we obtain r and r^. Let K be the number of degrees 

 equivalent to one length of the calibrating column, — this being of 

 course constant for each length along the tube on the supposition of 

 equal increments of volume for equal increments of temperature. 



* If this should be found slightly in error, it can produce no appreciable error 

 in the graduation, as an error could only arise from the alteration of the tube's 

 capacity, which might take place in a length equal to the difference found, this 

 fUrrerence being in perhaps no case more than or inch. 



