48 



THERMOMETER AND PYROMETER. 



inferior to that proposed by Dr. Gum- 

 ming, which preceded it in date ; al- 

 though until now no detailed account of 

 that invention has been published. 

 f 3. The Barometrical Thermometer of 

 the Rev. F. Wollaston is seen in fig. 

 47, A. It is a thermometer with a 



Fig. 47. 



large bulb, devised for the measurement 

 of altitudes, by observing the tempera- 

 ture at which liquids boil- on different 

 elevations ; * on the principle first 

 pointed out by Fahrenheit,-!' that the 

 boiling of a fluid varies with the pres- 

 sure of the atmosphere. Cavallo first 

 applied this principle to the measure- 

 ment of heights, $ and the instrument 

 proposed by Mr. Wollaston is intended 

 to facilitate this method. 



The bulb of the mercurial thermome- 

 ter he proposed to use, is one inch in dia- 

 meter, with a dilatation k, as seen in B, 

 and ending in a capillary tube, five inches 

 long, which is not closed in the usual 



* Phil. Trans, for 181?. 

 t Phil. Trans, vol. xxxii. 

 % Phil. Trans, vol. Ixvi.. 



manner, but, after being broken off 

 smoothly, is sealed by a little cap of 

 glass, as at i, i. ^The scale is 4.15 

 inches long, divided into 100 parts, and 

 may be subdivided by a vernier into 1000 

 parts ; giving 241 parts to each inch of 

 the scale ; and to facilitate observation, 

 these are read off by a small lens jointed 

 to the index, but not represented in our 

 figure. The index is moved by a mi- 

 crometer screw, d. The thermometer is 

 supported by means of stuffing between 

 two circular plates of metal, c, c, through 

 which it passes, and which are tightened 

 by screwing them together. They form 

 a metallic collar, that may be screwed 

 by either end into the top of the copper 

 boiler, f, g, which becomes the case of 

 the thermometer on inverting it; and 

 then the bulb is protected by a copper 

 cap C, which also serves as a measure 

 of the due quantity of water to be used 

 in the experiment. The portion of the 

 copper tube below the bottom, g', of the 

 boiler, is capable of holding the lamp, e, 

 which is attached to it by two sliding 

 wires. Thus the instrument becomes 

 very portable. The boiler is 5.5 inches 

 deep and 1.2 in diameter, with an aper- 

 ture at the top to permit the escape of 

 the steam, by which the heat is applied 

 to the bulb. 



When we have to determine an alti- 

 tude, the boiling point is noted at the 

 bottom of the eminence, and again when 

 we have ascended ; and the value of the 

 difference between those points on the 

 scale having been ascertained by expe- 

 riment, we can estimate the height 

 ascended, provided no change has, in the 

 mean time, taken place in the barome- 

 trical pressure ; or these points may be 

 simultaneously found by two observers. 

 The only correction required is, for the 

 specific gravily of air at different tem- 

 peratures, which may be found by Ge- 

 neral Roy's tables. The use of the di- 

 latation, h, is to receive the expanded 

 mercury, before it arrives at the boiling 

 point : and the small cap, i, i, is intended 

 to receive a globule of mercury, to be 

 detached occasionally from the "column 

 in the stem, when it is wished to alter 

 the range of the scale to suit various 

 altitudes. The method of separating 

 this globule is, to elevate the mercury, 

 by heating the bulb until the thread of 

 metal may be shaken over the flat end 

 of the capillary tube ; and when we wish 

 to join the globule again to the thread, 

 the two portions of mercury are brought 

 into contact by heat, and then as the in- 



