36 



THERMOMETER AND PYROMETER. 



retain the attached index at any part of 

 the tube, to which it is raised by the 

 column of mercury. The action of the 

 instrument will now be readily under- 

 stood. When an increase of tempera- 

 ture expands the spirit in the bulb, it 

 depresses the mercury in the limb a, 

 and proportionally raises it in the limb 

 b of the siphon : the mercurial column 

 in the latter raises the index d before it ; 

 and when the mercury sinks in that leg, 

 the bottom of the index d, retained at 

 that height by the glass spring, will in- 

 dicate how high the mercury had risen. 

 When the spirit in the bulb contracts 

 by cold, the mercury in the limb b de- 

 scends, and the consequence is a pro- 

 portional ascent of the column in the 

 side a ; which likewise carrying the in- 

 dex c before it, leaves its lower extre- 

 mity at the point to which the column 

 of that side had risen. In this manner 

 the maximum and minimum tempera- 

 tures are seen at any desired interval of 

 time ; and all that is necessary to pre- 

 pare the instrument for a fresh observa- 

 tion is to bring down both indices to 

 the surface of their respective columns 

 by means of a magnet, which will 

 act on the bit of iron wire included in 

 the body of each index. From the 

 above description, it is obvious, that 

 there must be an ascending scale to 

 measure the degrees of expansion in 6, 

 and a descending scale applied to a to 

 mark the contraction of the spirit. Mr. 

 Six graduated his thermometers by 

 placing them in water at different tem- 

 peratures, and marking on his scales 

 the heights corresponding to every 5 of 

 a standard mercurial thermometer im- 

 mersed in the same liquid. This elegant 

 invention has become a common instru- 

 ment ; and on account of the ease with 

 which the glass spring of the index may 

 be broken off, many instrument makers 

 substitute a slender bristle, tied to the 

 upper part of the index, and lapped 

 round its body, as at B. This renders 

 the spring less easily spoiled by the 

 careless shifting of the index ; but the 

 hair, by being long steeped in spirit, is 

 liable to have its elasticity destroyed ; 

 and a slender silver or platina wire 

 would be preferable. The usual dimen- 

 sions of the instrument are, a bulb 

 from 6 to 1 6 inches in length, and from 

 0.2 to 0.3 inch in internal diameter; 

 the siphon from the ~ to the ^ of an 

 inch in width, and of a length propor- 

 tioned to the size of the bulb ; the in- 

 dices about 1 inch long ; the terminal 



expansion of the tube is, in most of the 

 instruments now made, rather too small ; 

 in Six's original instrument, this part was 

 a cylinder of 2 inches in length, by half 

 an inch in diameter, to a bulb of 16 

 inches in length, and T f inch in internal 

 diameter. 



The chief defect of Six's thermome- 

 ter arises, as in most other contrivances 

 of this sort, from the unequal expan- 

 sion of the spirit, and the introduction 

 of two liquids of very different expansi- 

 bility in the instrument ; while, from the 

 construction, it would be difficult to ap- 

 ply any general correction to its indica- 

 tions. It does not indicate the expan- 

 sion of the spirit only, but also that of the 

 mercurial column ; which, where nice 

 observation is required, would be of some 

 moment ; and the necessary friction of 

 the indices will also tend to diminish 

 the effect of expansion. Yet this in- 

 strument is a valuable addition to me- 

 teorology ; and is probably the most 

 convenient for ascertaining the tempe- 

 rature of the ocean, at great depths, of 

 any hitherto given to the public. 



4. The day and night thermometers 

 of Dr. John Rutherford, from the sim- 

 plicity of their construction, and low 

 price, have in some measure super- 

 seded the register thermometer of Six. 

 This ingenious and elegant device was 

 first published in the Transactions of 

 the Royal Society of Edinburgh,* and is 

 represented in fig. 32 ; where A re- 

 presents a spirit, and B a mercurial 

 thermometer, each provided with its 

 own scale, placed 'horizontally on 

 the same piece of box wood or ivory. 

 B contains, as an index, a bit of steel 

 wire, which is pushed before the mer- 

 cury, and is left in that situation to 

 mark how high the temperature had 

 been. A contains a glass index half an 

 inch long, with a small knob at each 

 end ; it lies in the spirit, which can 

 freely pass beyond it when expanded by 

 heat ; when contracted by cold, from 

 the attraction between spirit and glass, 

 the last film of the column of spirit is 

 enabled to overcome the slight friction 

 of the index on the inside of the tube, 

 and to carry it back towards the bulb. 

 This attraction is so considerable, that 

 although the index will move freely up 

 and down in the spirit, on inclining the 

 instrument, it will rest on the last film, 

 and require several smart concussions 

 given to the thermometer, to make it 



* Edin. Phil. Trans, vol. iii. 



