THERMOMETER AND PYROMETER. 



35 



narrow neck / of the ball d, by which 

 the passage is still further contracted, so 

 that the mercury trickles through in 

 most minute division. The instrument 

 is prepared for a new observation by 

 being inclined so as to bring the mer- 

 cury in d to cover the orifice at /; the 

 bulb is then heated, and the mercury 

 is expelled from the ball into the short 

 leg of the siphon, until it be filled with 

 that fluid. 

 Fig. 30 is another form of the last 



Fig. 30. 



instrument, which has the advantage of 

 being more easily adjusted, and is less 

 liable, from slight motion, to have the 

 mercury which has passed into a 

 brought back into the tube. 



These instruments are extremelyjjin- 

 genious contrivances ; but there are 

 some practical difficulties in their con- 

 struction ; and the minimum thermo- 

 meter is rather liable to be broken from 

 the size of the bulb, and the several 

 bendings of the tube. Hence Lord C. 

 Cavendish's thermometers never appear 

 to have come into general use, although 

 very well adapted for certain purposes, 

 as for ascertaining the temperature of 

 the ocean at great depths. It is there- 

 fore unnecessary here to notice the cor- 

 rections pointed out by Mr. Cavendish 

 in its applications to various purposes. 



2. Next in point of time is the con- 

 trivance of Fitzgerald ; which has been 

 already noticed, as well as that of 

 Crighton ; for rendering their metallic 

 thermometers indicators uf the maxima 

 and minima of temperature during the 

 absence of the observer. 



3. The Register Thermometer in- 

 vented by Mr. James Six, of Colchester, 

 was h'rst described in the Philosophical 

 Transactions,* and is represented in 

 fig. 31. It is, in fact, a spirit of wine 



Fig. 31. 



thermometer, with a long cylindrical 

 bulb, and a tube bent in the form of a 

 siphon with parallel legs, and termi- 

 nating in a small cavity. A portion 

 of the two legs of the siphon from a 

 to b is filled with mercury ; the bulb, 

 and the remainder of both legs of the 

 siphon, as well as a small portion of 

 the cavity, are filled with highly recti- 

 fied alcohol. The double column of 

 mercury is intended to give motion to 

 the two indices c, d; the form of which 

 is better seen at A. Each index con- 

 sists of a bit of iron wire inclosed 

 in a glass tube, which is capped at each 

 extremity by a button of enamel. Their 

 dimensions are such, that they would 

 move freely in the tube, were it not for 

 a thread of glass drawn from the upper 

 cap of each, and inclined so as to press 

 against one side of the tube, forming a 

 delicate spring of sufficient power to 



Vol. Ixxii. 

 D 2 



