THERMOMETER AND PYROMETER. 



on applying the heat of the hand to d, 

 the mercury in the stem joins that in 

 the ball ; and the stem will' remain filled 

 with the mercury while the instrument 

 is vertical : but when it is brought into 

 a horizontal position by the machinery 

 above mentioned, and the bulbs ap- 

 proximated to the pencils ef, suspended 

 over them for that purpose, and supplied 

 with liquid through the channels in 

 them, the mercury in the ball will leave 

 the orifice of the stem, and that in the 

 latter will descend, as represented in the 

 figure ; and its subsequent contraction 

 is marked by an inverted scale ; which, 

 with the indications of the other ther- 

 mometer, will enable us to ascertain 

 the temperature at the moment of the 

 reclination of the instrument. Thus, as 

 both instruments are equal, if" the same 



diminution of temperature has sunk a b 

 to 50, and has produced a contraction 

 of 10 in ed; it is evident the sum of 

 both numbers will show the degree at 

 which the common thermometer stood at 

 the moment of the change of position ; 

 which in this case has been 60. 



This idea is most ingenious, and is 

 said in practice to work exceedingly 

 well. It promises to be useful in meteo- 

 rological investigations, although not so 

 complete as the register thermometer of 

 Keith, which continues to note its own 

 indications for a whole month; while 

 that requires to be readjusted for each 

 observation. 



7. We shall conclude this chapter by 

 a notice of another register thermometer 

 invented by Dr. Traill, and seen in fig. 

 35. It is a single spirit thermometer, 



Fig. 35. 



in which a column of mercury 5 of an 

 inch in length is introduced: at each 

 end of this column lies an index of fine 

 steel wire, gilded by means of a galvanic 

 circuit, to prevent oxidation in the 

 spirit. An inspection of the figure will 

 show how the variations of bulk of the 

 spirit in the bulb will move the column 

 of mercury ; and by this the indices are 

 pushed in opposite directions, but will 

 remain at the lowest and highest points 

 to which they are driven by the mer- 

 cury. The difference between the two 

 scales will be the length of the mercu- 

 rial column. The indices are brought 

 in contact with the mercury by a 

 magnet. 



This thermometer has the advantage 

 of giving the maxima and minima by 

 the changes in volume of a single fluid ; 

 for the expansion of so short a column 

 of mercury is quite inappreciable. The 

 defect of this construction is the liability 

 of the mercury to separate by sudden 

 motions of the instrument. This is 

 least likely to happen when the mercu- 

 rial column is short and the calibre 

 of the tube is minute ; and it is to 

 admit of a fine tube that gilded steel 

 wire is preferred to an index coated 

 with glass. 



CHAPTER IV. 



Differential Thermometers, and their 

 Modifications. 



THERMOMETERS of this kind are not 

 affected by general changes of tem- 

 perature in the surrounding medium ; 

 but are delicate indicators of partial 

 changes affecting one of their balls. 

 Some of the forms of the air ther- 

 mometer described by Van Helmont, 

 bear a general resemblance to the in- 

 strument known by the name of differ- 

 ential thermometer: but they were 

 rudely constructed, without a fixed 

 scale, and unsusceptible of accuracy, 

 or of application to the delicate inves- 

 tigations required by modern experi- 

 mental philosophy. We are indebted 

 to the ingenuity of Professor Leslie of 

 Edinburgh for a perfect differential 

 thermometer, and its application to 

 some very important purposes. 



In January 1800, he published a de- 

 scription of a new hygroineter and 

 photometer,* of which the principle de- 

 pends on the difference in the volume 

 of air contained in two equal balls of 

 glass, connected by a tube bent in the 



* NicholbOn'ti Jouraal, 4to, vol. iii. p. 461. 



