THERMOMETER AND PYROMETER. 



47 



wide a field ; but it may be proper to 

 state, that finished drawings of them all 

 have been in the possession of a dis- 

 tinguished member of the Meteorolo- 

 gical Society of London for upwards of 

 four years. The principle of them will 

 be readily understood from the pre- 

 ceding figure, (fig. 4 5,) in which a repre 

 sents an air thermometer ; b a barometer, 

 suspended from the opposite side of the 

 wheel c c, to compensate the influence of 

 variations in atmospheric pressure on the 

 instrument; dd is a siphon-cistern, in 

 both sides of which the mercury will 

 always remain on the same level ; / is 

 an index, to which a pencil may be 

 fixed, for tracing the variations of the 

 instrument on a plate revolving by 

 means of clock-work. 



The portions of the tubes which dip 

 into the mercury should be of equal 

 substance; and the tube of the air- 

 thermometer should be a cylinder ca- 

 pable of containing twice as much mer- 

 cury as the corresponding portion of the 

 barometer which counterpoises it. A 

 small correction may be required for the 

 varying immersion of the tubes, pro- 

 duced by the oscillations of the instru- 

 ment. This must be determined by ex- 

 periment, and allowed for in the gra- 

 duation of the scale. 



2. The Balance Thermometer of Mr. 

 Kewley is a contrivance for a similar 

 purpose, and is represented in fig. 

 46, A. 



This instrument is the subject of a 

 patent, the date of which is 1816. It 



Fig. 46. 



consists of a tube of glass, a, a, closed 

 at c, and terminating at e in a ball, 

 which communicates with another tube 

 of smaller diameter, which also termi- 

 nates in a ball at d, having a communi- 

 cation with the external air at /. The 

 tube , a, and one half of the ball, e, 

 are filled with spirit, or any light easily 

 expansible fluid. The other tube, from 

 e to d, is filled with mercury. The 

 whole is suspended in the iron frame, 

 h, i, /?, m, B, by means of two clamping 



Eieces, which are adjusted to the tubes 

 y the screws, o, o. The centre of gra- 

 vity is suitably adjusted by means of 

 the milled nut, sunk in the transverse 

 part of the frame, and receiving the 

 screw, i in order that the whole may 

 librate on the knife edges, m, n, des- 

 tined to rest on surfaces resembling the 

 suspension frame of a common balance. 

 A brass scale, p, is moved by the nut, k, 

 on the arbor of which is a pinion playing 

 in the teeth of the plate, p. 

 It is obvious, that by adjusting the 



n 



mercury in each arm of this balance, it 

 will be in cequilibrio ; but when the 

 spirit in a is expanded by heat, it will 

 force some more of the mercury into 

 the ball, d, and that arm of the instru- 

 ment will preponderate ; when it again 

 contracts, the atmospheric pressure will 

 cause the mercury to resume its original 

 situation. 



The instrument may be used as a 

 thermometer, by ascertaining at what 

 temperature it is in equilibrio, and 

 when either end preponderates, finding 

 how much is necessary to restore the 

 balance by the motion of the brass 

 plate, p ; but its chief value arises from 

 its applicability to shut and open doors 

 or windows, according to the tempera- 

 ture of the apartment ; in which case, 

 a lever, or tooth-wheel, is fixed on 

 one of its centres of oscillation. It is 

 almost needless to remark, that the 

 whole may be constructed of iron, and 

 of any convenient size. In point of 

 simplicity, and cheapness, however, it is 



