THERMOMETER AND PYROMETER. 



41 



fesses to have borrowed the idea of the 

 thermoscope from Leslie's hygrometer ; 

 but the latter has roundly charged him 

 with a more direct plagiarism from the 

 differential thermometer ; to which ac- 

 cusation we do not recollect that any 

 satisfactory answer has been published. 

 The differential thermometer has un- 

 dergone several alterations of form to 

 adapt it to particular purposes as an air 

 thermometer. One of the most com- 

 mon is seen at Jig. 37, where the 

 ball b is cemented to the tube, after 

 the introduction of the liquid, as in the 

 old air thermometer, but this form has 

 been rendered more elegant and con- 

 venient by the modification of Dr. De 

 Butts of Baltimore, (See Jig. 38,) in 

 Fig. 38. 



Fig. 37. 



which no cement is necessary ; for 

 the stem and both balls are united by 

 the blow-pipe. These instruments are 

 to be either fixed perpendicularly on a 

 stand, or suspended. The liquid is con- 

 tained in the lower ball, and the heat is 

 applied to the upper one ; so that the 

 stem is provided with a descending scale. 



3. Leslies, or the Thermometric 

 Hygrometer. 



When the ball of the differential ther- 

 mometer containing the supply of co- 

 loured liquid is coated with several folds 

 of tissue paper, and kept moist with 

 distilled water, the instrument becomes 

 an Hygrometer : for the descent of the 

 coloured liquid in the other stem will 

 mark the diminution of temperature 

 caused by the evaporation of the water 

 from the humid surface ; and as this 

 effect is proportional to the relative dry- 

 ness of the ambient air, it will give an 

 indication of the comparative quantity 

 of water suspended in the atmosphere, 

 at the different times of observation. In 

 most cases, two minutes are sufficient to 

 produce the full effect on the instru- 

 ment ; and the included liquid then be- 

 comes stationary, until the whole mois- 

 ture is exhaled from the ball. The 

 drier the ambient air is, the more 

 rapidly will the evaporation go on; 

 and the cold produced will be greater. 

 When the air is nearly saturated with 

 moisture, the evaporation goes on slow- 

 ly ; the cold produced is moderate, be- 

 cause the ball regains a large portion 

 of its lost heat from surrounding bodies ; 

 and the degree of refrigeration of the 

 ball is an index of the dryness of the air. 

 Could we ascertain with precision the 

 capacity of air for moisture, at different 

 temperatures, this hygrometer would 

 likewise afford a measure of the abso- 

 lute quantity of water suspended in the 

 air. The most approved form of the 

 instrument, according to Leslie, is seen 

 in fig. 39. The balls are parallel, and 

 bent from each other ; a is covered 

 smoothly with several folds of tissue 

 paper, which is to be kept continually 

 moistened with pure water, drawn from 

 the vase d, by the capillary attraction 

 of a few fibres of silk. In order to ob- 

 viate any inequality from the disturbing 

 effect of light, the ball b is formed of pale 

 blue glass ; and the papered ball is co- 

 vered with thin Persian silk of the same 

 hue. 



Should the water become frozen on 

 the ball, this hygrometer will still act ; 

 for evaporation goes on from the sur- 



