42 



THERMOMETER AND PYROMETER. 



Fig. 39. 



face of ice, in proportion to the dry- 

 ness of the air. Mr. ^Leslie estimates, 

 that when the ball is moist, air, at the 

 temperature of the ball, will take up 

 moisture equal to the sixteen thou- 

 sandth part of its weight, for each de- 

 gree of his hygrometer ; and as ice in 

 melting, requires l-7th of the caloric 

 consumed, in converting water into va- 

 pour, when the papered ball is frozen, 

 the hygrometer will sink more than 

 when wet by 1 in 7 ; and hence in the 

 frozen state, we must increase the value 

 of the degrees l-7th: so that each of 

 them will correspond to an absorption 

 of moisture, equal to one-fourteen thou- 

 sandth part of the weight of the air. 



When this hygrometer stands at 15, 

 the air feels damp ; from 30 to 40, we 

 reckon it dry ; from 5 to 60, very dry ; 

 and from 70 upwards, we should call 

 it intensely dry. A room will feel un- 

 comfortable, and would probably be 

 unwholesome, if the instrument in it did 

 not reach 30<>.* In thick fogs it keeps 

 almost at the beginning of the scale. 

 In winter, in our climate, it ranges from 

 5 to 15; in summer often from 1 5 to 

 55 ; and sometimes attains to 80 or 90. 

 The greatest degree of dryness ever 

 noticed by Leslie, was at Paris in the 

 month of September, when the hygro- 

 meter indicated 120. 



* Leslie " On the Relations of Air, Heat, and 

 Moisture," p. 70, 



The thermometric hygrometer is of 

 two forms ; the stationary, (Jig. 39,) 

 and importable, which resembles the in- 

 strument delineated in (figAl), without 

 its glass shade. This last form is de- 

 fended by a wooden case which screws 

 over it, to fit it for the pocket. 



4. Leslie's Photometer. 



This elegant instrument is the differen- 

 tial thermometer, covered by a case of 

 transparent glass, and having one of its 

 balls either painted black, or, what is 

 better, formed of black glass enamel. 

 The Stationary Photometer (Jig. 40) 



Fig. 40 



has both its balls at the same height, 

 and covered by a spherical shell of the 

 most transparent glass ; which, with the 

 annexed glass tube, defend the balls 

 from the disturbing influence of cur- 

 rents of air. 



The Portable Photometer (fig. 41) 

 has the balls in the same vertical line, 

 in order to admit a turned tube of 

 wood A, of the same form as its cover 

 a, a, to screw on the brass collar d, as 

 a defence to the instrument when in the 

 pocket; and for further convenience 



