THERMOMETER AND PYROMETER. 



44 



effect of the sun's rays noticed, as well 

 as the indirect reflection from the sky. 



5. Pyroscope. 



When one ball of the differential ther- 

 mometer is smoothly covered with thick 

 silver leaf, or inclosed in a polished sphere 

 of silver, and the other ball is naked, 

 it forms the pyroscope ; an instrument 

 intended by its inventor, Mr. Leslie, to 

 measure the intensity of heat radiating 

 from a fire into a room, or the frigorific 

 influence from a cold body. A figure 

 is unnecessary, as the instrument is 

 usually made either like the differential 

 thermometer, like that represented in 

 (fig. 37,) or the hygrometer, (fig. 39.) 

 The theory of its construction and 

 application is, that all the rays inci- 

 dent on the metallic surface, are re- 

 turned from it ; while those that reach 

 the transparent ball expand the air 

 within it, and depress the coloured li- 

 quid in the stem. In this way the com- 

 parative radiation from various bodies 

 may be ascertained ; and it is so delicate 

 an instrument, that in a warm room it 

 will be visibly affected by a pitcher of 

 cold water, at the distance of a few 

 inches. 



6. The JEthrioscope of Leslie is ano- 

 ther modification of the differential ther- 

 mometer which we shall here notice. 

 One of its most usual forms is given in 

 Jig. 42 ; and is what the inventor calls the 

 Pendant JEthrioscope. The ball a of the 

 thermometer is inclosed within a brass 

 sphere, d, d, without touching it ; and 

 for the convenience of adjustment, this 

 sphere may be unscrewed in the middle. 

 The other ball, b, which is about one half 

 the diameter of the first, is in the centre 

 of an oblong spheroidal cup, c, c, which 

 may be covered by a top that fits on at 

 /,/. The coloured liquid in the stem 

 is supported by capillary attraction in 

 the dilated extremity of the tube, where 

 it joins the ball a. The brass work is 

 highly polished, and the inside of the 

 spheroidal cup is well gilt.* 



This very elegant instrument is in- 

 tended, in the language of Mr. Leslie, 

 " to indicate the cold pulses emanating 

 from the sky ;" or, in other words, to 

 give a comparative idea of the radia- 

 tion proceeding from the surface of the 

 earth toward the region of perpetual 

 congelation in the atmosphere. The 

 brass coverings defend both balls from 

 the influence of the sun's rays, or 



* Edin. Phil. Trans., vol. viii. 



other adventitious sources of heat ; and 

 when the ball b is cooled by radiation 

 toward the heavens, the air within it 

 contracts, and the elasticity of that 

 within a, forces up the liquid in the 

 stem, the height of which marks the in- 

 tensity of the radiation. 



When the cover is on, the liquid re- 

 mains at ; but when it is removed, 

 and the instrument presented to a clear 

 sky, either by night or by day, it in- 

 stantly begins to rise, and continues 

 to mount until the ball b has sustained 

 the greatest diminution of temperature, 

 which radiation at that time can pro- 

 duce.t 



The circumstances which favour ra- 

 diation from the surface of the earth to- 

 ward the sky, namely, a clear and calm 

 atmosphere, are admirably pointed out 

 by Dr. Wells, in his excellent Essay on 

 Dew : and this instrument becomes a 



f Leslie appears to have been led to this invention 

 by some of his own experiments on radiant caloric ; 

 but it is proper to state, that Dr. Wollaston had 

 shown, that when a delicate thermometer, in the focus 

 of a concave metallic mirror, is presented to the sky, 

 cold is indicated. 



