THERMOMETER AND PYROMETER/ 



50 



black glass, about one and a quarter 

 inch in diameter, and is connected with 

 a ball d of the same size, by a bent tube 

 one-eighth of an inch in diameter. A 

 portion of sulphuric aether, sufficient to 

 fill three-fourths of the ball a, is intro- 

 duced ; a small mercurial thermometer, 

 with a pvriform bulb, is fixed in the 

 limb a o, the atmospheric air is ex- 

 pelled as completely as possible ; and 

 the whole is sealed at e. The ball d is 

 covered with muslin ; the whole is sup- 

 ported on a brass stand /, g, on which 

 is another delicate mercurial thermo- 

 meter. The tube can be removed from 

 the spring tube h ; and the whole, to- 

 gether with a phial of aether, packed 

 neatly in a box, that goes easily into the 

 pocket. The method in which the dew 

 point is indicated by this instrument, is 

 as follows : The aether is all brought 

 into the ball a, by inclining the tube ; 

 the balls are placed perpendicularly ; 

 the temperature of the ambient air is 

 now noted; aether is poured from a 

 dropping tube that fits the mouth of 

 the small phial, on the muslin cover of 

 d ; and, the cold produced by its eva- 

 poration, causing a condensation of the 

 elastic aethereal vapour within the ball, 

 produces a rapid evaporation from a, 

 by which the temperature of the ther- 

 mometer in it sinks ; and when the 

 black ball is thus cooled to the dew 

 point, a film of condensed vapour, like 

 a ring, surrounds the ball. If the ther- 

 mometer be at that instant noticed, we 

 obtain the true dew point of air at the 

 temperature indicated by the other ther- 

 mometer. 



The observation is made in a very 

 short period ; and much of the labour 

 required in the method of Le Roi is 

 saved. There seems but one objection 

 to this very ingenious instrument, and 

 it is one, which, even with much prac- 

 tice, is not easily obviated. The surface 

 on which the dew condenses, is small, 

 and requires a peculiar light to be well 

 seen; while the attention of the ob- 

 server, distracted between the close in- 

 spection of the surface of the black ball, 

 and the included thermometer, is not 

 always able to fix with absolute preci- 

 sion the dew point. 



6. This instrument has been modified 

 in 



Mr. Thos. Jones's Hygrometer ; 

 an instrument on exactly the same 

 principle as the original invention of 

 Daniell, but simpler in construction, 

 more compact, and less expensive. It 

 is seen in fg. 49, consisting of a deli- 



Fig. 49. 



cate mercurial thermometer, with its 

 tube at a, b, bent so as to bring its cy- 

 lindrical ball c, parallel with, and at a 

 little distance from, its stem. The bulb 

 is one inch long, and is terminated by 

 a flattened surface d, of black glass, 

 which projects a little beyond the sides 

 of the bulb. The bulb below the 

 flattened surface is covered with black 

 silk. The instrument is supported on 

 the wire ef, which is attached to the 

 scale by a pivot, that allows the black 

 surface to be inclined to the light, and 

 the whole, with a phial of aether, are 

 contained in a small case. * 



When used, the temperature of the 

 air is first noted ; then aether is poured 

 on the silk cover of the bulb ; and the 

 condensation of the dew is seen on the 

 black extremity of the bulb. 



The difficulty of marking the incipient 

 condensation on these instruments is 

 the same ; and it has produced various 

 modifications of the instrument. 



7. Dr. Cumming, of Chester, finds that 

 the dew point is most conspicuously 

 shown, by inclosing the bulb of a deli- 

 cate thermometer, covered by a sponge, 

 in a tube of planished tinned iron, silver, 

 or platina. When the sponge is moist- 



* Phil, Trans, for 1826, part ii. p. 23, 



