45, COKNHILL, E.G., AND 122, EEGENT BTRKET, W M LONDON. 71 



precipitation in the natural state of the atmosphere was twenty degrees. When 

 Darnell's Hygrometer is required to act merely as a weather-glass, to predict 

 the greater or less probability of rain, &c., the difference between the con. 

 stituent temperature of the vapour (shown by the interior thermometer), and 

 the temperature of the air (shown by the exterior thermometer), is all that is 

 necessary to be known. The probability of rain or other precipitation of 

 moisture from the atmosphere, is in inverse proportion to this difference. There 

 are several difficulties connected with the use of Daniell's Hygrometer that are 

 in a great measure overcome in Regnault's Instrument. Fig. 76 Price, 330 



86. Jones's Hygrometer (fig. 77). This instrument is the same in 

 principle as Daniell's Hygrometer, but simpler in its construction. The tube 

 of the Mercurial Thermometer is bent so as to bring its bulb vertical and 

 parallel with its stem. This bulb is one inch long, and of a conical shape, with 

 a flattened top or surface of black glass projecting a little beyond the sides. 

 Below the flat surface this bulb is covered with black silk. The Hygrometer 

 is mounted and supported on a brass stand in such a manner that the black 

 surface can be inclined towards the light. When used the temperature of the 

 air is first to be noted. ./Ether is to be poured on to the silk cover of the bulb, 

 and the condensation of moisture takes place upon the black surface of the 

 bulb. Then, by again noting the temperature, the dew point may be known. 



Price, 2 10 



87. Regnault's Condenser Hygrometer, (fig. 78), for ascertaining by 

 direct observation the dew-point, is superior to Daniell's, from its being more 

 certain in its indications, and economical in use. It consists of two highly- 

 polished silver cylinders, into the upper part of which are cemented thin glass 

 tubes ; these have brass covers, arranged to receive and support two delicate 

 Standard Thermometers, the bulbs of which descend nearly to the bottom of 

 the silver portion of these chambers. Each chamber has a small internal tube 

 carried down from the brass cap to within a short distance of the bottom, to 

 admit the passage of the air, which is drawn through both chambers by an 

 Aspirator, (fig. 78*) connected to the base of the hollow upright and arms 

 supporting the cylinders. 



To use this Hygrometer, aether is poured into one chamber sufficient to cover 

 the bulb of the thermometer, and then the thermometers being inserted into 

 both cylinders the instrument is now connected to the aspirator, and by it the 

 air is drawn through both cylinders down the internal tubes, passing in one 

 chamber in bubbles through the aether, and in the other chamber simply around 

 the thermometer. The tube in this empty cylinder is of such a diameter as to 

 ensure similar quantities of air passing through each chamber. 



After a short time the passage of the air through the aether will cool it down 

 to the dew-point temperature and the external portion of the silver chamber 

 containing the aether will become covered with moisture. The degree shown 



