45, CORNHILL, E.G., AND 122, REGENT STREET, W., LONDON. 



73 



FIG. 80. FIG. 82. FIG. 83. 



91. Wet and Dry Bulb Hygrometer with Wood or Zinc Scales, 

 mounted in a Japanned Metal Case, suited for out-door use, the Greenhouse 

 or Conservatory (fig. 82). Price, 14 



92. * Ditto Ditto with Negretti and Zambra's Patent 

 Porcelain Scale, in Japanned Case, as fig. 82. Price, 110 



93. Mason's Hygrometer, portable brass-jointed tripod-stand and metal 

 cover (fig. 83). Price, 3 3 



94. Negretti and Zambra's Standard Dry and Wet Bulb 

 Hygrometer, or Psychrometer (fig. 84), consists of two parallel 

 Thermometers, as nearly identical as possible, mounted on a wooden bracket, 

 one marked dry, the other wet. The bulb of the wet thermometer is covered 

 with thin muslin and round the neck of the stem is twisted or tied, as seen in 

 fig. 84*, conducting-threads of lamp-wick, or common darning- cotton, these 

 pass down into a vessel of water, placed at such a distance as to allow a length 

 of conducting-thread of about three inches ; the cup or glass being placed on 

 one side, and a little beneath, so that the water within may not affect the 

 reading of the Dnj Bulb Thermometer. In observing, the eye should be placed 

 on a level with the top of the mercury in the tube, and the observer should 

 refrain from breathing whilst taking an observation. The temperature of the 

 air and of the evaporation is given by the readings of the two thermometers, from 

 which can be calculated the dew point, Tables being furnished for that purpose 

 with the instrument. 



The dry bulb thermometer indicates the temperature of the air itself ; while 

 the wet bulb, cooled by evaporation, shows a lower temperature according to 

 the amount of and rapidity of evaporation. 



This instrument is used by the Members of the British Meteorological 



Society, and supplied to them by Negretti and Zambra (fig. 84) Price 220 



Glaisher's Tables for ditto 026 



