45, COBNHILL, E.G., AND 122, BEGENT STEEET, W., LONDON. 69 



83. Saussure's Hygrometer (fig. 74), for showing changes 

 in the hygrometric condition of the atmosphere on a graduated 

 arc, by the contraction and elongation of a human hair, this acting 

 the reverse of string or cord, stretching when moist and contract- 

 ing when dry. A thermometer is attached to the scale. 



Price, 1 10 



Although a most elaborate Treatise on the construction and 

 use of this Hygrometer was written by its inventor, M. Horace 

 Benedict de Saussure, Professor of Philosophy, at Geneva, in 1783, 

 this instrument may be regarded more as an ornamental curiosity 

 than of any scientific value. FIG 74. 



84. Leslie's Thermometric Hygrometer (fig 75). It will be seen 

 that Leslie's instrument is the elementary form of Mason's Wet and Dry Bulb 

 Hygrometer, by which it is entirely superseded. 



This instrument consists of a glass tube, terminated with a bulb at each 

 end, as fig. 75. The tube is partly filled with sulphuric acid, tinged by carmine. 

 One of the balls is covered with muslin, and kept continually moistened with 

 water, drawn from a vase placed near it by the capillary attraction of a few 

 strands of cotton- wick. The descent of the coloured liquid in the other stem 

 will mark the diminution of temperature caused by the evaporation of the water 

 from the humid surface. The drier the ambient air is, the more rapidly will 

 the evaporation go on ; and the cold produced will be greater. When the air 

 is nearly saturated with moisture, the evaporation goes on slowly ; the cold 

 produced is moderate, because the ball regains a large portion of its lost heat 

 from surrounding bodies. The degree of refrigeration of the ball is an index of 

 the dryness of the air. 



When this hygrometer stands at 15, the air feels damp ; from 30 to 40, 

 we reckon it dry ; from 50 to 60, very dry ; and from 70 upwards, we should 

 call it intensely dry. A room would feel uncomfortable, 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. Price, 110 



85. Daniell's Hygrometer, for ascertaining the dew-point by direct 

 observation (fig. 76), invented about the year 1820, by the late Professor Daniell, 

 of King's College, London. 



It consists of a glass tube, bent twice at right angles, and terminating, at 

 each end, in a bulb. In the long limb of the tube is enclosed a delicate 

 thermometer, which descends to the centre of the bulb, which is about three- 

 parts filled with sulphuric aether. All the other parts of the tube are carefully 

 freed from air, so that they are occupied by the vapour of the aether. This bulb 

 is made of black glass ; the other bulb on the shorter limb is transparent, and 



