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BEXJ. PIKE S, JR., DESCRIPTIVE CATALOGUE. 



Sctussvre'k Hygrometer (Fig. 617) consists of 

 a human hair prepared by boiling it in caustic 

 ley ; one end of the hair is fixed to the upper 

 part of a frame, usually about ten inches long, 

 and four wide, and the other end passes round 

 the axis of a small grooved wheel or pulley, the 

 axis of which carries a counterpoise by which 

 the hair is kept stretched, and an index which 

 moves over a graduated arch ; on the upper 

 part of the scale there is usually fixed a thermo- 

 meter. When the surrounding air becomes 

 more humid, the hair absorbs an additional 

 quantity of moisture, and is elongated ; the 

 counterpoise consequently descends and turns 

 the pulley, whereby the index is moved towards the one 

 hand or the other. On the contrary, when the air becomes 

 dryer the hair loses a part of its humidity, and is shortened. 

 The counterpoise is consequently drawn up, and the index 

 moves in the opposite direction. Price, $5.50. 



Hygrometers by Condensation. The two preceding hygro- 

 meters are constructed on the principle of absorption, and 

 for meteorological researches are of but little use. Those 

 remaining to be described are constructed on the principle 

 of condensation, and form valuable instruments for the me- 

 teorologist, of which Mason's is the simplest and most modern 

 construction. In order to give an idea of the general prin- 

 ciple on which they depend, let us conceive a glass jar, hav- 

 ing its sides perfectly clean and transparent, to be filled with 

 water, and placed on a table in a room where the tempera- 

 ture is, for example, 60 ; the temperature of the water 

 being the same as that of the room. Let us next suppose 

 pieces of ice, or a freezing mixture, to be thrown into the 

 water, whereby the water is gradually cooled down to 55, 

 50, 45, &c., degrees. As the process of cooling goes on, 

 there is a certain instant at which the jar loses its transpa- 

 rency, or becomes dim ; and on attentively examining the 

 phenomenon, it is found to be caused by a very fine dew, or 

 deposition of aqueous vapor on the external surface of the 

 vessel. The precise temperature of the water, and conse- 

 quently of the vessel, at the instant when this deposition 

 begins to be formed, is called the dew point, and is capable 

 of being noted with great precision. Now this temperature 



