METEOROLOGY. 



269 



priuc 

 nyi^rometers 

 constructed ? 



of air may be made to pass from a state of dampness to one of extreme dry- 

 ness, by merely elevating its temperature, and tills, too, without altering tlio 

 amount of moisture it contains in the least degree. 



AVhatareHy- Instruments designed for measuring the 

 groniet«rs? quantity of moisture contained in the atmos- 

 phere, are called Hygrometers."* 



_ Many organic bodies have the property of absorbing vapor, 



principle are and thus increasing their dimensions. Among such may bo 

 mentioned hair, wood, whalebone, ivory, etc. Any of these 

 connected with a mechanical arrangement by which tho 

 change in volume might be registered, would furnish a hygrometer. 



A large sponge, if dipped in a solution of salt, potash, soda, or any other 

 substance which has a strong attraction for water, and then squeezed almost 

 dry, will, upon being balanced in a pair of scales suspended from a steady 

 support, be found to preponderate or ascend according to the relative damp- 

 ness or dryness of the weather. 



The beard of the wild oat may also serve as a hygrometer, as it twists 

 around, during atmospheric changes from dampness to dryness. 



If we fix against a wall a long piece of catgut, and hang a weight to the 

 end of it, it will be observed, as the air becomes moist or dry, to alter in 

 length ; and by marking a scale, the two extremities of which are determined 

 by observation when the air is very dry, and when it is saturated with moist- 

 ure, it will be found easy to measure the variations. 



. An instrument called the " Hair Hygrom- 



"Hair Hy- eter," is constructed upon this principle. It 

 grometer. consists of a human hair, fastened at one 



extremity to a screw (see Fig. 223), and at the other pass- 

 ing over a pulley, being strained tight by a silk thread and 

 weight, also attached to the pulley. To the axis of the 

 pulley an index is attached, which passes over a graduated 

 scale, so that as the pullej^ turns, through the shortening or 

 lengthening of the hair, the index movea When the in- 

 strument is in a damp atmosphere, the hair absorbs a con- 

 siderable amount of vapor, and is thus made longer, while 

 in dry air it becomes shorter ; so that the index is of 

 course turned alternately from one side to the other. 



The instrument is graduated by first placmg it in air ar- 

 tificially made as dry as possible, and the pomt on the 

 scale at which the index stops under these circumstances, 

 is the point of greatest dryness, and is marked 0. The 

 hygrometer is then placed in a confined space of air, which 

 is completely saturated with vapor, and under these cir- 

 cumstances the index moves to the other end of the scale : 

 this point, which is that of greatest moisture, is marked 



• Hydrometer, from the Greek words v>/)oj (moist) and nirpiv (measure). 



Fig. 223. 



