METEOROLOGICAL INSTRUMENTS. 225 



EVAPORATION. 



Instruments for the measurement of evaporation are called 

 " atmometers," or more correctly, " atmidometers." They con- 

 sist in principle of an open dish filled with water, and allowed to 

 remain for a length of time exposed to the air. The amount of 

 v/ater removed by evaporation is measured either by volume 

 (Von Lament, Dufour, Dr. A. Mitchell), or by weight (Osnaghi 

 [exhibited] and S. H. Miller). Most atmometers are only fitted 

 for exposure in a screen, SQ, as to be sheltered from rain and 

 wind, and so can only indicate in a very modified degree the true 

 evaporation in the open air. 



Moreover, it is doubtful whether the results of experiments 

 made with small instruments are applicable to large water 

 surfaces. fr| ^ 



WIND. 



Wind is measured either by its pressure on a definite surface, 

 or by its velocity. 



Of pressure gauges, the best known is Osier's. In this a plate 

 of known area is driven back against a spring, and its motion 

 recorded on paper. In Gator's instrument (exhibited) the resist- 

 ance is furnished by levers instead of springs. The principle of 

 these arrangements was first devised by Bouguer. 



A very simple form of Pressure Anemometer is Wild's (exhi- 

 bited among the Russian instruments), in use in Russia. Norway, 

 and Switzerland. It consists of a rectangular plate hung on 

 hinges on a horizontal axis. The angle which this makes with 

 the vertical indicates the force of the wind. The first propounder 

 of this idea is unknown. It is evident that this instrument is not 

 self-recording. 



The idea of employing the vanes of a windmill to measure the 

 velocity occurred to Ch. Wolff, 1743, and was afterwards adopted 

 by Whewell. 



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