AN IMPROVED COG PSYCHROMETER 



HARRY B. SHAW 

 Bureau of Plant Industry, Washington, D. C. 



In the field and at experiment stations scientific observers have 

 occasion to note the relative humidity. An instrument commonly- 

 used for this purpose is the sling psychrometer, as furnished by the 

 United States Weather Bureau. To secure readings this instru- 

 ment must be whirled. 



The student of plant life often has occasion to ascertain the 

 humidity conditions of restricted areas, as among foliage, or in 

 small frames, etc. The shng type of instrument is useless under 

 such conditions. Dr. F. E. Clements designed a small, portable 

 cog psychrometer (see Research Methods in Ecology, 1905, p. 39) 

 and later Prof. P. J. O'Gara designed one along very similar fines 

 (see Monthly Weather Review, January 1909, pp. 22, 23). It is 

 true of both these instruments that the necessity to whirl the entire 

 instrument is avoided and that some protection is afforded the 

 thermometers themselves, but both the protective device and 

 thermometers are whirled, thus causing risk of entanglement 

 with foliage. The mechanical parts of these instruments are not 

 well constructed nor durable. In the design of Dr. Clements the 

 thermometers are so short as to render rapid and accurate read- 

 ing almost impossible, and in addition, the scale is Centigrade, 

 whereas the conversion tables of the Weather Bureau are in Fah- 

 renheit degrees. They also have the defect that both bulbs are 

 made to whirl in the same plane. This may have the result that 

 the ''wet" bulb wiU leave a trail of vapor in which the "dry" 

 bulb must travel — a condition that may influence the dry bulb 

 to indicate incorrectly. 



The writer has had occasion to study the humidity conditions 

 of plants shaded by tents and glass covered frames, and among 

 dense foliage in the open. Under these conditions it became neces- 



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THE PLAXT WORLD, VOL. 17, NO. 6, 1914 



