40 THE HABITAT 



the sling psychrometer in field work, and has entirely supplanted the latter 

 in the writer's own studies. It is smaller, more compact, and the danger of 

 breaking in carriage or in use is almost nil. It has the great advantage of 

 making it possible to take readings in a layer of air less than two inches 

 in thickness, and in any position. Fairly accurate results can even be ob- 

 tained from transpiring leaves. The instrument can readily be made by 

 a good mechanic, at a cost for materials of $1.75, which is less than half the 

 price for the sling form. A single drawback exists in the use of short, 

 Centigrade thermometers, inasmuch as tables of relative humidity are 

 usually expressed in Fahrenheit. It is a simple matter, however, to con- 

 vert Centigrade degrees into Fahrenheit, mentally, or the difficulty may be 

 avoided by the conversion table shown on page 47, or by constructing a 

 Centigrade series of humidity tables. The fact that the wet and dry bulbs 

 revolve in the same path has raised a doubt concerning the accuracy of the 

 results obtained with this instrument. Repeated comparisons with the 

 sling psychrometer have not only removed this doubt completely, but have 

 also proved that the standardization of the thermometers has been efficient. 



64. Construction and use. A convenient form of egg-beater is the Lyon 

 (Albany, New York), in which the revolving plates can be readily removed, 

 leaving the axis and the frame. The thermometers used are of the short 

 Centigrade type. They are 43^-^ inches long and read from -5 to 50. 

 Eimer and Amend, 205 Third Ave., New York city, furnish them at 75 

 cents each. The thermometers are carefully standardized and compared, 

 and then grouped in pairs that read together. Each pair is used to con- 

 struct a particular psychrometer. Each thermometer is strongly wired to 

 one side of the frame, pieces of felt being used to protect the tube and in- 

 crease the contact. The frame is also bent at the base angles to permit 

 free circulation of air about the thermometer bulbs. The bulb of one ther- 

 mometer is covered with the proper cloth, and the psychrometer is finished. 

 Since the frame revolves with the thermometers, it is necessary to pour the 

 water on the wet bulb, or to employ a pipette or brush. The thermometer 

 bulbs are placed in the layer to be studied, and the frame rotated at an even 

 rate and with moderate rapidity. The observation is further made as in the 

 case of the sling psychrometer. As the circle of rotation is less than three 

 inches in diameter, and the layer less than an inch, in place of nearly three 

 feet for the sling form, the instrument should not be moved at all for ex- 

 tremely localized readings, but it must be moved considerably, a foot or 

 more, if it is desirable to obtain a more general reading. 



65. Hygrometers. While there are instruments designed to indicate the 

 humidity by means of a hygroscopic substance, not one of them seems to 



