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 4%4 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 
