A COG PSYCHROMETER 185 



sary to protect the whirling thermometers, therefore an outer, 

 stationary cage was constructed to protect the inner, revolving 

 parts of the instrument. Instead of the fragile and cheaply con- 

 structed commercial egg beaters, a substantial hand drill, such as 

 may be obtained at any hardware store at a cost of about one dol- 

 lar, was used. This is strong, not unduly heavy or large, runs 

 very smoothly and is durable. The shank to which the chuck of 

 this drill is attached is sufficiently long for the application of the 

 outer protective frame or cage by means of small machine screws, 

 as shown in the illustrations. 



In figure 2 is shown a second and still more specialized form of 

 cog psychrometer. This was designed and used for relative 

 humidity studies among dense foliage, where it was desirable to 

 avoid disturbing the foliage during a series of readings. The pro- 

 tective additions to both these instruments are made of strips of 

 sheet brass of commercial sizes. In the second type of instrument 

 the revolving supporting frame is identical with that of the first 

 mentioned design, except that the axis is prolonged considerably 

 at the lower or free end (e, fig. 2). In this case the protective 

 cage is separate. Supporting arms, instead of the protective cage, 

 are attached to the shank of the drill. The psychrometer is in- 

 serted within the cylindrical cage, the free end of the prolonged 

 axis being inserted into a hole in the center of a strip {i. fig. 2) 

 . rivetted diametrically across the lower portion of the cage, while 

 the ends of the supporting arms rest upon the upper rim of the cage. 



To use this instrument, the cage is first set among the foliage 

 whose humidity conditions are to be studied, care being taken 

 that none of the leaves enter the cage; after moistening the "wet" 

 bulb, the instrument is set into the cage as above indicated and 

 the thermometers are revolved to obtain the reading. This opera- 

 tion may be repeated as often as desired without disturbing the 

 foliage. 



