THE WATER OF THE HABITAT 29 



venient and satisfactory. It is smaller, more compact, and the 

 danger of breakage in use or in carriage is extremely small. It has 

 the further advantage of making it possible to take readings in a 

 layer of air less than 3 centimeters in thickness, and in any posi- 

 tion. The use of the sling psychrometer is attended with grave 

 danger to the instrument in a free space less than two yards across. 

 The cog psychrometer has a single disadvantage owing to the neces- 

 sary use of short thermometers. To secure the proper range. Centi- 

 grade thermometers must be used, and the readings thus obtained 

 must be converted into Fahrenheit temperatures before the humid- 

 ity can be determined from the usual tables.^ 



38. Making a reading. In general, observations should be 

 taken facing the wind. It is also a wise precaution to shift the 

 position of the instrument a foot or more during the reading, except 

 when the humidity of a definite layer is desired. The cloth of the 

 wet bulb is first mxoistened with water carried in a small 50-cc. 

 bottle for this purpose. Distilled water is preferable, but tap- 

 water and the water of streams may be used without appreciable 

 error, if the cloth about the wet bulb is changed occasionally to 

 prevent the accumulation of dissolved material. The water is 

 poured slowly upon the cloth of the bulb until it is completely 

 wetted, care being taken not to wet the dry bulb. As the cloth 

 absorbs water reluctantly when perfectly dry, a pipette or a brush 

 is usually a valuable aid in wetting it quickly. The temperature 

 of the water used is of slight consequence, though readings can be 

 made more quickly when the temperature is not too far from that 

 of the air. The psychrometer is held in the proper position, i.e., 

 the bulbs are placed in the layer of air to be studied, unless a gen- 

 eral reading is to be made, and are then rotated at an even rate and 

 at a moderate rapidity. As the reading must be made when the 

 mercury of the wet bulb reaches the lowest point, the instrument 

 is usually stopped after 100 revolutions, and the position of the 

 column is noted. The lowest point is often indicated by the ten- 

 dency of the mercury to remain stationary. As a rule, the lowest 

 point can be known with certainty only when the next glance shows 

 a rise in the column. Check readings of this nature must be made 

 every 25 or 50 revolutions in order to make sure that the mercury 

 has not reached the minimum and then begun to rise while the 

 instrument is in motion. In noting the final reading, care must be 



' Researcli Methods, 39. 



