26 EGGS IN COLD STORAGE. 



first whirling, as this is done only to make sure 

 that the muslin is thoroughly saturated with water. 

 If the water used is of nearly the same temperature 

 as the room, correct readings are sooner obtained. 

 If the psychrometer and water are at a much higher 

 temperature than the air of the room, it will take a 

 proportionately longer time to reach a correct read- 

 ing, but the accuracy will not be impaired, if sufficient 

 time is allowed for the mercury to settle. It is very 

 important that the muslin covered bulb should not 

 become dry in the least; it should be saturated with 

 water during the full time of observation. There will 

 be no difficulty in getting accurate readings down to 

 29 F., as indicated by the dry bulb. At about this 

 temperature, and with the wet bulb at about 27 F., 

 ice will form on the wet bulb and cause the psychro- 

 meter to become somewhat erratic in its behavior.* 



Jt is difficult to describe the proper movements 

 psychrometer. f or whirling the sling psychrometer, a little practice 

 being the best instructor. The handle is held in 

 a horizontal position, the frame mounting the ther- 

 mometers revolving around the pivot, after the man- 

 ner of the weapon with which David slew Goliath, and 

 from which our moisture-tester gets the easy part 

 of its name. A high rate of speed is unnecessary, a 

 natural, easy motion of the forearm or wrist being all 

 that is required. When stopping the psychrometer 

 the arm should follow the thermometer from the high- 

 est point of the circle of rotation, whereby the radius 

 of the path of the psychrometer is increased, and the 

 momentum overcome. The stopping can be accom- 

 plished in a single revolution, after a little practice. 

 The psychrometer will come to rest very nicely by 

 simply allowing the arm to stand still, but the final 

 revolution will be quite irregular and jerky. 



* The writer is in receipt of a special report on this point, prepared by 

 Prof. C. F. Marvin, in charge of the instrument division of the Weather Bureau, 

 and will gladly give any one having difficulty with the psychrometer at these 

 temperatures information so far as he can; but the point involved is somewhat 

 intricate, and so few are using temperatures as low as 29 F. that it is thought 

 best to omit a discussion of this phenomenon. 



