444 H. A. Hazen — Condensing Hygrometer 



and it may be necessaiy to seek another. It may be that at 

 low temperatures the ice refuses to conduct the very last part 

 of the heat from the bulb ; however, this explanation does not 

 answer when the temperature begins rising, as in that case the 

 ice-covered bulb ought to again lag and be slightly cooler than 

 the liquid. 



The writer's attention was first called to this matter by the 

 astonishingly high relative humidities reported from the north- 

 western stations, whenever a high-pressure area with very low 

 temperatures appeared. At such times the relative humidity is 

 almost invariably reported at 100 per cent, and yet ordinarily 

 these same high areas, with N.W. winds, frequently reduce the 

 humidity 50 and even 60 per cent when they reach places 

 toward the S.E. where the temperature is above 20°. Again, 

 many observers have reported the ice-covered bulb reading 

 higher than the dry on cold days, even when they had cor- 

 rected thermometers and very high winds: it was thought that 

 many times the trouble arose from a lack of ventilation and an 

 improper coating of ice. Later experiments, however, with 

 perfectly correct thermometers, most careful ice-coats, good 

 ventilation and a nearly saturated air at temperatures clown to 

 0° have repeatedly confirmed these abnormal results, showing 

 in one instance the iced bulb "5° higher than the dry. When 

 we consider that at —20° a difference of *5° between dry and 

 wet amounts to 36 per cent in the deduced relative humidity, 

 the great importance of a thorough investigation of this point 

 becomes very apparent. We must know the amount of this 

 contraction with different coatings of ice and at different tem- 

 peratures; knowing these facts, we may apply a correction to 

 the wet bulb-reading before using it for computing the moist- 

 ure content of the air. The best method of testing the question 

 would seem to be by a rigid series of comparisons at very low 

 temperatures between the ice-covered bulb and a condensing 

 hygrometer. 



In the use of the psychrometer for making the best observa- 

 tions the most important consideration is that of a thorough 

 ventilation, and this is especially to be attended to if readings 

 are to be made in a room or very still air. Out of doors, if 

 there is a good air circulation about the wet bulb, artificial ven- 

 tilation is not needed in the summer, but with an ice-covered 

 bulb the action is so sluggish that it is very convenient to have 

 a movable fan for increasing the air circulation and expediting 

 the observation. . The most convenient method of ventilating 

 the wet bulb is by swinging in a circle of some two feet radius. 

 This method seems to have been first adopted by Saussure at 

 the end of the last century ; he employed a swung wet bulb for 

 obtaining the temperature of evaporation but drd not join with 



