552 MR A. J. HERBERTSON ON 



worm before reaching the bulbs. Several experiments were tried, but a stronger fan 

 and clockwork than those in the ordinary instruments are necessary to maintain a 

 satisfactory current of air. By means of this, the amount of supersaturation of the 

 air can be determined, and the humidity determined by a liquid and not an ice-coated 

 bulb in frosty weather. 



In conclusion, the writer was greatly impressed by the difference between the 

 conditions of the laboratory and the open air. The changes observed in the conditions 

 of the atmosphere increased with the delicacy of the instruments used for investigating 

 its properties. It becomes a practical question what degree of refinement of investiga- 

 tion is most profitable, and the answer will obviously vary according to the problem 

 to be solved. The aspiration-psychrometer gives the means of determining the pressure 

 or mass of water vapour in the air much more accurately than the ordinary dry or wet 

 bulb thermometer in a Stevenson screen. For an accurate determination of the mass 

 or pressure of water vapour at any minute, undoubtedly the aspiration or sKng 

 psychrometer should be used. In calm weather the Stevenson screen thermometers, 

 while indicating the conditions of the air surrounding them, may not give so reliable 

 an indication of the general atmospheric conditions. On the other hand, when the air 

 is changing very rapidly, as so often happens on mountain tops, the more sensitive 

 aspiration-psychrometer responds practically instantaneously to those variations, and 

 the difference of a minute or so in the time of observation may give a considerable 

 difference in the temperatures and humidities, while the Stevenson screen instruments 

 do not show these extremes so clearly, but at any one moment give a reading which is 

 nearer the average. In the case of observations for the humidity of rooms or factories, 

 the aspiration-psychrometer gives the more reliable observations for the determination 

 of vapour mass or pressure, but the stationary thermometers probably yield truer 

 indications of physiological effects ; but, as has been pointed out before, the physio- 

 logical influences can be best deduced from aspiration readings correlated with 

 observations of movements of air. 



The rapid variations of humidity are clearly shown in the records of the ordinary 

 registering hair hygrometer. These are so great that it was found impossible to obtain 

 precise values from the records of this hygrometer at the minutes of any psychrometer 

 reading. In future experiments it might be profitable to use a hair hygrometer with a 

 clock which turned the paper-covered drum in 24 hours or even in a shorter time. 



