HYGROMETRIC RESEARCHES. 543 



the third day. On the calm, sunny 4th September 1893 the range of dry bulb tempera- 

 ture was considerably greater in the case of the dry bulb and nearly five times as great 

 in the case of the wet bulb than on the sunny but windy 10th September of 

 the following year. The dry bulb range was greatest on the 11th September 1893, 

 when fog was constantly passing, and the temperature conditions very variable 

 from foggy to the clear spells. The humidity on the 11th September, however, 

 was great, and the range of wet bulb variations was less than half that of the 4th 

 September. 



The range of the thermometers in the Stevenson screen was roughly one-half that 

 of the aspiration-thermometers, except in the case of the windy 10th September 1894, 

 when it was as much as four-fifths. 



From all these examples it is clear that in dry and almost calm weather, when the 

 aerial conditions vary most, considerable differences of temperature are registered 

 by a Stevenson screen and by a thermometer past which air is aspirated, and that the 

 aspiration instrument responds more rapidly to changes in the temperature of the 

 atmosphere. 



In the case of the wet bulb thermometer, the same general law of the greater 

 sensitiveness of the aspiration arrangement holds, but it has to be modified by a con- 

 sideration of the influence of the passing air on evaporation. In the case of the 

 aspiration instrument this is constant ; in the case of the Stevenson screen it is variable. 

 In the latter, in windy weather, the air is constantly renewed, and the conditions as 

 far as regards this one factor resemble more closely those of the aspiration-psychrometer, 

 and the results are comparable. They are all the more so, as we have seen, that the 

 air is well mixed, and more uniform in its properties under such conditions, but an 

 allowance must be made for the greater sensitiveness of the aspiration instrument. 

 The value of the aspiration instrument in times of calm, clear weather, when atmospheric 

 conditions may vary considerably both in space and time, is obvious. It indicates the 

 humidity of the air in such a way that the results can be compared with those taken 

 under windier conditions. Stevenson screen thermometers indicate in addition the 

 influence of the wind, or want of wind, on the rate of evaporation from a moistened 

 surface. 



For physical determinations the aspiration instruments give much the more 

 satisfactory results. It may be urged that the unventilated thermometers give nearer 

 indications of the physiological effects of humidity in calm conditions, such as those by 

 factories ; but it would be better, both from the point of view of human health and of 

 the behaviour of organic fibres under different temperatures and humidities, that the 

 use of aspiration or sling-psychrometers should entirely replace that of stationary 

 instruments. Any evaluations of the physiological effect can be deduced from these as 

 adequately as from those of the fixed and unventilated thermometers. 



