64 



SCIENCE. 



[N. S. Vol. IV. No. 81. 



section of the instrument under considera- 

 tion, say from. 20 to 85 per cent., only a 

 difference of from one to three per cent. 

 could be observed. 



But for the extremities of the scale, from 

 to about 20 per cent, and from about 85 

 to 100 per cent, the reading indicating the 

 relative humidity seemed unreliable, and 

 especially so at low humidities, differing in 

 some cases as much as 10 per cent, from 

 the calculated degree of saturation. 



Thus, in cases of either very low or very 

 high humidity, when two or more hair hy- 

 grometers were placed in the same atmos- 

 phere, their readings were very apt to in- 

 dicate different relative humidities, and 

 also when the same hair hygrometer was 

 placed at different times in an atmosphere 

 of a constant hygrometric state (of either 

 very low or very high relative humidity) 

 it gave different percentages. 



These variations of course presented a 

 difficulty in drawing a correction curve for 

 the extremities of the scale on the hair 

 hygrometer. 



Prof. Rood called my attention to an 

 article in the Beihldtter zu den Annalen 

 der Physih und Chemie, Vol. 19, No. 11, 

 page 875 — ' Theorie des Haarhygrometers, 

 by B. Sresnevsky,' in which it is stated 

 that the change in the length of the hairs 

 for degrees of saturation of the air ceases to 

 be regular when the relative humidity be- 

 comes as low as 7.8 per cent. This is in 

 agreement with the statements just made. 



In the tests which were carried on, it 

 was also invariably found that if the de- 

 gree of saturation of the air was altered, 

 some time had to elapse before the hairs 

 became adjusted to the new conditions sur- 

 rounding them, and therefore ample time 

 was always given for the hairs to become 

 adjusted when the hygrometric state of the 

 air was changed. Thus, when the hair 

 hygrometers were placed in an entirely 

 new atmosphere differing 15 per cent. 



or more in relative humidity, 5 to 25 

 minutes elapsed before the hairs re- 

 sponded perfectly to the change and gave 

 approximately correct readings. It was 

 further observed that the length of this 

 time depended on whether the change was 

 to a higher or to a lower percentage of 

 humidity, and also in what part of the 

 scale the change occurred. 

 For example: A change from 



15 to 90 per cent, required about 10 minutes 



which indicated that it takes longer for the 

 hairs to dry out than for them to take up 

 the moisture, and that the change is slower 

 at the lower parts of the scale than else- 

 where. 



A knowledge of the relative humidity of 

 the air is important, not only in various 

 branches of science and the arts, but also 

 in the treatment of the sick, particularly in 

 cases of certain pulmonary disorders. 

 • J^Ithough it may be probable that a per- 

 fectly accurate direct reading hygrometer 

 cannot be obtained, this drawback should 

 not condemn the hair hygrometer, for such 

 great exactness is seldom required, a 

 knowledge of the relative humidity of the 

 air to within two or three per cent, being, 

 in most cases, all that is necessary. 



The precaution of allowing considerable 

 time to elapse for the hairs of hair hygro- 

 meters to become adjusted to a changed 

 atmosphere, before taking a reading, is only 

 really necessary where a possibility exists 

 >that a decided change in the relative 

 humidity has suddenly occurred. 



The table which is given below has been 

 constructed from results obtained by com- 

 paring the readings of relative humidity 

 shown by the hair hygrometer with those 

 calculated from observations made with wet 



