July 17, 1896.] 



SCIENCE. 



63 



now it has reached a higher place among 

 hygrometrical apparatus by virtue of cer- 

 tain improvements in its construction. 



I have recently tested a form of hygrom- 

 eter based on the Saussure principle, with 

 a view of observing its action when sub- 

 jected to different changes in the degree of 

 saturation of the air. 



Fig. 1. 



Fig. 2. 



The hygrometer which was tested is 

 shown in the accompanying cut, Figs. I. 

 and II. ; the essential parts of the instru- 

 ment are designated by letters, and the 

 parts so indicated are explained in the fol- 

 lowing paragraph : 



A and A' are two thin brass supports, re- 

 spectively 23.7 centimeters and 22 centi- 

 meters long and 2 centimeters broad. 



B, small rivets which connect the sup- 

 ports A and A', but leave an air space 

 which separates the latter by .7 of a centi- 

 meter. 



C, six fine hairs about 18 centimeters 

 long, placed parallel, and laid close to- 

 gether, so as to hang like one large hair in 

 the air space between A and A'. 



D, an adjusting screw, from which the 

 six hairs, C, are suspended. 



E, a short lever, to which the lower ends 

 of the hairs, C, are fastened (a small weight 

 is attached to this lever in such a way that 

 the hairs are kept at a slight tension) . 



F, an indicator, 4 .7 centimeters long, 

 fastened to the lever, E, which shows chan- 

 ges in the length of the hairs, C ; by the use 

 of this indicator the actual change in the 

 length of the hairs is multiplied a conven- 

 ient number of times. 



G, a scale of percentages of relative 

 humidities, from to 100 per cent. 



H, a thermometer fastened to the support 

 A', which is used in dew-point determina- 

 tions. 



It was important that the scale of per- 

 centages, G, Fig. II., should be carefully 

 tested and either verified or the corrections 

 obtained throughout; therefore observations 

 were made with all possible different per- 

 centages of saturation of the surrounding 

 air. 



The readings of the air hygrometers in 

 each case were compared with the relative 

 humidity obtained from observations made 

 with wet and dry bulb thermometers, and 

 the percentage of saturation deduced from 

 the Smithsonian hygrometrical tables of 

 Guyot. This was 'done as a relative com- 

 parison, since the wet and dry bulb ther- 

 mometers, or Auguste's psychrometer, is 

 the means almost entirely used at the U. 

 S. Weather Bureau stations for obtaining 

 the relative humidity of the air. 



Of course, in the present investigation, 

 the determinations made with the wet and 

 dry bulb thermometers were themselves 

 subject to some error ; yet this method is so 

 generally accepted, and is the means which 

 is so often used for obtaining the relative 

 humidity, that it seemed fair to compare 

 the readings of the hair hygrometer with 

 those calculated from observations made 

 with wet and dry bulb thermometers. 



The results of the comparisons which 

 were obtained indicated that for the middle 



