June 8, 1883.] 



SCIENCE. 



505 



Hygrometric observations at Iowa Circle, "Washington, at 7 A.M., during March, 1883. 



elevated stations, will be needed before these 

 formulae can be improved. 



The following directions maj' be given as 

 essential to the satisfactory working of the 

 dr}-- and wet-bulb hj'grometer : — 



In order to obtain accurate results, an in- 

 duced air-current from 1.5 to 5 metres j)er 

 second (3.4 to 11.2 miles per hour) is essential. 

 This is needed even with moderately high 

 wind ; as experiment has shown, that, in a 

 double-louvred shelter, with a wind of 12 

 miles per hour blowing directly through it, a 

 velocity of only 1 to 1.5 miles per hour was 

 recorded in the most favorable spot. 



The thermometers sliould be preferablj^ cj'lin- 

 drical, with the bulb removed an inch or more 

 from the scale ; and no metallic substance should 

 be permitted near the wet bulb. The dry 

 thermometer should be kept clean, as dust 

 and grit would cause a deposition of moisture 

 in foggy weather. 



The muslin should be fine, and tied smoothlj'^ 

 over the bulb. It needs cleaning as often as it 

 appears to be turning j-ellow. If dust settles 

 upon it, it can be easilv cleaned with water. 



Clean rain or melted-snow water should be 

 used for wetting. A strip of cotton three- 



eighths of an inch wide, or a wick, will serve 

 to make connection between the muslin and 

 the reservoir in warm weather. If the air is 

 veiy drj', this strip will cease acting ; and in 

 such case the bulb may be immersed for a 

 moment. It will be found, that if the reser- 

 voir is kept full, and the angle of the cotton 

 is not too great, the latter difficultj' will sel- 

 dom be encountered. 



If anj- moisture is seen on the dry thermome- 

 ter, it should invariably be wiped off. 



If the air-temperature approaches freezing, 

 the reservoir should be removed ; though the 

 wick may be left, its end being carried up and 

 fastened to the frame in such a way as to per- 

 mit of immersing the bulb. The water in the 

 reservoir should be kept in the open air until a 

 film of ice forms upon it, the intention being to 

 keep it as near freezing as possible. The bulb 

 should be repeatedly wet by immersion till a 

 coating is formed, the thickness of which should 

 depend on the difference between the diy and 

 wet bulbs and the velocitj' of the air-motion ; 

 i.e., the greater the difference and the velocity, 

 the thicker the coating. There is no difficulty 

 with an induced air-current in obtaining accu- 

 rate results with a coating 1 mm. in thickness. 



