118 



MR. W. N. SHAW ON HYGBOMETBIC METHODS. 



untrustworthy are set forth. DOBEREJNER'S modification* is cited as one of many 

 given in GEHLER'S dictionary. The author then goes on to describe a method of using 

 a dew-depositing arrangement which consists in reading the temperature of the 

 boundary of a dew deposit upon a surface whose temperature varies continuously, 

 instead of reading the temperature of a surface assumed to be at a uniform tempera- 

 ture at the instant at which a deposit occurs. 



It is unnecessary to give a lengthy description ; it will be sufficient to say that the 

 lower end of a vertical column of mercury in an iron cylindrical tube is maintained by 

 ice or by a freezing mixture at a very low temperature ; after some time the tempera- 

 ture of the column becomes steady, gradually increasing in the vertical direction. 

 Dew is deposited up to a certain height, and the temperature at that height is read by 

 a thermometer with a small bulb sunk in the mercury to the required level. The 

 height of the thermometer bulb can be adjusted, and its position defined by two index 

 points which are on the same level as the bulb. The precautions necessary for 

 accuracy are described in due course. This instrument was used by BELLI as a 

 standard hygrometer. 



After this discussion he passes on to his evaporation hygrometer, and the wet-and- 

 dry-bulb. His experiments showed (1) a depression of the wet-bulb reading of about 

 '7 R. as corresponding to a relative motion of the air of three metres per second ; 

 (2) in still air wet-bulb thermometers with large bulbs take a longer time to arrive at 

 their final temperature than those with small bulbs, and the final temperature is 

 higher in the former than in the latter case. I was interested to notice that he 

 included in his comparison observations a brass ball 80 mm. in diameter, containing 

 the bulb of a thermometer, as I had made a similar experiment with the same object 

 before I knew of BELLI'S work. I quote his results. He concludes from them that, 

 provided there is a sufficiently good air current, the reading is independent of the size 

 of the bulb. 



TABLE I. Air still. 



* 'Gilbert's Annalen,' vol. 70, 1822, p. 136. 



