HYGROMETRIC RESEARCHES. 



541 



example of one of the most notable records of rapid variation, the following table is 

 obtained : — 



Change per Minute. 



















Average 



Time 



16 h 5 m 



6 



7 

 ■ 



8 



9 



10 



11 



neglecting 

 signs. 



Dry Bulb- 













in Aspiration-Psychrometer 



13-60 



+ 0-25 



-0-20 



-0-60 



dz0-<>0 



+ 0-45 



+0-20 



0-28 



in Stevenson Screen 



13-83 



±0-00 



-0-22 



-0-17 



+ 0-06 



±o-oo 



+ 0-11 



0-09 



Wet Bulb— 



















in Aspiration-Psychrometrr 



8-80 



+ 0-20 



=F0-00 



+ 0-45 



+ 0-65 



-2-30 



-0-90 



0-75 



in Stevenson Screen 



8-44 



+ 0-17 



+ 0-22 



+0-23 



+ 0-88 



-1-55 



-0-61 



0-61 



Departure of Wet Bulb from Dry — 



















Aspiration-Psychrometer . 



4-80 



+ 05 



-0-20 



- 1-05 



-0-65 



4 2-75 



+ 1-10 



097 



Stevenson Screen 



5-39 



-0-17 



-0-44 



-0-40 



-0-82 



+ 1-55 



+ 0-72 



0-68 





From this it is obvious that the instruments in the aspiration-psych rometer change 

 more rapidly and to greater extremes. The change per minute in the dry bulb in the 

 air current varies from —0*6 to +0*45, or, neglecting signs, averages 0'28 per minute, 

 compared with a variation from — 0"22 to +0 , 11 and an average of 0'09 per minute in 

 the case of the dry bulb in the Stevenson screen. The wet bulb shows a similar varia- 

 tion in sensitiveness under these different conditions — in the aspiration-psychrometer its 

 variation per minute ranges from — 2*30 to + 0'65, and averages 0*97 ; in the Stevenson 

 screen only from — 1 '55 to + 0'88, and averages 0"61. The relative humidit}^, as evidenced 

 by the departure of the wet bulb readings from those of the dry, shows even more marked 

 changes — the aspiration variations being from +2"75 to — 1'05, and the average 0'97 

 per minute; the Stevenson variations being from 4- 1*55 to — 0'44, and the average only 

 0'68. The higher temperature of the Stevenson screen dry bulb is, as we have seen, a 

 function of the time of day, and the lower humidity registered at the beginning of the 

 observation in the Stevenson screen is probably part of a drier phase still actively 

 affecting the Stevenson screen instruments. 



Another instance of quicker response to changed atmospheric conditions on the part 

 of the aspiration instrument may be cited. At 20 hours 4 minutes on 2nd September 

 1893 the aspiration dry bulb thermometer was at 8°'0 C, and fell practically steadily to 

 6° '8 C. during the next 12 minutes. The Stevenson screen dry bulb, on the other hand, 

 began at 5°"94 C, or ended at 6°'17 C. That the former represented the true state of 

 the atmosphere is confirmed by the changes of temperature in the large cylindrical 

 aspirator used for drawing air through the drying tubes. The thermometer in this 

 aspirator sank steadily from ll°-67 C. to 8 o, 00 C. in the same period. 



The following table (page 542) shows the range of temperature on Ben Nevis in 

 the minute to minute observations of three different days. The observations lasted, on 

 an average, from 4 to 6 minutes on the first two days, and from 10 to 1 1 minutes during 



