1892 - 93 .] A. J. Herbertson on Atmosphere at Ben Nevis. 179 
The aspirator was set about 5 feet to the windward side of the 
Stevenson screen, and the box containing the bulbs was placed on 
the top of the aspirator stand so that the mouth of the tube was 
about 4 feet from the ground and faced the wind. 
The cap was then removed from tube 1, and the aspirator allowed 
to act. Usually 14 or 15 litres were drawn through the tubes at 
the rate of 2 litres per minute in the case of the bottle aspirator, 
and of J litre per minute in the case of the reversible one. 'No 
difference could be detected between results obtained by aspirating 
fastly and slowly. In very dry weather 20 and 24 litres of air were 
sucked through the tubes. 
During the experiment the dry and wet bulb thermometers were 
regularly read, usually every two or every three minutes. In some 
experiments, when temperature and evaporation conditions were 
very constant, observations were made every five or six minutes ; 
hut, when these varied very rapidly, readings were taken every 
minute. In frosty weather the wet-bulb required constant attention, 
and indeed when the air was very dry and the wind strong, a new 
coating of ice had to be put on every few minutes. The sensitive- 
ness of the wet-bulb under these conditions was remarkable, some- 
times varying as much as 0°*7 F. in a minute. It was difficult to 
determine at times when the bulb was giving the true evapora- 
tion. When the readings began to show a steady rise the bulb was 
moistened, and if, after waiting for some time, the temperature 
registered before that rise was again reached, it was assumed that 
such temperature was the true wet-bulb reading. When there was 
little wind this difficulty was much less, except in a calm, wdien, of 
course, the sluggishness of the instrument was great. In future 
experiments it will be advisable to observe the dry and wet bulb in 
a current of air of known velocity. 
At the time of the earlier experiments the dust-counter was not 
in working order, but afterwards the number of dust particles was 
estimated, except when the wind was south-westerly. 
The general weather conditions during each experiment were 
noted, and a copy made of corresponding observations made at the 
Low-Level Observatory at the same time. 
After exposure, the cap was replaced on tube 1. Inside the 
observatory each tube was capped, and then placed in a box, kept 
