HYGROMETRIC RESEARCHES. 551 



The weather at both stations was dry and warm on September 4th, but the air at 

 the summit was relatively much warmer, — the difference between the two stations 

 varying from o, 9 to 5° -3. The vapour curve for the low level shows a slow but 

 irregular increase in the mass of water in each cubic metre of air ; that for the summit 

 station curves in the same way as the movements of the wet bulb, showing a minimum 

 at 10 a.m., two maxima at 2 and 4 p.m., and a marked minimum at 5 p.m., while at 

 this hour the dry bulb reached a second maxima almost as high as the first at 2 p.m. 

 The vapour present at 7 p.m. in the air at the summit was only 0*41 gramme per cubic 

 metre more than at 8 a.m., but the range for the twelve hours was 2"96 grammes per 

 cubic metre. 



On both the 4th and 11th September 1893 there was a constant increase in the excess 

 of water vapour at the base station compared with that at the summit until 11 a.m., and 

 then a sudden drop at noon. On the more normal day there was a gradual increase in 

 the amount of water vapour during the forenoon at both stations, but it was much more 

 rapid at Fort-William ; whereas, on September 4th, an increase of water vapour took 

 place at the base, but a diminution of it at the summit of the mountain, until between 

 10 and 11 a.m. On the 11th, from noon onwards, there was a steady increase in this 

 difference, which was very irregular on the afternoon of the 4th, due principally to the 

 variations at the summit station already noted. 



The atmospheric movements at the summit in radiation weather are very complex, 

 and the mass of vapour is perhaps the most marked index of this variety of aerial 

 samples, which appear to depend on the direction of the air current up or down the 

 side of the mountain. 



The difference between the mass of vapour per cubic metre at the two stations, 

 under abnormal conditions, varies very much, the more humid atmosphere being 

 pressed down by drier and warmer air flowing in from above, as Mr Omond has shown, 

 and this concentration of water vapour in the lower regions below the summit of Ben 

 Nevis, acting as a screen, intensifies the abnormal temperature relations between the 

 two stations. At Ben Nevis the boundary between such drier and moister layers must 

 be often well marked, and an observer, in climbing or descending the mountain, might 

 with advantage make observations with the aspiration-psychrometer at intervals of 

 every few hundred feet. The variations are so great that it is obvious that the 

 conception of an independent atmosphere of water vapour following Dalton's law for a 

 gaseous atmosphere is untenable. 



In the Montpellier experiments the vapour curve was very uniform, and sometimes 

 remarkably constant. 



With the aspiration-psychrometer it is possible to carry on work on the summit of 

 Ben Nevis indoors in bad weather, but the observatory is so small that when the 

 weather outside made work impossible, the apparatus could not be properly installed. 



Mr Eankin has suggested that the aspiration-psychrometer might be employed 

 with advantage in foggy or frosty weather, the air being first drawn through a heated 



