550 



MR A. J. HERBERTSON ON 



The difference between calculated and determined values is less under drier 

 conditions, and, as far as the present experiments go, they indicate that the simple 

 formula in the ordinary cases may be accepted as satisfactory. 



Mass of Water Vapour (Absolute Humidity) under Different Weather Conditions. 



The determinations made at Montpellier, at Fort- William, and on the summit of 

 Ben Nevis, all agree remarkably — so much so as to give every confidence in the aspira- 

 tion-psychrometer's action under very different conditions. As has just been remarked, 

 there is no appreciable difference between the results of high and low level determinations, 

 but more experiments are necessary, — especially in warm, dry weather at the top, and 

 in cold, dry weather at the base of Ben Nevis. 



In the autumn of 1893, simultaneous experiments were carried on at the two Ben 

 Nevis Observatories. Two days may be chosen for the comparison of the water 

 vapour at high and low levels, under very different conditions. The figures for dry 

 and wet bulbs and mass of water vapour per cubic metre at both stations are plotted in 

 the accompanying diagrams. 



The following table shows the differences between the temperatures of the dry and 

 of the wet bulb thermometers, and the mass of water vapour in a cubic metre of the 

 atmosphere at Fort-William and Ben Nevis at different hours on the 4th and 11th of 

 September 1893 :— 



Hours. 



8 



9 



10 



11 



12 



13 



14 



15 



16 



17 



18 



19 



4th September 1893. 

























Dry Bulbs (F.W.— B.N.) C.°, . . 



0-94 



1-57 





3-04 



3-89 



4-53 



3-94 



5-26 



4-47 



3-87 



4-81 



4-40 



Wet Bulbs do. C.°, . . 



4-99 



8-30 





8-83 



7-50 



6-55 



5-95 



6-92 



6*52 



8-55 



7-84 



7-45 



Water Vapour do. 



























Grammes per cubic metre, . 



4-83 



6'66 





7-38 



5-65 



5-25 



4-57 



5-52 



5-48 



7-08 



5-83 



5-70 



11th September 1893. 



























Dry Bulbs (F.W.— B.N.) C.°, . . 





4-85 





8-09 



9-40 



9-34 



9-87 



P-52 



10-11 







6-79 



Wet Bulbs do. C.°, . . 





471 





6-57 



6-12 



6-56 



676 



6-54 



7-20 







6-34 



Wiiter Vapour do. 



























Grammes per cubic metre, . 





1-58 





2-07 



1-16 



1-20 



1-63 



1-91 



2'26 







2-54 



On September 11th, the vertical temperature gradient was almost normal save in 

 the early morning, the atmosphere at Fort-William dry, that at Ben Nevis nearly 

 saturated. At Fort- William the amount of water vapour increased until 11 a.m., and 

 fell at noon, rising steadily until 4 p.m. At Ben Nevis, except at noon, the water 

 vapour increased until 2 p.m., and fell regularly thereafter except at 5 p.m. The bulges 

 in the curve at noon, 2, and 5 p.m. are explained by the detached fog which appeared 

 in the valleys around the Ben during the forenoon, and rose to the level of the hill top 

 about noon, and just touched at these hours. 



