INTER-RELATIONS OF BAROMETRIC PRESSURE. 521 



then the calculated sea-level pressure for the top is less than that at Fort- William, 

 and the meteorological conditions are cyclonic and the accompanying weather dull, 

 humid, and rainy. The large result here arrived at empirically from correlated 

 observations of the two Ben Nevis Observatories is in accordance with the principle laid 

 down by Dalton in 1834 — viz., "that air charged with vapour, or vaporised air, is 

 specifically lighter than when without the vapour ; or, in other words, the more vapour 

 any given quantity of atmospheric air has in it, the less is its specific gravity." x 



The intimate relation thus disclosed between the varying temperatures and sea- 

 level pressures of a high-level and a low-level station is of prime importance in 

 forecasting weather, inasmuch as it reveals, in a way not hitherto attempted, the 

 varying conditions of the hygrometric states of the atmosphere, particularly at high- 

 levels, upon which changes of weather so largely depend. The setting in of a process 

 of saturation of the atmosphere at great heights may thus be made known, when even no 

 cloud has yet been formed to indicate any such saturation. The intimate and important 

 bearing of these results on such practical problems in meteorology as the forecasting of 

 the great seasonal changes of weather such as the Monsoons of India is very evident. 



In the course of this discussion it has been noted that the reduced hourly values 

 for the summit of the mountain often indicate that the transition from the anticyclonic 

 to the cyclonic type of weather, and vice versa, is slow, extending over several days, thus 

 prolonging the time for the framing of predictions of the more important weather 

 changes. 



The precursor and accompaniment of the heaviest, most widespread, and long- 

 continued rain is when the sea-level pressure for the summit is very greatly lower than 

 the sea-level pressure at Fort- William, thus indicating the saturation of the atmosphere 

 to a great height, while at Fort- William the point of saturation due to the advancing 

 cyclone has not yet taken place. But, on the other hand, when this point of saturation 

 has been reached, the sea-level pressure for the summit shows only a very small 

 difference from the sea-level pressure at Fort- William. 



It is remarkable that comparatively few observations, when the difference of the 

 mean daily temperatures at the two Observatories has exceeded 21° - 0, could be utilised 

 in this inquiry, since in most of such cases high winds prevailed resulting in ' pumping ' 

 of the barometer. These differences of daily mean temperature, rising for example 

 to 22°*2 on 18th May 1900, are, however, important in their connection with the pre- 

 diction of weather, inasmuch as they generally precede and accompany cyclones with 

 severe storms of wind and rain. They often result from a great lowering of tempera- 

 ture at the summit of the mountain, while at Fort- William no such lowering of 

 temperature occurs. 



There is, however, another distinct class of very strong winds which occur on the 

 top of Ben Nevis, of which the following show the cases which were noted in April and 

 June 1891 :— 



1 Dalton's Meteorological Observations and Essays, 2nd ed., Manchester, 1834, p. 100. 

 TRANS. ROY. SOC. EDIN. — VOL. XLIII. 3 U 



