64 REPORT — 1903. 



sunshine strong, humidity high, night temperatures unusually high, and 

 dews heavy, with calms or light winds. On these days while at the top 

 temperature was very high and the air clear and very dry, at Fort William, 

 under a sky equally clear and temperature high, the air showed a large 

 humidity, and this state of moisture extended to a height of about 2,000 

 feet, or nearly halfway to the summit. Thus, then, while the barometer 

 at the top was under an atmosphere wholly anti-cyclonic, with its accom- 

 panying dry dense air, the barometer at Fort William was not so circum- 

 stanced. On the other hand, it was under the pressure of such dry dense 

 air, above the height of 2,000 feet only, whereas from this height down 

 to sea-level it was under the pressure of air whose humidity was large and 

 pressure therefore much reduced. The result was that the sea-level pres- 

 sure at Fort AVilliam was 0-050 inch lower than it would have been if the 

 dry dense air of the anti-cyclone had been continued down to Fort William. 

 This is confirmatory of what is to be expected, that the greater density of 

 dry air as shown in our laboratories prevails equally in the free atmosphere. 

 The first part of the discussion is virtually finished, the chief result of 

 which is this : — 1. When the difference of mean temperature of the day 

 is only r2°-0 or less, then the sea-level pressure calculated for the top of 

 the mountain is markedly greater than at Fort William ; 2. When the 

 difference of temperature is 18°-0 or greater, then the sea-level pressure 

 for the summit is markedly lower than at Fort William. In the former 

 case the meteorological conditions are anti-cyclonic, the weather being 

 then clear, dry, and practically rainless ; and in the latter case the condi- 

 tions are cyclonic, the accompanying weather being dull, humid, and rainy. 

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

 values from day to day often indicate that the transition from the anti- 

 cyclonic to the cyclonic type of weather, and vice versa, is slow, sometimes 

 extending over several days, thus prolonging the time for the prediction 

 of the more important weather changes. 



It may be remarked that the result here empirically arrived at is in 

 accordance with the principle laid down by Dal ton, 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.' 



The precursor and accompaniment of the heaviest and most wide- 

 spread rains is when the sea-level pressure for the summit is very greatly 

 lower than the sea-level pressure at Fort William. This indicates the 

 saturation of the atmosphere to a great height, while at Fort William, and, 

 say, 2,000 feet higher, the point of saturation due to the advancing cyclone 

 has not yet taken place. 



On the other hand, when this point of saturation has been reached, 

 then the sea-level pressure for the summit shows less difference from the 

 sea-level pressure at Fort William. The changes of pressure which occur 

 at the two Observatories as a cyclone advances and passes on are particu- 

 larly interesting and instructive. 



It is remarkable that comparatively few observations, when the differ- 

 ence of the temperature has exceeded 22°-0, could be utilised in this 

 inquiry, because in such cases high winds prevailed, resulting in ' pumping ' 

 of the barometer. These differences of temperature, rising even to 27°-0, 

 are, however, extremely valuable for weather prediction, inasmuch as they 

 often precede and accompany very severe storms of wind and rain. They 

 arise from an extraordinary lowering of the temperature at the summit, 



