THE BRITISH ISLANDS IN NOVEMBER 1838. 4^3 



the fact. It is only on the supposition that there is a central axis of revolution, 

 which causes the bands of wind that are nearest to it to revolve more rapidly than 

 those which are more distant, that a diminished atmospheric pressure can there 

 be brought about. 



But there are other causes which may contribute to the observed effect. By 

 the rapid rotation of the central parts, and consequent centrifugal force produced 

 there, the air acquires a certain degree of attenuation, which must diminish the 

 pressure on the mercurial column. This view is strongly supported by the fact, 

 that the barometer has been observed to sink in the immediate vicinity of water- 

 spouts,* which, by their vertiginous motion, must necessarily attenuate the at- 

 mosphere in contact with them. 



The reasons now assigned or suggested for a fall of the barometer during 

 storms, are applicable to them generally, from whatever quarter of the globe they 

 proceed. But when a storm comes from the southward, it brings alongst with it 

 a warm temperature, which speedily diminishes the weight of the atmosphere. 

 The column of air, on being heated, expands and rises, flowing off laterally into 

 cooler parts, the effect of which is immediately to lessen the atmospheric pressure 

 on all places within the column, and to increase the pressure on places situated 

 beyond its verge. 



When the two storms described in this paper approached the British islands, 

 all the causes now noticed, probably combined to depress the barometer. Pre- 

 vious to their arrival in these latitudes, there had prevailed for some days a 

 strong north-easterly gale, which had caused the barometer to continue high. The 

 whirling columns of warm air, as they advanced northwards, had therefore to con- 

 tend against the cold wind blowing in an opposite direction. Now, it is obvious, 

 that, when these two winds met and mixed, the cold air would continue to oc- 

 cupy the surface, even in its retreat before the southern storms. The latter 

 would therefore affect the higher regions of the atmosphere in Europe, and es- 

 pecially in Britain, before the north-east wind was entirely arrested. The upper 

 part of the revolving column would precede and overhang the under part, which 

 would be farther impeded in its course by the surface of the sea or land over 

 which it traversed. From this cause, both storms must have affected the upper 

 regions of the British atmosphere, for some days before they began to sweep over 

 the British islands, and these upper regions being heated, would immediately 

 cause the barometer to sink. Accordingly it has been seen, that, before the arrival 

 of either of the storms above described, the barometer began to sink in all parts 

 of Great Britain. It has been shewn that the track of the storm's centre was 

 most probably about 200 miles to the west of Ireland. On its arrival there, the 

 atmosphere would, of course, attain a maximum temperature, and at that period, 



* Oersted on Water-spouts. His memoir is translated in the Edinburgh Philosophical Journal 

 for July 1 839. 



VOL. XIV. PART II. 4 1 



