636 MR MILNE ON A REMARKABLE OSCILLATION OF THE SEA. 



bably the most correct, seeing that it agrees best with the Plymouth observations, 

 and that it corresponds nearly with the ratio of the relative densities of mercury 

 and sea water. 



If, therefore, the storm, as it approached Great Britain, was accompanied 

 with a diminution of atmospheric weight, to the extent of an inch and a half, it 

 would have the effect of raising the sea comprehended within its limits at least 

 2 feet. It might be more ; for, as Sir HENRY DE LA BECHE remarks, a sudden 

 impulse given to the particles of water, would cause them to rise beyond the 

 height strictly due to the amount of the atmospheric change. 



In order to prove that the view now suggested was really applicable to the 

 circumstances of the case, it would have been desirable to have shewn, that the 

 storm in question was accompanied by a considerable barometrical depression. 

 If, as appears probable, the storm was one of a whirling character, it may almost 

 be assumed, that, in its central parts, there must have been a considerable diminu- 

 tion of atmospheric pressure ; and, acccordingly, it appears that at Sheffield, over 

 or near which the centre of the storm passed, the barometer was seen to sink a 

 whole inch. Such would be the effect produced by the mere whirling of the 

 storm. But the centrifugal action which produced a depression of the barometer 

 at and near the central parts of the storm, would probably produce an opposite 

 effect near its limits. There, a condensation of the air would to some extent take 

 place. It may be reasonably expected, therefore, that, except at places over or 

 near which the very centre of the storm passed, the barometrical returns would 

 shew no great depression. 



From the circumstance here alluded to, it may be inferred, that the rise of 

 the sea under the central parts of a whirling storm, would be greatly aided by 

 the depression of the sea towards the limits of the storm, caused by the atmo- 

 sphere being there in a condensed state. But, independently of these effects, due 

 to the whirling character of the storm, it should be remembered that, during 

 sudden changes in the electrical state of the atmosphere, great changes take 

 place in its density. BECCARIA mentions, that he once saw the mercurial column 

 descend during a flash of lightning. 1 



Now, considering the extensive and extraordinary exhibition of electrical 

 phenomena which accompanied the storm in question, it is not unreasonable to 

 suppose, that, from this separate cause also, a sudden and considerable diminution 

 of atmospheric pressure accompanied the storm as it moved over the waters, thus 

 heightening the effects produced by the whirling character of the storm. This 

 separate cause is the more deserving of attention, seeing that, in so many of the 

 instances recorded of similar oceanic oscillations, thunder and lightning occurred 

 abundantly about the same time. 



By these combined causes, the pressure of the atmosphere on those parts of 



1 ROBERTSON on the Atmosphere, vol. i. p. 105. 



