634 MR MILNE ON A REMARKABLE OSCILLATION OF THE SEA. 



sible that such a disturbance in the atmosphere could produce such an effect on 

 the ocean ? 



In answer to this question, I would submit the two following considerations, 

 either of which would be sufficient to accomplish the effect observed. 



(1.) That wind is capable, by its mere mechanical pressure, to alter the level 

 of the ocean is well known. Mr LUKE HOWARD alludes to this, in his explana- 

 tion, above quoted, of what occurred at Portsmouth and Hull on the 4th March 

 1818. This effect is greatly increased when the wind, after blowing in one direc- 

 tion, suddenly shifts, and blows in an opposite direction ; an example of which 

 has already been given, under date 13th Februaiy 1756. 



Now, the storm of 5th July was of this character, and must have acted on 

 the surface of the ocean, so as to impel its waters first in one direction, and soon 

 after in an opposite direction, thereby causing an alternate flux and reflux on 

 the adjoining continents. 



It is true that the gale at Plymouth does not appear to have been so severe 

 as at Mountsbay, and especially in the midland counties. This may be explained 

 by supposing that the storm, whilst moving through the atmosphere, descended, 

 in some parts of its track, close to the earth's surface, and in other parts affected 

 only the upper regions. This seems to have been the case at Mackerston, where 

 a low scud from the NE. was generally prevailing, as the whirling currents 

 passed along in the upper regions. Before reaching Plymouth, the storm may 

 have been acting on the surface of the ocean with the same fury which it displayed 

 at Mountsbay, Sheffield, Liverpool, and Longtown, and thereafter risen, by re- 

 bounding, as it were, into the higher regions. 



By such gusts, then, it is quite possible to conceive how the sea, which was 

 affected by them, may have been thrown into a state of oscillation. That is to 

 say, the superficial waters may have been pushed forward by pressure, and which, 

 on reaching the shore and falling back from it, would produce an alternate flux 

 and reflux. 



(2.) The second consideration which Iwish to submit, is founded on the 

 well-known fact, that the level of the ocean rises with the fall of the barometer. 



Mr WALKER, the Queen's Harbour-Master at Plymouth, to whom I am in- 

 debted for an account of the oscillation observed there on the 5th July last, was, 

 I believe, the first person to ascertain, by a long series of observations, that 

 depressions of the barometer are generally accompanied by elevations of the 

 ocean's surface; and he has been able even to calculate the amount of such 

 elevation, with reference to the amount of barometrical depression. These obser- 

 vations of Mr WALKER, are thus referred to and explained by Sir HENRY DE LA 

 BECHE, in his Report on the Geology of Cornwall. 1 



1 Eeport on the Geology of Cornwall and Devon, p. 11. 



