Observations on the Storm of Dec. 15, 1839. 115 



lieved to contribute, largely to the clouds and rain which usually 

 accompany a storm or gale ; and is probably due, in part, to the 

 excess of external atmospheric pressure on the outward portions 

 of the revolving storm. 



8. In storms which are greatly expanded there is sometimes 

 found an extensive area of winds of little force and variable di- 

 rection, lying within the circuit of the true gale, and attended 

 throughout with a depressed state of the barometer. This more 

 quiescent portion of air in the centre of a gale has been found to 

 extend, in some cases, to a diameter of se^/eral hundred miles. 



In the case now before us, the direction of the arrows repre- 

 senting the course of the wind at noon, as carefully drawn on a 

 larger map, shows an average convergence, or inward inclination, 

 of about six degrees. But it is not deemed safe to rely upon this 

 result in a single case, which is liable to be affected by the errors 

 of observation and the deflecting influences of the great valleys 

 and lines of elevation, as well as by the errors of approximation 

 which often arise from referring all winds to eight, or, at most, to 

 sixteen points of the compass. 



It is not intended, on this occasion, to support the foregoing 

 characteristics by such extended details of evidence as their dis- 

 cussion would necessarily demand ; and they are mentioned here 

 only because the true character of the rotation in these gales, as 

 well as the necessary or incidental connexion of this rotation 

 with other phenomena which attend them, has seemed to be of- 

 ten misapprehended. 



As relates to the whirling or rotary action in the case before 

 US, it may be remarked, that had we obtained no observations 

 from the northwestern side of the axis of this gale, it would 

 have been easy, in the absence of more strictly consecutive ob- 

 servations than are usually attainable, to have viewed the initial 

 southeasterly wind of the gale,* and the strong northwesterly 

 wind which soon followed, as two distinct sheets or currents of 

 wind, blowing in strictly opposing directions ; and if we could 

 so far lose sight of the conservation of spaces and areas, the laws 

 of momentum and gravitation, together with a continually de- 

 pressed barometer within the storm, we might then have suppos- 

 ed one of these great winds, if not both, to have been turned 



* Observed between the coast of Massachusetts and latitude 25° N. 



