OBSERVATIONS ON THE STORM OF DECEMBER 15, 1839. 79 



irregularities, is less noticeable in the storms which are traced solely on the 

 ocean. 



4. The barometric indications of a gale commonly extend much beyond the 

 observed limits of its action. 



5. The body of the gale constitutes a determinate sheet or stratum of moving 

 air; and of this sheet or stratum a large portion sometimes overlies another and 

 more quiescent stratum of air, the latter having, perhaps, a different motion; as 

 in common winds of the temperate and higher latitudes : in which case the gale 

 is either not felt at the surface of the earth, or the observed changes of wind 

 are found, in part, unconformable to the whirlwind theory. 



6. Owing to the convergent and somewhat variable courses of storms in the 

 extra tropical latitudes, as well as to their unequal rates of progress, two storms 

 will sometimes cover, in part, the same field, one of which will overlie the 

 other, and, perhaps, thin out at its margin, in the same manner as common 

 winds. This, also, may occasion a different order of change in the observed 

 winds and weather from that which is more commonly noticed in a regular 

 whirlwind storm. 



Owing to such causes, the oscillations of the barometer are often irregular; 

 and this is particularly noticeable in the higher latitudes. 



7. In most gales of wind there is, probably, a subordinate motion, inclining 

 gradually downward and inward in the circumjacent air, and in the lower por- 

 tions of the gale; and a like degree of motion, spirally upward and outward, in 

 the central and higher portions of the storm. This slight vorticular movement 

 is believed 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 atmosphe- 

 ric pressure on the outward portions of the revolving storm. 



8. In storms which are greatly expanded there is sometimes found an exten- 

 sive area of winds of little force and variable direction, lying within the circuit 

 of the true gale, and attended throughout, with a depressed state of the baro- 

 meter. This more quiescent portion of air in the centre of a gale has been 

 found to extend, in some cases, to a diameter of several hundred miles. 



In the case now before us, the direction of the arrows representing the course 

 of the wind at noon, as carefully drawn on a larger map, shows an average con- 

 vergence, 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 



