•290 



It appears from an examination of tliis list that the period 

 of maximum frequency of unusually high pressures is in Ja- 

 nuary, and that of the minimum in July. This is precisely 

 what might have been expected, the former period being that 

 of the maximum range of the irregular oscillations, and the 

 latter that of the minimum. 



Of the barometric oscillations contained in this list there 

 are some which deserve particular notice. 



The oscillation of January, 1825, is (as has been already 

 remarked) the most considerable; and its features resemble, 

 in many respects, those of the wave of the present month. 

 The barometer began to rise Dec. 27, after which the mercury 

 executefl a series of rapid oscillatory movements. On Jan. 4 it 

 began to rise continuously, and attained the height of 30"93 

 on the morning of Jan. 9. The subsequent descent was gra- 

 dual and regular. The entire wave occupied a period of 

 twenty-two days in its passage. During the minor oscillations 

 at its commencement, the wind was exceedingly variable; it 

 settled in the NW., (January 1-4). From the 4th to the 

 6th, during the passage of a minor oscillation, it shifted from 

 NW. through E. to SE. ; and the movement continued in 

 the same direction from the 6th to the 9th, during the pas- 

 sage of the anterior slope of the great wave, when it com- 

 pleted an entire gyration. From the 9th to the 1 1th the wind 

 continued in the NW., and then retrograded through a qua- 

 drant to SW. during the passage of the posterior slope, it 

 was high at the commencement and end, and calm during 

 the passage of the crest, as in the wave of the present month. 



The oscillation of March, 1840, is the next in magnitude, 

 as respects the height attained ; but is much the most consi- 

 derable of any recorded in the duration of the oscillation, 

 which embraced a period of forty-five days (February 15- 

 April 1). Owing to this continuance of high pressure, the 

 mean pressure for the month of March, 1840, amounted to 

 30-383, the highest monthly mean of which the writer was 



