S9S 



Second Report of the Meteorological Committee of 



[April 



At the meetings of the Institution of Wednesday September 3, and 

 October 1, Sir J. Herschel stated that he had examined the Meteorolo- 

 gical Journal kept at the Port-Office by Mr. M'Cleod, under the direc- 

 tion and superintendence of Captain Bance, during 58 months, com- 

 mencing with October 15, 1828, in which are registered the heights of 

 the barometer with the temperature of the instrument, for the hour of 

 D a. m.j noon, and 3 p.m., with the usual notices of wind and weather, 

 and that having reduced and interpolated them by graphical projection, 

 he had been led to the following conclusions : — 



1st, That the atmospheric pressure at Cape Town is subject 

 to a considerable and very regular annual fluctuation, amount- 

 ing (when reduced to a temperature of 32° Fahrenheit) to 0.287 in. 

 — the highest level being attained about the 16th of July, and the 

 lowest about the 16th of January, on an average of five years. 



2d, That the barometric pressure is also subject to a regular diurnal 

 fluctuation, whose average amount, on a mean of the whole year, may 

 be stated at 0.027 in.: the highest pressure taking place at or about 

 9 a. m., and the lowest (so far as can be gathered from observations 

 made only at the hours above named) at 3 p. m. 



3d, That this daily oscillation is : self subject to an annual alternate 

 increase and diminution — the limits being 0.0 198 in. and 0.0322— the 

 former, or lesser diurnal variation, corresponding to the middle of 

 January, and the latter or greater to the beginning of July. 



4th, That these fluctuations are maintained with such regularity, 

 that there is not a single month in the fifty-eight examined/m the mean 

 of which the daily oscillation does not appear; and that in the annual 

 oscillation (with exception of one remarkable anomaly, produced by 

 the tremendous storm of July 1831) not only does every year exhibit 

 the fluctuations in question, but its progress is marked by similar stages, 

 or phases of increase and diminution ; the most remarkable of which 

 is a temporary suspension of the regular rapid rise of the mercury 

 towards its maximum, usually taking place about the latter end of May 

 or beginning of June. 



5th, That, contrary to usually received notions, the rainy season at 

 the Cape corresponds to a generally elevated state of the barometer, 

 although it is true that particular storms of wind and rain are often 

 marked by a temporary depression. 



Sir J. Herschel further observed, that the amount of the annual 

 barometric variation at the Cape corresponds pretty nearly with the 

 amount of a depression of the mercury, which he stated to have been 

 observed by himself in his voyage hither, at and near the equator, 

 below lis habitual st^io in the extra-tropical regions — a depression then 



