the South African Literary and Scientific Institution. S45 



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

 the lowest about the 16th of January, on an average of ^vq 

 years. f' 



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 itself subject to an annual 

 alternate increase and diminution — the limits being 0.0198 in. 

 and 0.0322 — the former, or lesser diurnal variation, correspond- 

 ing to the middle of January, and the latter or greater to the 

 beginning of July. 



4th, That these fluctuations are maintained with such regu- 

 larity, that there is not a single month in the fifty-eight exa- 

 mined, in the mean of which the daily oscillation does not ap- 

 pear ; 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 ques- 

 tion, 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 winc^ 

 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 its habitual state in the extra- 

 tropical regions — a depression then noticed, as he at that time 

 supposed for the first time, but which it appears had also been 

 (very recently) noticed and made the subject of inquiry and 

 numerical computation by Professor Schow of Copenhagen, in 

 a paper published in the Annales de Chimie for June 1833, 



