the South African Literary and Scientific Institution, i^ 



their different extent and relation to the continents adjacent to 

 them. 



If, in a report like this, it be allowed to speculate on the 

 causes of meteorological phenomena, it appears extremely pro- 

 bable that the equatorial depression in question arises from the 

 same cause which produces the trade winds, viz. the rarefaction 

 and consequent ascent of the equatorial air, which, although 

 constantly supplied from the extra-tropical latitudes, is yet not 

 supplied instanter^ nor without a due dynamical motive force, 

 which, in a free elastic fluid, can be no other than an excess of 

 pressure on the side from which the supply is drawn, or (which 

 comes to the same thing) a diminution of it, in the nature of a 

 " suction," on that side towards which the superficial currents 

 rush ; which excess and diminution obviously arise from the 

 overflow of the unsustained portion of atmosphere above the 

 equatorial zone into the regions beyond. The inquiry, there- 

 fore, connecting itself as it does, with all the greater phenomena 

 of meteorology, assumes a high degree of interest, and will no 

 doubt be studied with the perseverance and exactness it merits. 



A series of observations of the heights and times of high and 

 low water at Simon's Bay, extending from January 26. to 

 June 30, has been obligingly submitted to the consideration of 

 the Meteorological Committee, by J. Deas Thomson, Esq. and 

 the Astronomer-Royal. It has not yet been possible to com- 

 pare them with any theory, and indeed it would be premature 

 to attempt it here, as they will require to be combined with the 

 mass of knowledge now accumulating on this subject in Europe, 

 to render them in any degree available. One remarkable result, 

 however, may be mentioned here, which offers itself on a very 

 cursory inspection of the heights, as compared with the declina- 

 tions of the Sun and Moon, viz. that while the monthly fluctu- 

 ation of the mean sea-level, arising from the moon's alternate 

 occupation of the northern and southern hemisphere, is scarcely 

 perceptible, amounting hardly to two inches, its annual variation, 

 due to the similar approach of the sun to the northern and 

 southern solstice, is much more considerable, and forms indeed 

 a prominent feature in the Tides of this coast, amounting to no 

 less than eight inches, or nearly a fifth of the average difference 

 between high and low water — as the following brief Table will 



