ofihe South. African Institution. 38§ 



ibe looked upon as unequivocally established, and it is hoped 

 that it will henceforth attract the attention of all voyagers, and 

 that observations will be diligently accumulated for the purpose 

 of ascertaining the law of variation of atmospheric pressure in 

 all latitudes both within and beyond the tropics, and in cither 

 hemispliere, since it is very possible that the same exact law may 

 not be found to apply to both, and that the Atlantic, Indian, 

 and Pacific Oceans may offer differences depending on 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 (he 

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

 and consequent ascent of the equatorial air, which although 

 constantly supplied from the extratropical latitudes, is yet not 

 supplied ivstanter, nor without a due dynamical motive force, 

 which, in a free elastic fluid can be no otter 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, therefore, 

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

 raetcorologj'^ assumes a high degree of interest, and will no 

 doubt be studied with the perseverance and exactness it merits. 



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

 lo^v water, at Simon's Bay, extending from January 26 to 

 .Jt/ne 30, has been obligingly submitted to the consideration ot 

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

 tbe Astronomer Royal. It has not yet been possible to compare 

 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 IMoon, viz. that while the monthly fluctu- 

 ation of the mean Sea-level, arising from the Moow's alternate 

 occupation of the northern and southern hemisphere, is scarcely 

 perceptible, amounting hardly to 2 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 8 inches, or nearly a fifth of the average difference 

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

 show — in which the interval embraced by the obscr\ ations is 



3 c 



