344 PHYSICAL GEOGRAPHY OF THE SEA, AND ITS METEOROLOGY. 



643. Difference in tons of (he harometric pressure upon the north-east 

 and south-east trade loinds. — With those barometric observations, 

 and the assumed fact that the mean pressure of the atmosphere is 

 15 lbs, upon the square inch, \se may readily determine in tons 

 the total by which the superincumbent pressure upcjn the south- 

 east trade-winds between the parallels of 5'^ and 20"" exceeds that 

 upon the north-east between corresponding parallels. For the 

 whole girdle of the earth, the excess of pressure upon the south- 



" That the atmospherical circulation is more active in the sonthcru than in 

 the northern hemisphere appears to be indicated also by the " brave west winds " 

 of the extra-tropical south.^ If the air performs its circuit more rapidly througli 

 one system of trade-winds than the other, then it follows that it must perform 

 its circuit more rapidly also along those regions through which it has to pass in 

 order to reach such rapid trades. Consequently, there should be a great differ- 

 ence between the gales of the northern and those of the southern hemisphere. 

 If we suppose the general circulation of the northern hemispliere to be sluggish, 

 the air in its circuits there would have time to tarry by the way, as it were, and 

 to blow gales of wind from all points of the compass. On the contraiy, if the 

 general circulation of the southern hemisphere be brisk and active, the air in its 

 general circuits, like a fast train on the railway, w^ould not have so much time to 

 tarry by the way, because, like the cars, it must he up to time. Hence, admit- 

 ting this view of the matter to be correct (and you perceive that for the want of 

 the experiments alluded to we are groping in the darkness of conjecture), though 

 we might expect gales of wind in the extra -tropical regions of the south, yet tliey 

 Avoiild for the most part blow loith the prevailing direction of the wdnd, and not 

 against it. Thus the gales on the polar side of Capricorn should, particularly at 

 sea, have westing in them always — almost. 



" In corroboration of this view, I may mention, on the authority of ajjaper just 

 received from Lieutenant Van Gogh, of the Dutch Navy, that the gales of wind 

 which take place between the meridians of 14° and 32° E., and between the 

 parallels of 33° and 37° S., have been discussed at the Meteorological Institute 

 of Utrecht. For this purpose he tabulated the results for the whole year of 

 17,810 observations — an observation comprehending a period of eight hours. 

 According to these observations, it is blowing a gale of wind oif the Cape of 

 Good Hope 7"16 per cent, of the whole year, and from the following quarters; 

 namely, between N.N.W. and S.S.W. 6.43 per cent. : from all other quarters, 

 0.73 per cent. 



" Perhaps you may find it convenient to institute, with some of the vessels of 

 your fleet, a regular series of experiments in the south-east trades upon speed, 

 when sailing at various angles with the course of the wind. Besides answering 

 our immediate purpose, the results might enable lis to convert ships into very 

 good anemometers for all winds excej)t gales. 



" Pardon me for being so tedious upon this subject. If vou have felt me so, 



1 S.e also PUtf Xir. and v) 63*> and ^ C3; 



