643. Difference in tons of the barometric pressure upon the north-east and south-east trade winds.—With those barometric observations, and the assumed fact that the mean pressure of the atmosphere is 15 lbs, upon the square inch, we may readily determine in tons the total by which the superincumbent pressure upon 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 southern than in the northern hemisphere appears to be indicated also by the "brave west winds" of the extra-tropical south.[fn 1] If the air performs its circuit more rapidly through 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 difference between the gales of the northern and those of the southern hemisphere. If we suppose the general circulation of the northern hemisphere 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 contrary, 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, would not have so much time to tarry by the way, because, like the cars, it must be up to time. Hence, admitting 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 they would for the most part blow with the prevailing direction of the wind, 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 paper 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 off 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 us to convert ships into very good anemometers for all winds except gales.
"Pardon me for being so tedious upon this subject. If you have felt me so,
- ↑ See also Plat XII. and §632 and §633
