330 PHYSICAL GEOGRAPHY OF THE SEA, AND ITS METEOROLOGY. 



643. With these barometric observations, and the assumed fact 

 DiEFerence in tons of that the mean prcssuie of the atmosphere is 15 lbs. 



the barometric pres- ,-, • -, i-i t i • • 



sure upon the north- upon the squaro mch, we may readily determme in 

 Se'^"itii?r^''"''^' toils the total by which the superincumbent pres- 

 sure 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-east trade-winds is 1,235,250 millions of tons. 



" In order to treat this question thoroughly, it is very desirable to know the dif- 

 ference in the speed of vessels when sailing with the same wind aft, with it 

 quartering, with it a point or two abaft the beam, and with it close hauled. With 

 a good series of experiments upon this subject, we should be able to arrive at defi- 

 nite conclusions with regard to the average difference in force not only of the two 

 systems of trade- winds, but of the winds generally in various parts of the ocean. 



" If we assume that a wind wliich, being dead aft, drives a vessel at the rate of 

 six knots, will, when brought nearly abeam, drive her eight knots— as in this 

 chapter I have supposed — and then if we apply the dynamical law of the resist- 

 ance increasing as the squares of the velocity of the ship, we should be led to the 

 remarkable conclusion that the average velocity of the north-east to south-east 

 trades of the Atlantic is as 36 to 64. Tlierefore, in conducting tliese experiments, 

 it would be very desirable to know the area of canvas that fairly feels the wind 

 wlien it is aft, and the area upon which the wind blows when the sliip is hauled 

 up. Suffice it to say, that the facts which we abeady have indicate that the 

 south-east trades, both of tiie Atlantic and Indian Oceans, are fresher than the 

 north-east trades of the Atlantic. May we infer from this that the south-east 

 trades of the Pacific are also fresher than the north-east trades of that ocean ? If 

 we may so infer, and be right, then there is another step which we may take 

 with boldness, and pronomice the atmospherical circulation of the southern hemi- 

 sphere to be much more active than that of the northern. And having reached 

 this round in the ladder up which I am solicitnig you to accompany me, we are 

 prepared to pause and take a view of some of the new physical aspects which these 

 facts and this reasoning spread out before us. 



•' Tliat the atmosplierical circulation is more active in the southern than in the 

 northern ]>emisphere appears to be indicated also by the " brave west winds " of tlie 

 extra-tropical south.i If the air performs its circuit more rapidly through one 

 Bj^stem 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 tlie northern and those of the southern hemisphere. If we suppose the 

 general circulation of the northern hemispliereto 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 tiie experiments aUuded to we are 

 groping in the darkness of conjecture), though we mighi expect gales of wind in 

 the extra -tropical regions of the south, yet they would for the most part, blow ivith 



^ See also Plate XIII. and § 632 and § 633. 



