432 niYSICAL GEOGRAPHY OF THE SEA, AND ITS METEOROLOGY. 



the fleets that are out reconnoitring the seas for us, returns with 

 additional facts for our storehouse of knowledge. A\'hether these 

 tend to confirm or disprove the theory a restatement is often 

 called for; hence the repetition, of which the case before us is an 

 example. 



812. Tlie S.E. and N.E. trade-winds jmt in a balance.'- — The facts 

 stated in Chap. XV. go to show that the south-east trade-winds 

 are stronger than the north-east. The barometer tells us (§ C43) 

 that between the parallels of 5° and 20° the south-east trade- 

 winds bear a superincumbent pressure upon the square foot of 

 nearly 4 pounds greater than that to which the north-cast trades 

 are subjected. Such an excess of superincumbent pressure upon 

 a fluid so elastic and subtle as air, ought to force the south-east 

 trade- winds from under it more rapidly than the lighter pressure 

 forces the north-east. Observations showing that such is or 

 is not the case should not be ignored. 



813. Observations by 2235 vessels. — I have the separate and 

 indejoendent evidence from every vessel in a fleet numbering no 

 less than 2235 sail to showr that the S.E. are stronger than the 

 N.E. trades. All of these vessels passed tlirough both systems of 

 trade-winds. The knots run -per hour by each one of them, 

 as they passed through the south-east trades of the Indian Ocean 

 and through both systems of the Atlantic, have been measured 

 and discussed from crossing to crossing. The average result in 

 knots is expressed in the annexed table, P. 433. The comparison . 

 is confined to the rate of sailing between the parallels of 10° and 

 25°, because this is the belt of steadiest trades. 



814. Ships used as anemometers. — It is well to observe, that on 

 each of these three oceans, though the direction of the wind is 

 the same, the course steered b}^ each fleet is different ; conse- 

 quently, these sailing anemometers are at difi'erent angles with the 

 wind ; through the south-east trades, the wind is nearly aft in 

 the Atlantic, and quartering in the Indian Ocean, giving an 

 average sailing speed of 7 knots an hour in the latter, and of 6 

 in the former ; while through the north-east trades the average 

 speed is 6J knots an hour one way (N.W. J W.), with the wind 

 just abaft the beam, and of the other (S.S.E.), with the wind at 

 a point not so favourable for speed. Indeed, most of the shijDS 

 which average a S.S.E. course through this part of the north- 

 east trade-wind belt are close haided ; therefore the average 

 strength of the trades here cannot be fairly compared with the 



