Page:Physical Geography of the Sea and its Meteorology.djvu/458

432 the fleets that are out reconnoitering the seas for us, returns with additional facts for our storehouse of knowledge. Whether 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. The S.E. and N.E. trade-winds put 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 (§ 643) 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 independent evidence from every vessel in a fleet numbering no less than 2235 sail to show that the S.E. are stronger than the N.E. trades. All of these vessels passed through 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 by each fleet is different; consequently, these sailing anemometers are at different 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 6¼ knots an hour one way (N.W. ¼ W.), with the wind just abaft the beam, and 5¾ of the other (S.S.E.), with the wind at a point not so favourable for speed. Indeed, most of the ships which average a S.S.E. course through this part of the northeast trade-wind belt are close hauled; therefore the average strength of the trades here cannot be fairly compared with the