Page:Physical Geography of the Sea and its Meteorology.djvu/460

434 average strength where the fleet have free winds. What is the difference in the strength of such winds, which impinging upon the sails, each at the particular angle indicated above, imparts the aforesaid velocities? Moderate winds, such as these are, give a ship her highest speed generally when they are just abaft the beam, as they are for a north-west course through the north-east trades of the North Atlantic. So, to treat these ships as anemometers that will really enable us to measure the comparative strength of the winds, we should reduce the average knots per hour to the average speed of a mean ship sailing through average "trades" in each ocean, with the wind impinging upon her sails at the same angle for all three, as, for example, just abaft the beam, as in the North Atlantic.

815. Velocity of the trade-winds.—Let us apply to the average speed through the South Atlantic and Indian Oceans such a correction. Through the former the wind is aft; through the latter, quartering. If we allow two knots as a correction for the one and one as a correction for the other, we shall not be greatly out. Applying such corrections, we may state the speed of a mean ship sailing with average trades just abaft the beam to be as follows: