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:
| Through the | N.E. | of the | N. Atlantic | 6¼ | knots per hour.[1] |
| ,, | S.E. | ,, | S. Atlantic | 8 | ,, |
| ,, | S.E. | ,, | S. Atlantic | 8 | ,, |
- ↑ That this correction is not too large is indicated not only by the experiments which Admiral Chabaunes, in command of the French fleet on the coast of Brazil, has kindly caused to be made by the brig " Zebra," but by experiments which he has subsequently made with the frigate "Alceste," as per the following extracts of a letter, just received from that distinguished
officer, dated—
"Frigate ' L'Alceste,' at Monte Video, May 10, 1860.
"Dear Sir,—I am replying to your letter of the 30th of January last, which I have received with the duplicates of that dated January 15th, 1851), which you have had the goodness to send me through M. de Montholon, as well as the interesting nautical monograph No. 1, and, finally, the manuscript passage from an article relative to the force of the winds to the height and velocity of clouds and waves. I regret exceedingly that I have not under my orders a squadron of sailing vessels with which I could put to sea, and devote myself to all these researches. Unfortunately, I have only steamers in my division, with the exception of the frigate which carries my flag, and political circumstances have retained me almost constantly in the La Plata. I am now approaching the end of my command, and in a few days must leave Monte Video to engage in some hydrographical works on the coast of Brazil, and afterwards I shall return to France, where I expect to arrive in the beginning of September. I shall not fail during this voyage to make a series of experiments such as you
