Proceeding towards the centre, the next inter-circular space contains the winds, according to their direction, for the three spring months. The figures in the S.E. corner of the circumscribed square show the total number of winds recorded for each month, as 159 for March; 227 for April, and 301 for May, exclusive of 4, 5, 12, calms in the S.E. quadrant for the same months, and so on, around with the sun, for calms and number of winds, in the order of the months; beginning with the first number in the N.E. corner and quadrant, as the total winds and calms for December, the 2ⁿᵈ numbers for January and so on.
The third space between the circles contains the winds for the three summer months in the same order, June, July and August, between the radii. The space between the two inner circles is for Autumn; the outer round of figures being the winds for Sept, and the inner for Nov. Diagram A is referred to for further explanation. In it the months are written instead of the number of winds and calms for each month and point of compass.
The method, in which the number and direction of the winds have been ascertained, is this: As many logs as could be obtained have been examined; the 24 hours have been divided into three parts of eight hours each; and, according to the square in which the vessel was, the prevailing direction of the wind for every eight hours has been entered as one wind.
Thus, in the square between 5° & 10° N. and 15° & 20° W. we see, in the S.W. corner 916 winds, and in the S.W. quadrant, 15 calms. These are the total number of intervals of eight hours each for which the winds and calms for August have been examined in this 5° square of the ocean. Consequently, all the vessels passing through this square in the month of August were in it, 310⅓ days. Compare the totals for August in the adjacent squares, with this 916 and reasons will appear for the conclusion that the winds here in August, are particularly light and battling, and that the vessels were detained so long in this square on that account.
The blank spaces, or the spaces that are filled up in some of the squares and circles and not in others, mean that no winds have been reported in the logs from these points of the compass, or that no calms prevailing for eight hours together, have been reported during those months.
Diagram B is for the convenience of the navigator. Make a fac simile and cut out the black part. Now to ascertain the chances for head and fair winds in any part of the ocean, lay this card over the circles in the square in which the vessel may be, with the white pointer mid-way between the two radii that represent the course to be sailed. The winds that can be counted for the month, in the segment that has been cut from the card, will show the chances for head winds; and this number, subtracted from the total for the month, will show the chances for fair winds. Calms speak for themselves.
Navigators using this chart, either to lay off their best rout for the month, or to decide upon which tack to go, when the winds come out ahead, will know what difference to make between the chances for winds that will enable them to lay within 4 points of their course, and the chances for winds that will enable them to lay within two points of their course. In sailing 10 miles, a ship, within six points of her course, makes but 3.8 good. Within 4 points and 10 miles, she makes 7.1 good, and within 2 points, she makes good 9.2 miles out of ten.
M. F. Maury. Lt. U.S.A.
National Observatory,
Washington, May 1858.