Matthew Fontaine Maury, the Pathfinder of the Seas

by Charles Lee Lewis

PILOT CHART OF THE NORTH ATLANTIC

(Chart from inside front cover)

PILOT CHART
OF THE
NORTH ATLANTIC
M. F. Maury. L.L.D. Lieut. U.S. Navy.
National Observatory, Washington
1853.
New Edition corrected 1857.
SHEET NO. 2.
SERIES C.

EXPLANATION.

The object of this chart is to show the relative number of times in every 5° square of the ocean that the wind blows from the several points of the compass for each month. As an example, take the square between 5° & 10° N. and 20° & 25° W. The figures in the N.E. corner of the circumscribed square show that in this square, there have been examined 294 records of the winds in Dec; 212 in Jan; & 161 in Feb; and the numbers 33, 18, 8, in the N.E. quadrant of the inner circle, show that it was calm 33 times in addition to the 294 winds in Dec; 13 times in Jan; and 8 times in Feb.

These are the winter months; and the number of times that the wind has been found to blow from the several points of the compass, in the winter months, is entered in the space between the two outer circles. The radii show the points of the compass; the figures between the radii show the number of times, for each month, the wind has been timed to prevail from such points for as much as eight hours together. Thus, between the two heavily drawn radii opening to the North, the winds from N. by W. to N. by E. are entered as North winds. Between the right hand line of these two and the broken radius to the right of that again, the winds between N. by E. & N.E. by N. are entered as winds from N.N.E. Between this broken radius and the next one on the right are the winds from N.E. and so on, with the sun, around the sixteen points of the compass. All the winds between N. by W. and N. by E. are called North winds; those between E. by N. and E. by S. are entered as from E. & so on.

Referring again to the same square, and to the two heavily drawn radii opening to the North, the numbers 20, 8, 16, between the two outer circles, mean, that 20 of the 294 winds in Dec, 8 of the 217 in Jan, and 16 of the 161 in Feb. were North. In Dec, the winds blew also 36 times from N.N.E; 37, N.E; 51, E.N.E; 60, E; 21, E.S.E; 17, S.E; 10, S.S.E; 6, S; and so on.

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.

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(Chart from inside back cover)

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