_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._ [Signature: _M. F. Maury. Lt. U.S.A._] _National Observatory, Washington, May 1858._ [Illustration: Chart from inside back cover] PILOT CHART OF THE NORTH ATLANTIC OCEAN The following is a summary of information contained on this chart, and in order that certain items may be more readily identified, they have been numbered and a duplicate number inclosed in a circle placed on the chart as close to the item or feature as practicable: FRONT