Page:Popular Science Monthly Volume 16.djvu/203

 could be obtained relative to the expanse of ocean under consideration, the next step is to classify these, and to reduce the observations. The classification consists in grouping together all the sheets of the same square for the same month, without distinction as to year or ship; and, to reduce, all the observations on the blanks of each group are first transcribed to a single sheet of another blank. Form D—a kind of ledger, as it were, to which the separate sheets of Form B bear the relation of so many entries in a day-book.

It may not be entirely devoid of interest to some of my readers, to have here such a description of Form D as will enable them to reproduce it: a fac-simile before one would render the details of the reduction more easily intelligible.

At the top of the form are spaces for entering the number of the square and the name of the month. Under this are six vertical columns crossed by thirty-four horizontal lines. The headings of the columns in succession, from the observer's left to right, are as follows: 1st column, "Total No. of hours of wind from every alternate point"; 2d, "No. of hours of wind from every point"; 3d, "True direction of wind" (under this heading the points of the compass, beginning with north, are printed—one point on each line—down the first thirty-two lines of the blank, and "calms" and "variable winds" are on the last two lines ); 4th, "Mean force of wind from every point"; 5th, "Final mean force of wind from every alternate point"; and, 6th, "Percentage of wind from every alternate point."

To illustrate the use of Form D, suppose that for any square for any month—say No, 643 for July—there are fifty sheets of Form B. To collect the numerous observations of the wind scattered throughout these, each point is considered separately and in succession. The sum of the different periods that the wind was of the same force from the same point on all the sheets of Form B is found; the direction of the wind on this Form being magnetic, it is corrected for the variation of the compass, and then the sum is entered on Form D opposite that point which it becomes as a true direction.

Take a specific case, and let it be the magnetic north of Form B, with the variation one point westerly for the square under reduction: suppose that all the hours the wind was a force of 5 amounted to 80; then "80" would be entered in the 2d column, and "5" in the 4th column of Form D, on the horizontal line on which "N. W." is printed; because a magnetic north wind becomes a true north-by-west wind when the variation is applied. Again, if all the hours the wind was of another force, say 3, from the same point (magnetic north) amounted to 120, then "120" would be placed in the 2d column, on the right of the previous sum 80, and "3" in the 4th column, on the right of the other force, 5, Similarly, with all the hours of each force, and with every point of the compass, the order being preserved throughout of having the first sum of hours in the 2d column