Page:Popular Science Monthly Volume 16.djvu/201

 is evident that all the observations between these lines were taken in that square.

The limits of all the squares traversed are determined and marked in like manner, the number of each square is written between its bounding lines, and, when thus wholly prepared, the observations taken in each square during whatever length of time the ship was in it—whether one hour or several, provided it was continuous time in the same month—are compiled on one blank of Form B. This is a sheet of thick, durable paper, so ruled into columns with appropriate printed headings that each of the following-named items has a place for entry: the limits in latitude and longitude of the square, its number, and the number of hours the ship was in it; date of passing through the square, name of the ship, and the period covered by her log-book—all, that direct reference may be had at any time to the original sources. In a series of columns the thirty-two points of the compass are printed in regular order; blank spaces are provided on the right and left of each point—those on the left for the number expressing the wind's duration from that point, and those on the right for the figure denoting its mean force for the period of that duration. To compile the direction of the wind, the number of hours it was from the same point, whether consecutively or at different intervals, are counted, and the sum total is entered on the left opposite the point; for the force, the mean of the several hourly forces corresponding to the period of direction just mentioned is found and entered on the right of the point. Similarly for each point from which the wind blew while the ship was in the square.

As the entries regarding the wind in the log-book are hourly estimates of both its direction and force, for the hour—an interval that permits little variability in either quantity—accuracy to this degree is insured in both the compilation and charts. There is no averaging the force or direction for longer periods than an hour, and, as far as I am aware, this is the only system pursued by any nation wherein these items are compiled with such detail.

When calms or light, variable airs occur, the number of hours of each is counted, and the sum entered in its proper place. When a gale—that is, wind of a force of 8 and upward—happens, the number of hours it blew from any of the eight principal points of the compass (supposing it to have varied in direction), together with the force during each period, are duly tabulated.

The total number of hours of fog, of rain (including snow and mist), and of squalls (heavy, moderate, and light); the state of the weather by symbols; quantity of clear sky; and variation of the magnetic needle, with the location in which it was observed, are all entered in their respective places.

The mean of the mercurial barometer, attached thermometer, dry-bulb, wet-bulb, and temperature of the sea-water at the surface—that