Page:Popular Science Monthly Volume 16.djvu/200

 weather that can occur in an hour, may be succinctly and accurately described:

We now arrive at the method of compilation, and for this several blank forms are used which will be described as the necessity arises to speak of each kind.

First, however, every log-book and journal is closely examined for any errors it may contain, and, if such exist, they are scored with a red pencil, and hence do not enter into the compilation or charts.

Whatever judgment of the log-book this scrutiny warrants, is entered in it for guidance of the compiler.

The first blank to be used is Form A—a sheet 36 inches by 20. It is a projection according to Mercator of that part of the sphere between the 70th parallels of latitude north and south. Beginning with the equator and the meridian of Greenwich, parallels and meridians are drawn at every fifth degree, thus dividing the ocean, like a chess-board, into small squares. These are numbered consecutively from 1 to 1,667, so that, either by its number or its limits in latitude and longitude, the expanse of ocean covered by any square can be designated.

When a ship makes a passage under sail, she will cross certain of these squares on devious courses, and be in each square a short or a long period according as the winds and weather favor or oppose her. The particular square in each day can be ascertained by referring the noon position to Form A; and the hour of entering and of leaving it can be found by working the ship's run, by means of the courses and distances, backward or forward as necessary, from the noon position. Then, drawing heavy lines across the left-hand or meteorological page of the log-book at the hours of entry and of exit, it