Page:Popular Science Monthly Volume 16.djvu/195

 either from the office directly, or from one of its agents in the principal commercial ports of the world. It is essentially like the log-book of the navy, and is for observations at sea only. When full, it is to be transmitted to Washington at the expense of the office. A number of sailing charts and all the latest hydrographic information are supplied gratis as an inducement to keep the journal. Hundreds of them are already afloat on ships of various nationalities, and are being filled with valuable data regarding every sea known to commercial enterprise.

Before proceeding to describe the method of compilation, I shall dwell for a moment on one of the items of record in the log-book, viz., the ascertainment of the ship's speed. Besides probably being of interest to many who yearly cross the sea in quest of either pleasure or health, a knowledge of this will tend to elucidate another matter of which I shall speak hereafter—the determination of whatever currents are drifting a ship, it may be, into serious danger.

The ship's speed is found by "heaving the log." The principle involved is the same as if one were to fasten the end of a tape-line, which is coiled on a spool, to a post, and then, holding the spool in his hand, he walked from the post at a uniform pace, allowing the line to easily roll off, but not become slack. If at the end of one minute he had walked 300 feet, in an hour he would have walked (at the same rate) 18,000 feet, or about three nautical miles.

Now, no stationary point exists in the ocean from which to measure, but this desideratum is attained by means of a thin flat board, sector shape, of eight inches' radius, and with the rounded edge loaded with lead to keep it upright in the water. Short lines connect the three corners of this "log-chip," as the sector is called, with the "log-line"—one of them by means of a wooden plug which is gently forced into a hole in a piece of wood fastened to the log-line about two feet from the chip. After well soaking and stretching, the log-line is marked as follows: A length of it about 100 feet from the chip is allowed for "stray-line," and then the length of a "knot" (for the sand-glass that runs for 28 seconds) is determined by this proportion. As the number of seconds in an hour is to the number of feet in a nautical mile, so is the length (in time) of the sand-glass to the length (in feet) of a knot; or 3600: 6086 $$=$$ 28: 47·33.

The limit of stray-line from the log-chip is marked by a piece of red bunting six inches long, and each length of 47·33 feet after that by a piece of fish-line with one, two, three, etc., knots in it, according to its number from the limit of stray-line. Each length of 47·33 feet (the "knot") is subdivided into five equal parts, and a small piece of white bunting two inches long is turned into the line at every two-tenth division thus formed.

"To heave the log" is performed thus: one person holds the sand-glass, another the reel on which the log-line just described is coiled.