OCEAN METEOROLOGY. i8i 



either from the office directly, or from one of its agents in the princi- 

 pal commercial ports of the world. It is essentially like the log-book 

 of the navy, and is for observations at sea only. AVhen 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 sup- 

 plied 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 enter- 

 prise. 



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 in- 

 terest 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 in- 

 volved 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 mea- 

 sure, 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 : 0086=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, 



