i82 THE POPULAR SCIENCE MONTHLY. 



and a third throws the log-chip with the line attached over the ship's 

 stern ; the chip, floating upright, is kept stationary by the resistance 

 of the water, while the vessel moves on, and the line runs out ; the 

 midshipman watches it until the limit of stray-line just passes the rail, 

 when he sharply says " Turn " ; the glass is quickly reversed, the sand 

 begins to run into the lower compartment, and both time and space 

 are reckoned from the word turn. The " knots " reel off slowly or 

 rapidly according to the ship's velocity, and, when the last grain of 

 sand runs out, the line is instantly stopped. The number of knots 

 and tenths run out denotes the speed at the moment of making the 

 experiment, and, according to the conditions of wind, sea, and sail for 

 the whole hour, the speed is deduced for the hour, and so entered in 

 the columns. 



To draw in the line, a quick, strong jerk on it frees the plug, when 

 the chip floats horizontally, and can be hauled aboard with little re- 

 sistance. 



The " course steered," which is always a coordinate entry with the 

 velocity, is obtained from a standard compass, whose every error is 

 found and tabulated, to be applied when necessary. As the course 

 and velocity are entered every hour in the log-book, we have thus a 

 continuous record of each direction in which the ship headed, together 

 with the distance she proceeded in that direction. 



The courses and distances are the data by which, with the aid of a 

 traverse-table, the ship's position may be found at any time — the 

 position by " dead reckoning," or " account," as it is called. Inde- 

 pendently of this, the position — " by observation " — is daily found by 

 the navigator by altitudes of the sun, the moon, or the stars. 



Suppose a ship to leave New York at noon of any day, and that 

 her " run " is accurately kept until noon of the next day, when the 

 latitude and longitude by account are found. The ship may not really 

 be in this position : currents may have borne her along or athwart her 

 course, yet we can not discover them ; they act on log-chip and vessel 

 alike : but let the position " hy observation " be determined for the 

 same instant that it is " by account,^'' and we have at once a standard 

 of comparison whereby the treacherous streams are made known. If 

 none exist, the position by the two methods should agree within the 

 small limit of error due to the unavoidable imperfection of both ob- 

 servers and instruments. 



A third mode of ascertaining the ship's run is by the patent log — 

 an instrument constantly towed astern at the end of a long line. It 

 has a small propeller which the motion through the water causes to 

 revolve. This revolution is communicated to a series of cogged wheels 

 connected with hands that point to a circular scale — an arrangement 

 not imlike a gas-meter. Every noon the log is hauled aboard, read, 

 reset, and then thrown overboard, to record again the number of miles 

 by which the ship nears her port. Being entirely independent of both 



