Transportation on Water 377 



the vessel east or west of London? How many degrees? On 

 the trip to Japan from San Francisco, at what longitude does 

 the international date line occur? How many degrees west 

 of the prime meridian ? What is the hour by the chronometer 

 when the sun crosses its meridian? How can one adjust dates 

 without changing the chronometer? 



168. Determination of latitude. Daily observations of 

 the sun or other heavenly bodies are made on all ships at sea 

 to determine their location unless clouds prevent. The sextant 

 is used to make these observations accurate. The sextant 

 measures the angular distance of the sun above the horizon at 

 noon. Suppose that on March 21, at longitude 30 west, the 

 sun is found to be 70 above the horizon or 20 from the ob- 

 server's zenith. Obviously the ship is north or south of the 

 equator just 20, for on this day the sun is directly over the 

 equator. How can one determine whether the ship is north 

 or south of the equator? With the latitude and longitude 

 determined, it is a simple matter to find the exact location of 

 the ship by means of the chart. 



169. The log. The log resembles in certain respects a 

 cyclometer or speedometer. Its mechanism is operated by a 

 cord attached to a small propeller, which like a spoon hook is 

 made to whirl or spin by being dragged through the water. 

 The mechanism is connected with a hand that indicates upon 

 a small dial the rate or speed and the distance traversed. For 

 example, a log shows that a ship is moving at twenty knots. 

 (A knot means nautical miles per hour.) If this rate were 

 maintained for a day of twenty-four hours, it would be equiva- 

 lent to a distance of about 540 miles. Modern invention has 

 greatly improved the log of former times. The first kind of 

 log used was a trailer, from the speed of which the experienced 

 sailor could judge the rate of progress. To-day the log of some 

 of the great liners is electric and operated by the revolving 

 screw or attached to the propellers, every revolution of which 

 is known to mean a certain distance. 



