Transportation Water and Air 277 



rivers and the entrances to harbors. In addition, most ships are 

 now equipped with radio apparatus so that if they are endangered 

 in any way they may call for help. There are also many modern 

 inventions that safeguard travelers in case of disaster. Sea travel 

 today is infinitely safer than ever before. 



Instruments Used in Navigation. In the navigation of 

 ships the compass, of course, plays an important part. The mod- 

 ern compass is very complicated in structure, in order to make 

 it reliable on any ship, and in order that it may always occupy a 

 horizontal position no matter how much the ship rolls. 



In sea travel, due to winds and tide and other factors, the 

 actual position of a ship out of sight of landmarks is often un- 

 known unless accurately determined by instruments. 



To locate the position of a ship at sea two instruments are com- 

 monly used. One is the chronometer, a clock which keeps accu- 

 rate time. A ship usually has two chronometers, one telling 

 actual Greenwich time, and the other telling the ship's local time, 

 determined at noon of each day as the sun reaches its high- 

 est point in the sky. By comparing the times of the two chro- 

 nometers the ship's longitude can be determined, as each hour 

 of difference in time indicates 15 difference in longitude from 

 Greenwich. 



Latitude is determined by a rather difficult process involving 

 an instrument, called a sextant, for making observations of the 

 sun which are then compared with certain tables in the standard 

 Nautical Almanac. The sextant can be used only when the sun 

 is visible. It measures the angle formed by lines running from 

 the position of the observer to the sun and to a point on the hori- 

 zon directly beneath. By means of the almanac table, this angle 

 is translated into terms of latitude. 



The speed of a ship is determined by a form of propeller 

 dragged after the ship on a cable. The propeller revolves as it is 

 pulled through the water. This apparatus is connected through 

 the cable to a dial on which it records its revolutions in terms of 

 miles. This instrument is known as the taffrail log. It can be 

 best understood if we think of it as acting much as does the 

 familiar speedometer of an automobile. 



