96 NAVAL SCIENCES. 



ocean being ruffled by the slightest wind ; the ship, with all its sails 

 unfurled, seefns to rest upon the waves (/?/. 26, fig. 4) ; when often, as if by 

 a freak of Neptune, a stormy wind springs up from the black clouds which 

 rise from the sea, and the masts are broken, the sails are shivered, and the 

 rigging is torn in pieces. 



The mariner who wishes to navigate the ocean must be acquainted with 

 all its peculiar features. For this purpose he makes use of charts, which 

 point out the reefs which he must avoid and the course which he is to 

 follow. When he is once at sea, the compass is his only guide. This alone 

 can tell him the direction which he is to keep, when nothing but sky and 

 water are before him. It is well known that the point of the magnetic 

 needle always turns to the north, whatever be the direction of the ship, and 

 consequently enables the mariner to ascertain its true position on the ocean. 

 For this purpose, a thin plate of isinglass is cemented under the needle, 

 turning with it about its centre. This circular plate, like all circles, is 

 divided into 360 degrees. If, then, the deviation of the line of direction 

 necessary for the voyage from the meridian line is measured by means 

 of a circle divided in the same manner on the chart, the ship can easily 

 be so turned, that its line of direction shall deviate the same number of 

 degrees from the meridian, and it will thus reach its object without any 

 other guide. In order to facilitate the observation of the compass, the 

 circle is again divided into thirty-two parts, called points of the compass. 

 These thirty-two points are named as follows. The four cardinal points 

 are called north, east, south, and west. Between these are four others, 

 north-east, south-east, &c. Between these eight points are eight others, 

 north-north-east, east-north-east, &c., and between these sixteen are six- 

 teen more, north by west, east by south, &c., completing the full number 

 thirty-two. These last are again subdivided into fourths, for greater pre- 

 cision of steering, and designated thus : North ^ east, north-west by north 

 ^ north, and so on. The mariner has also his hour-book, giving the true 

 position of the stars for every hour, the artificial horizon, and the sextant, 

 which enable him, according to the angle which the vessel makes with the 

 stars and the horizon, to ascertain her place when he can get a good obser- 

 vation of the sun or of a star, as he can thus ascertain the true time of the 

 spot where he is and calculate his position by the difference of time from 

 his home or from Greenwich. For this purpose he has the most accurate 

 watches and chronometers, some of the latter keeping time with so much 

 precision that they do not lose a minute in a voyage round the world. 

 When he can get no astronomical observation on account of cloudy weather, 

 he calculates his position by the speed of his ship, measured hourly by the 

 log, and by the mean direction in which he has steered. This is called 

 dead 'reckoning, and is necessarily less accurate, as the influence of currents 

 and of leeway can only be conjectured. 



2. Practical Navigation. Practical Navigation, which we are now to 



consider, teaches the use of the sails and rudder, on every occasion, so as to 



produce the suitable motion, speed, and direction of the ship, in order to reach 



the end of her voyage. For this purpose we first have recourse to the rudder. 



748 



