16 A TEDIOUS BRIEF EXPLANATION 



to make a safe approach to land, but he also 

 requires to be informed of any local peculiarities of 

 current and tide ; and, moreover, if he be new to 

 a country, he wants to know something of what its 

 harbours and roadsteads promise him in the way 

 of shelter and supplies. All this information it is 

 the business of survey-ships to furnish him with, up 

 to date. 



The next question is. How does a survey-ship set 

 about obtaining this information? For answer, let us 

 suppose that we are dealing with an island about 

 which little is known except its geographical position, 

 which we will take for granted. Then the first thing 

 to be done is to examine its topography — to sketch 

 or '*plot" its coast-line, and to fix, for beacons, the 

 position of all conspicuous stationary objects ashore. 

 The next thing is systematically to sound out the 

 surrounding sea, in order to determine its bottom- 

 contours, and so to discover any hidden dangers in 

 the way of rocks and shoals. This is done by 

 running line upon line of close soundings, at regular 

 intervals, from near shore to the open depths. Every 

 time a sounding is made the position of the ship is 

 fixed, generally by taking angles between certain 

 stationary natural objects, or certain marks that have 

 been set up ashore ; or, when this cannot be done, then 

 in the ordinary way, by chronometer and observation 

 of the sun, or even by dead reckoning ; but the usual 



