THE SURFACE OF THE EARTH. 



leaden weight sinking in the water draws with it the line, which 

 thus unrolls itself from the reel, and this continues until the lead 

 strikes the bottom. When that takes place the reel ceases to 

 revolve and the line to sink, and the seaman who observes the 

 sounding, notes the mumber of the knot which is nearest the 

 surface, and thus obtains the depth, which is always expressed in 

 fathoms. 



The lead, suspended from the extremity of the sounding-line, 

 is cup-shaped at its lower end, and grease, technically named the 

 arming, is put into the cavity, so as to be capable of taking up 

 by adhesion a portion of the shells, sand, or other substance, which 

 is at the bottom, with which it comes into contact. This being 

 drawn up, the navigator is informed not only of the depth, but 

 of the quality and character of the bottom, which often serves 

 him as a guide to his position. 



In this manner surveys are made of the bottoms of all seas 

 which are much navigated, and charts are drawn and engraved, 

 upon each part of which is marked the number of fathoms of 

 depth in the corresponding parts of the sea, and frequently the 

 character of the bottom. 



It happens fortunately that the general depth of the oceans and 

 open seas is so considerable as to be attended with no danger to 

 navigation. Such charts, therefore, as are here described are 

 only necessary for navigation in enclosed seas and tracts of water 

 near to coasts. 



28. Lali.es are sheets of water, of greater or less magnitude, 

 completely surrounded by land, and having no superficial com- 

 munication with the sea. They are, therefore, to the water what 

 an island is to the land, and, like an island, the name is generally 

 restricted to magnitudes which are not very great. A lake of great 

 magnitude is generally called an inland sea. 



Like other geographical terms, these, however, are arbitrary ; 

 some sheets of inland water called seas being less than others 

 called lakes. 



29. Rivers are large streams of fresh- water, formod by the rain 

 which falls on elevated parts of the land, descending the de- 

 clivities in streams, which, gradually uniting one with another, 

 form at length a large course of water, which receives the name of 

 a river. 



30. The Bed of a River is a groove formed in the land, de- 

 scending in a direction varying with the level of the surface, 

 until it reaches the coast, where its water is discharged into 

 the sea. 



31. The Banks of a River are the land which confines its 

 course on either side, and are distinguished as the right and left 



136 



