Hydrography^ and the Art of Navigation. 43 



For this purpose, it is necessary to draw sea- water from a 

 great depth, at a distance from Jand, and to bring it to the sur- 

 face with all the air which it contains. This air must then be 

 disengaged by boiling, its volume measured under the ordinary 

 pressure of the atmosphere, and finally subjected to a chemical 

 analysis. In these operations, the only difficulty is to draw the 

 water from the desired depth, and bring it to the surface with- 

 out allowing any of its contents to escape. Care must be taken 

 not to employ vessels which are empty or filled with air merely, 

 designed to open at the assigned depth and admit the water ; for 

 the pressure to which they will be subjected will cause the wa- 

 ter to filter through the joints of the most perfect closing 

 plates {pbturateurs)y or break the vessel if these resist. And, 

 lastly, if the gaseous mixture contained in the deep-lying beds 

 is subjected to the same pressure which they undergo, it will 

 expand in the inverse proportion when the apparatus is brought 

 near the surface, and will either escape by the closing plate 

 (ohturateur) or burst the vessel containing it. In order to avoid 

 these effects, a hollow glass cylinder ought to be employed, clos- 

 ed at one extremity by a solid plate of metal, thus forming a 

 true bucket provided with a handle, to which a cord is attached 

 that it may be Jet down to the bottom of the sea. This bucket 

 being empty, and open to the surrounding water, descends into 

 the different beds without being injured by the pressure. When 

 it has reached the required depth, a cord is drawn which is at- 

 tached to its lower part, and by which it is reversed. This second 

 cord is then employed to draw up the apparatus, and in order 

 that it may not get entangled with the other, it is kept at the 

 other end of the ship. But this cylinder of glass has a double 

 bottom, one fixed, the other moveable. The latter is in reality 

 the piston of an air-pump, which descends alone, by its own 

 weight, when the bucket is drawn back ; and, at the same time, 

 the bottom fixes to a small hole furnished with a valve, which 

 opens from without inwards by the pressure of the surrounding 

 water, and allows it to enter into the empty space made by the 

 descending piston. When the latter has descended, and the 

 space filled, the valve in the bottom closes by its own spring, 

 and the enclosed water is thus separated and kept apart till it 

 arrive at the surface. But, if this water contain compressed air, 



