258 THE SEAS 



directions printed on it in five different languages instruct- 

 ing the finder to place it in the nearest letter box after 

 having filled in the place and date of recovery. Some 

 of these bottles travel for enormous distances, and recently- 

 one which was liberated in the middle of the English 

 Channel was picked up away on the coast of Norway having 

 journeyed a distance of 1,440 miles in 190 days. 



Currents can also be directly measured by means of 

 special recording instruments. Several instruments have 

 been invented, but there is one in general use to-day the 

 principle of which is as follows. The machine is lowered 

 to a given depth in the water and to it is fixed a vane which 

 at once sets the instrument facing in the direction of the 

 current. The current then acts on a small propeller which 

 it rotates. Fixed to the instrument underneath is a small 

 circular tray divided into a number of compartments 

 corresponding to the divisions on a compass. After 

 ever^^ so many revolutions of the propeller a little lead 

 shot is released which falls on to the centre of a grooved 

 magnetic needle, runs down it, falls off its North point 

 into the compartment of the tray that lies immediately 

 beneath it, according to the direction of the current. 

 After a certain time the instrument is hauled up on 

 board, and by counting the number of shot in the differ- 

 ent compartments of the tray the number of revolu- 

 tions of the propeller can at once be calculated, and 

 from this the speed of the current ; also at the same time, 

 by noting the number of shot in each separate compart- 

 ment, the direction in which the current has been flowing 

 is at once given. 



The indirect methods of attack depend upon the fact 

 that the physical and chemical characters of the water 

 vary from place to place. For instance the Gulf Stream 

 water is notably saline, and by an examination of the 



