THE CONQUEST OF THE ZONES 



ship's propeller that supplies the model for the mod- 

 em log, in which the primitive piece of wood is re- 

 placed by a torpedo-like piece of metal with miniature 

 propeller-Kke blades at its extremity. This apparatus 

 is towed at the end of a long line, and its blades, whirling 

 more or less rapidly according to the speed of the ship, 

 communicate their motion to a recording apparatus, 

 adjusted at the ship's stem, to which the line is attached 

 and the face of which ordinarily presents a dial on which 

 the speed of the ship may be observed as readily as one 

 observes the time by the clock. 



Some recent modifications of the log employ an elec- 

 trical device to register the progress, but the principle of 

 the revolving vanes, which owe their speed to the rate 

 at which they are dragged through the water, is the 

 fundamental one upon which the action of the log 

 usually depends, though attempts have been made 

 to substitute pressure-gauge systems. 



WTiile the modem log records the speed of the ship 

 with a fair degree of accuracy, its register shows at 

 best only an approximation of the facts. As already 

 mentioned, the rate of revolution of the ship's propeller 

 blades furnishes what most navigators regard as a rather 

 more dependable test of speed. An apparatus for re- 

 cording this is found on the bridge of the modem ship. 

 But due allowance must of course be made for the 

 effect of winds, waves, and ocean currents. These con- 

 stantly variable factors obviously make the estimate as 

 to the precise distance traversed by a ship in a given 

 time a matter not altogether devoid of guess work ; and 

 no navigator who has been obliged to sail for several 

 VOL. VII. — 2 [17] 



