SURVEYING THE ROUTE. 



29 



position is checked by taking an altitude for longitude and the 

 latitude at noon, giving the actual distance travelled over 

 ground and the correct course from last position. The positions 

 are carefully marked on the chart for future reference. 



Two forms of patent logs (Massey's and Walker's) are shown 

 in Figs. 20 and 21 from illustrations kindly supplied by Mr. 

 Louis Casella, of Holborn. The fans or propellers rotate at 

 a speed proportional to the rate they are towed by the ship 

 through the water, the tow line communi- 

 cating the rotation to the indicator on 

 board (Fig. 22), which shows the number 

 of knots in any period from the time of 

 setting the apparatus. A flywheel is 

 placed in the tow line which acts as a 

 governor and steadies the revolutions of 

 the indicator spindle. The indicator is 

 made to strike a bell as each knot is 

 recorded. Massey's propeller log (Fig. 20) 

 registers on the log itself. The tube A A 

 (coned at both ends for the purpose of 

 giving a smooth action and preventing 

 jumping from the water at high speeds) 

 contains the register, and revolves with it 

 on a fixed axis passing through the cone 

 points. The axis is geared with the train 

 of wheels of the rotating register through 

 an endless screw, thus recording the 

 distance run on the dial-plate C. A 

 milled head is provided at D for turning 

 a sleeve by which the dial-plate can be 

 covered when in use. The towing line 

 is attached at the thimble F, and should 

 be long enough to allow the log to fall sufficiently astern to 

 clear the eddy of the wake. 



One disadvantage of these logs is that they sink at slow 

 speeds and do not register, and it is not practicable to apply 

 any floating appliance, as this would need adjustment according 

 to speed, and the log is too far astern for any such apparatus 

 to be controlled from the ship. A modified form of log 

 registering from the tail, and an improved method of 



Fig. 20. 

 Massey's Propeller Log 



