loS POPULAR LECTURES AND ADDRESSES. 



siderable margin for error. In the recent court- 

 martial regarding the loss of the Vanguard, the 

 speed of the Iron Duke was estimated by one 

 of the witnesses at ten and a half knots according 

 to his mode of reckoning from revolutions of 

 the screw and the slip of the screw through the 

 water, while other witnesses, for reasons which 

 they stated, estimated it at only 8*2 knots. It 

 was stated in evidence, however, that the only 

 experiments available for estimating the ship's 

 speed in smooth water from the number of 

 revolutions of the screw, had been made before 

 she left Plymouth. If the old log-ship and glasses 

 had been used, there could have been no such 

 great range of doubt : or the Massey log, which 

 may be held to do its work fairly well if it 

 gives the whole distance run by the ship in 

 any interval within five per cent, of the truth. 



60. Consider further the steerage. In a wooden 

 ship a good ordinary compass, with proper pre- 

 cautions to keep iron from its neighbourhood, 

 may be safely trusted to within a half-quarter 

 point ; but, reckoning the errors of even very 



