28 
A most excellent paper “ On the Efficiency of Single and 
Twin-screw Propellers ” was read and discussed at the In- 
stitution of Naval Architects, April 11th, 1878, by W. H, 
White, Esq., Assistant Constructor to the Navy, Member of 
Council, in which is recorded two cases of negative slip. 
The Invincible, with twin-screw, displacement 5,563 tons, 
70*8 revolutions per minute, 4,882 I. H.P., with four screw 
blades whose dimensions are 16ft. 2in. diameter, 20ft. 9|in. 
pitch, speed 14*093 knots per hour, gave a negative slip of 
1*89 per cent. The Alexandra, twin-screws, 9,432 tons dis- 
placement, 64*2 revolutions per minute, 8,615 I.H.P., with 
four screw-blades, 21ft. diameter, 22ft. 2 fin. pitch, speed 15 
knots per hour, gave a negative slip of 6*5 per cent. 
