24 
14. Experience in'screw propulsion has recorded several 
cases of the quantities (H' — H) having been negative, giving 
rise to what is known to practical naval architects as 
negative slip. Hannibal, a screw steamer of 8136 tons, 
speed 7'999 knots per hour, 64'87 revolutions per minute, 
propeller 17ft. diameter, pitch 12ft. 6in., length 2ft. lin., 
indicated horse power 1071, had a negative slip of ’601 
knots per hour. Plumper, 8129 tons, speed 6*627 knots 
per hour. 111 revolutions per minute, propeller 8ft. 8 Jin. 
diameter, 6ft. Of in. pitch, ll|in. length, indicated horse 
power 135, had a negative slip of *591 knots per hour. 
Many other instances of this peculiar kind of slip have 
been recorded in the history of screw propulsion ; but, its 
occurrence has generally provoked, amongst those interested 
in the progress of naval architecture, considerable discussion 
as to its real cause, some affirming its impossibility by an 
exclamation somewhat as follows how can the cart go 
faster than the horse, &c.? while others have endeavoured 
to see a cause for it, either in the flexibility of the propeller 
blade when it is subject to pressure, or, in the state of the 
water in which the propeller revolves. The history of this 
discussion is curious, and many of the leading events in it 
are recorded in the pages of the transactions of the society 
of Naval Architects. With respect to the importance 
attached to the subject of slip, it may be said that its 
introduction into the investigations of screw propulsion has 
been regarded by many in the light of an untoward event ; 
inasmuch as its diminution does not necessarily imply an 
increase in the speed of the ship produced by the same 
effective horse power of engines. And, moreover, the advo- 
cates of “ slip resistance ” would be sorely perplexed to 
assign properly the element called slip in a propeller with a 
variable pitch. 
15. With a view, therefore, of freeing the question of slip 
as much as possible from the elements which appear to me 
