Oil the Various Modes of Propelling Vessels. 427 



faster than if the screw worked in a solid substance ; and this 

 excess of velocity of the vessel above that of the screw has 

 been termed the "negative slip/'' 



The negative slip has been accounted for in various ways, 

 and is, perhaps, due to a variety of causes. It may, to some 

 extent, be explained by a twisting of the blades which is con- 

 sequent on the strain, and is equivalent to an increase of pitch. 

 Screws with a line pitch are more liable to it than those with a 

 coarse. A more effective cause of the negative slip is perhaps 

 found in the column of water that always follows a ship to fill 

 up the space which is left vacant by the progress of the vessel. 

 The forward motion of this current may more than counter- 

 balance the positive slip which must, in every case, occur. 

 More pressure, and therefore more power of resistance, is given 

 to the water in which the screw is immersed, than are found 

 in the surrounding water, also, by the mass of fluid which is 

 elevated, at the stern, by the screw itself. The constant action 

 of these two causes must always render the apparent less than 

 the real positive slip. 



The last mode of propulsion to which we shall direct the 

 attention of the reader is the application of that reaction which 

 is produced by fluid issuing from apertures. I shall say nothing 

 of the numberless contrivances that have been constructed on 

 the principle of ducks' feet— opening when intended to act on 

 the water, and folding up when they were to be drawn in the 

 opposite direction. Some of these are very ingenious, but all 

 of them, if liable to no other objection, are of necessity so 

 complicated, and so liable to injury, that their adoption would 

 be out of the question. 



The reaction produced hj a stream of fluid is the source of 

 motive power in Hero's engine, invented more than two thou- 

 sand yea.rs ago at least, and of Barker's mill ; and it is used in 

 the rocket, and many other kinds of fire- work. Nature herself 

 employs it as one of her sources of propulsive power; the 

 stream of water emitted by the gills of fish acting as an 

 auxiliary to the action of their fins. The application of this 

 principle to the propulsion of vessels holds out peculiar advan- 

 tages. Like the screw, it affords a means of aiding the rudder, 

 and even rendering it unnecessary j it is very easily applied, 

 the apparatus used with it being very simple, and less liable 

 than even the screw to injury from external causes. Contri- 

 vances founded upon it were among the earliest which have 

 been proposed as substitutes for the ordinary modes of pro- 

 pulsion; and, although they have been so long unsuccessful in 

 the contest, there is some reason to suppose that they may yet 

 supersede all others. They are beset, however, essentially with 

 great difficulties, which the unskilfulness of the earlier experi- 



