On the Various Modes of Fropelling Vessels. 423 



four vanes would be in reality a screw with four threads ; and 

 Bouquer, in a treatise published in 1747, mentions the applica- 

 tion of revolving arms, like those of a windmill, to the pro- 

 pulsion of a vessel. It would be suggested, likewise, by the 

 smoke-jack. Bramah, in 1785, patented the application, to the 

 stern of a ship, of a wheel with inclined fans or wings, like 

 those of a smoke-jack. He was the first to use a stuffing-box 

 for connecting the screw with the prime mover within the 

 vessel. In most of its early applications the screw was sus- 

 pended at a distance from the stern, and motion was com- 

 municated to it by very clumsy means. 



In its more perfect form, the screw propeller consists of 

 threads or blades placed on an axis parallel to the keel, and 

 forming segments of a helix or spiral. Its jpitch is the dis- 

 tance in the direction of the axis between any one thread and 

 the same thread at the point where, if continued, it would 

 complete its next convolution. When there is but one screw, 

 it can be fixed only at the bow or the stern. If at the former, it 

 acts on water at rest, which increases its effect ; but it throws 

 water against the bow, which retards the vessel. If at the 

 stern, to prevent interference with the rudder, it is placed in 

 the dead wood, or that portion of the ship which is immediately 

 behind the rudder. In 1768, Pancton suggested the use of 

 one screw, either in the bow or the stern, or a screw at each 

 side. But one of the earliest practical applications of the 

 screw propeller was that by Bushnell, of Connecticut, in 1776. 

 He employed one screw for raising or depressing, and another 

 for propelling a submarine boat, which was intended for the 

 fixing of torpedoes to the sides of hostile ships. 



Perhaps no contrivance has afforded more occupation to 

 ingenious minds than the screw propeller; it has been made of 

 every conceivable form, and fixed to the vessel in every con- 

 ceivable way. The number of patents to which it has given 

 rise are counted with difficulty. It came into very general use 

 soon after it had attracted the serious attention of experimen- 

 talists and projectors. But the Government were slow to 

 adopt it. Smith, an Englishman, and an amateur, and Ericsson, 

 a Swede, and an accomplished engineer, may be considered to 

 have practically introduced it into use. Both of them en- 

 deavoured to secure the patronage of the Admiralty, but only 

 Smith, whose experiments appear to have been more satis- 

 factory, succeeded ; and Ericsson left the country in disgust. 

 Smith required gearing; Ericsson's professional resources 

 enabled him to do without it. Smith used a single screw, con- 

 sisting of one whole convolution, and also a double threaded 

 screw, each thread of which was equal to half a convolution, — 

 one-sixth of a convolution for each has since been found to 



