298 OF STEAM NAVIGATION. [SECT. x. 



to raise and propel water out behind the vessel : this mode was proposed by Ber- 

 nouilli, and afterwards by Mr. Linaker. 1 These, with numberless variations, the 

 greater part of which are obviously inferior to the methods in their simple form, 

 have been proposed. Some of the best we propose to notice. Our selection must 

 however be limited, because it must be confined to those which afford sufficient 

 power in a convenient manner, and without being liable to injury by the violence 

 of the waves, or to get out of order. 



625. The species may be divided into two classes ; viz. 1. those in which the 

 action is continuous, or nearly so ; 2. those which act at intervals. To the first 

 class belong the second, sixth, and seventh methods ; to the second, the first, 

 third, fourth, fifth, and eighth. 



Since, when the action is continuous, the area of the surfaces in action, multiplied 

 by their resistance, must be equal to the area of the vessel, by the vessel's resistance 

 when reduced to the same direction, it is obvious that all those which act at inter- 

 vals only, must require to be of greater area than those which act continually. 

 Hence, unless there be some other manifest advantage, this circumstance alone 

 must determine us to reject all except the first class, and of this to take only the 

 second, sixth, and seventh methods. Most of the others would require complicated 

 action, and be inconvenient in practice. The first class also reduces to two ; for 

 the two opposite water screws without a cylinder give about the same effect as one 

 with a cylinder ; and this method, though it has not been used, deserves attention, 

 from the circumstances of its being capable of acting wholly below the surface of 

 the water, and in a direction parallel to the motion of the vessel, and only so far 

 above the centre of resistance as is deemed necessary to stability. I can easily 

 conceive that the trial of an experiment may be the means of condemning a very 

 useful principle, merely through inattention to the proportions and mode of action. 



OF THE SPIRAL PROPELLER OR WATER SCREW. 



626. The acting portion is a spiral surface projecting from a cylindrical axis ; 

 and, in order that it may be at all effective, each point in the surface must revolve 

 so rapidly, that the motion of that point in the direction of the axis must be greater 

 than that of the vessel. Also if the angle of the spiral to the axis be constant, it is 

 obvious, that by having more than one revolution, the rest add little to the effect, 

 perhaps not equivalent to the additional friction. 



1 Buchanan on Propelling Vessels by Steam, p. 40. 



