PRIME-MOVERS. 359 



This employment of the induced current as a prime-mover is 

 described by Venturi in the record of his experiments made at the 

 latter end of the eighteenth century, and within the last few years 

 Professor James Thomson has applied the same principle with great 

 success in his jet pump. 



The next mode I shall notice of obtaining motive power from water, 

 is also one where it operates by an induced current this is, the 

 trombe d'eau, an apparatus wherein water, falling down a vertical 

 pipe, induces a current of air to descend with it, and the lower end 

 of the vertical pipe being connected with the top of an inverted 

 vessel, the bottom of the sides of which vessel is sealed by a water 

 joint, the water dashing upon a block placed below, the mouth of 

 the pipe, is separated from the air, so that while the water descends 

 and escapes from under the sides of the vessel the air rises and is 

 accumulated in the upper part, and can be led away to blow a forge 

 fire. These machines are described in Belidor's work. 



The utilization of the rise and fall of the tide is also fully described 

 by Belidor, who gives drawings of channels so arranged that during 

 both the rise and fall of the tide the water-wheel, notwithstanding the 

 reversal of the current, revolves in one and the same direction. The 

 tide is a source of power which it is highly desirable should be 

 utilized to a greater extent than it is ; if we consider the enormous 

 energy daily ebbing and flowing round our shores, it does seem to 

 be a matter of great regret that this energy should be wasted and that 

 coal should be burnt as a substitute. 



The last mode in which power may be obtained from water, to which 

 I have to allude, is that of the employment of the waves. 



Earl Dundonald, better known as Lord Cochrane, proposed by his 

 patent of 1 833, to utilize this power for propelling a vessel. This he hoped 

 to accomplish by means of cylinders containing mercury, the oscilla- 

 tions of which were to cause a vacuous condition in the cylinders, and 

 thereby give motion to an air-pressure engine ; and lately we have had 

 produced before the Institution of Naval Architects, and also before 

 the British Association at Bristol, the apparatus of Mr. Tower, by which 

 the motion of the waves is to be utilized. A model constructed on this 

 principle has driven, it is said, a boat against the wind at some two or 

 three miles an hour. 



