VELOCITY OP CURBENTS* 



*ide of the vessel, the stream not only ascends this surface without 

 leaving any portion of itself behind, but carries also with it the 

 whole of the water of the vessel, until its surface becomes level with 

 the lowest part of the stream. 



The effect of a jet of water, in drawing towards it a current of 

 air, is in some measure illustrated by an experiment which is often 

 exhibited among the amusements of hydraulics. A ball of cork, or 

 even an egg, being placed in the middle of a jet, which throws up a 

 pretty large stream to a moderate height, the ball, instead of falling 

 or being thrown off, as it might naturally have been expected to 

 do, remains nearly either stationary, or playing up and down, as 

 long as the experiment is continued. Besides the current of air 

 which Venturi has noticed, and which tends to support the ball in a 

 stable equilibrium, the adhesion of the water, combined with its 

 centrifugal force in turning round the ball, assists in drawing it 

 back, when it has declined aliltle on either side, so that the stream 

 has been principally in contact with the other side. A similar effect 

 may be observed in the motions of the air only, as Dr. Young has 

 shown by some experiments of which an account is published in the 

 Philosophical Transactions. Thus, if we bend a long plate of metal 

 into the form of the letter S, and suspend it in the middle by a thread, 

 so that it may move freely on its centre, and if we then blow on its 

 convex surface with a tube directed obliquely towards the extre- 

 mity, instead of retreating before the blast, it will on the contrary 

 appear to be attracted ; the pressure of the atmosphere being di- 

 minished by the centrifugal force of the current, which glides along 

 the convex surface, because it finds a readier passage in the neigh- 

 bourhood of the solid, towards which it is urged by the impulse of 

 the particles of the air approaching it on one side, and by the de- 

 fect of pressure on the other side, occasioned by the removal of a 

 certain portion of the air which it carries with it. 



From considerations similar to those by which the velocity of a 

 river is determined, we may calculate the quantity of water dis- 

 charged from a pipe of any given dimensions, and in any position. 

 The same expressions will serve for estimating the magnitude of the 

 friction in both cases ; the pipe being considered as a small river, 

 of which the mean depth is one-fourth of its diameter: but a part 

 only of the force of gravity is now expended in overcoming the 

 friction, the rest being employed in producing the momentum of 



