ON THE FRICTION OF FLUIDS. 297 



four times as much air as it naturally contained had been condensed by means 

 of a syringe, and causing the current to pass through a small box, in which 

 the bason of a barometer was placed, the mercury was depressed more than 

 two inches, in consequence of the rarefaction which the current produced iu 

 the air of the box. ( Plate XXI. Fig 268.) 



Professor Venturi has also made several experiments of a similarnature on the 

 motion of water : he observes that not only the water in contact with a stream 

 is drawn along by it, but that the air in the neighbourhood of a jet is also made 

 to partake of its motion. When the mouth of a pipe, through which a stream of 

 water is discharged, is introduced into a vessel a little below the surface of the 

 water which it contains, and is allowed to escape by ascending an inclined sur- 

 face placed opposite to the pipe, and leading over the side 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 be- 

 comes level with the lowest part of the stream. (Plate XXI. Fig. 269.) 



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 either nearly 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 equilibrimn, 

 theadhesionof the water, combined with its centrifugal force in turning round 

 the ball, assists in drawing it back, when it has declined a little 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 I have 

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

 sophical 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 extremity, instead of retreating before the 

 blast, it will on the contrary appear to be attracted ; the pressure of the at- 

 mosphere being diminished by the centrifugal force of the current, which 



VOL. I. Q q 



