TRANSACTIONS OF THE SECTIONS. 229 



tlie atmosphere; in that case, of course, the pipe which enclosed that portion of 

 the stream ■would have become simply an inert envelope, and might have been 

 removed without aiFecting- the dynamic properties of the stream. Theoretically, 

 indeed, with the frictionless fluid the contraction of jet might he carried so far as 

 not merely to obliterate all positive pressure, but to produce a negative pressure 

 equal to that of the atmosphere. For, in fact, the conditions thus brought into 

 operation would be in efiect identical with those which would exist if the experiment 

 were performed in vacuo, and the head in cistern and at the outlet were both in- 

 creased by 34 feet ; but the theoretical possibility thus indicated is greatly curtailed 

 by friction ; and the illustrative experiment I am about to exhibit deals only with 

 the case in Avhich the pressure at the contraction is reduced apparently to zero, or 

 in reality, as I have said, to that of the atmosphere. 



In the apparatus as here arranged (see Plate XI. fig. 25), consisting of the dis- 

 charging and the recipient cistern, with the intervening jet-orifice and recipient- 

 orifice, the overflow of the recipient cistern is at 18 inches above the centre of the 

 orifices. 



As I continue to fill the discharge cistern, you observe the jet shoots across the 

 open space between the orifices, and the water-level continues to rise in the recipient 

 cistern ; and so long as the head in the former is maintained at a moderate height 

 above that in the latter, the whole of the stream enters the recipient-orifice, and 

 there is no waste except the small sprinkling which is occasioned by inexactness of 

 aim, and hj the want of exact circularity in the orifices. 



When the head in the recipient has reached the o-^erflow, and thus remains at a 

 steady height of 18 inches above the orifices, the virtually complete reception is 

 insured by maintaining a head of 20^ inches in the discharging cistern, or an excess 

 of head of 2| inches on the discharge side ; and this excess, in efl'ect, represents the 

 energy wasted in friction. 



You observe that as I diminish the supply of water and allow the excess of head 

 in the discharger to become reduced, a steadily increasing waste becomes established 

 between the orifices ; and it is interesting to trace exactly the manner in which the 

 friction operates to produce this result. 



If the conoids of discharge and reception are tolerably short as they are here, it 

 is the outer annuli or envelopes of the stream which are in the first instance affected, 

 that is to say retarded, by friction ; and the escape or waste between the orifices 

 implies that this surface-retardation has reduced the velocity of those envelopes 

 below that due to the head in the recipient ; thus an annular counter-current is 

 able to establish itself, and in fact constitutes a counter discharge from the recipient. 



As the head in the discharger is more and more reduced, the diminishing velocity 

 of the central inflow into the recipient offers less and less frictional resistance to the 

 annular counter-current which envelops it, and the waste continually increases ; 

 it is probable, however, that to the last the velocity of the central zones of the jet 

 remains equal to that due to the head in the discharger ; and hence you wiU observe 

 that unless this is reduced below the level of the overflow, the head in the recipient 

 is fully maintained to that level, though the whole quantity discharged is wasted 

 between the orifices. 



When the supply is altogether cut ofl", both cisterns simultaneously empty them- 

 selves, the two jets meeting between the orifices and becoming spread into a beau- 

 tifvU plane disk or film of water at right angles to the line of discharge ; but you will 

 notice that from some inequality in the commencement of the action, and to some 

 extent probably from a quasi-instability in the equilibrium of the double discharge, 

 one of the jets will presently for a moment get the better of the other and drive it 

 back so as almost to an-est its flow, and thus for the moment arrest also the waste of 

 head on that side ; but the momentary excess of head thus occasioned, almost in- 

 stantly asserts its superiority, producing a jet of superior force, and thus driving- 

 back for a moment the opponent by which it had just before been mastered. Thus 

 a curious oscillation of discharge ensues, which is to a large extent a tiiie dynamic 

 phenomenon somewhat analogous to that which becomes established in an inverted 

 siphon partly filled with water, if for a moment the head is increased in one of tlie 

 legs ; the reaction which in the siphon is furnished by the other leg, beyond the bend, 

 is, in the case before you, furnished by the dynamic reaction of the jets ; but the cir- 



