August 2, 1877] 



NATURE 



273 



involves serious difficulties when greatly magnified, owing to the 

 i;reat amount of heat ; and it was chiefly in order to escape this 

 difficulty that Mr. Froude sought some fresh viodiis operandi, 

 and ultimately felt his way to the arrangement to which we 

 desire to draw attention. 



Under this arrangement, the engine, in utilising its power, will 

 still be virtually winding up a weight ; but the weight, instead 

 of being constant, will vary with the speed of rotation, much in 

 the same way as the resistance of the propeller itself does ; and 

 thu? the work performed by the engine under trial will more 

 closely resemble its natural work. 



The reaction, instead of arising from the continuous friction 

 of two solid surfaces, will consist of a multitude of reactions 

 supplied by the impact of a series of fluid jets or streams, whicli 

 are maintained in a condition of intensified speed by a sort of 

 turbine revolving within a casing filled with water, both being 

 mounted on the end of the screw-shaft in place of the screw, 

 the turbine revolving while the casing is held stationary ; the 

 jels being alternately dashed forward from projections in the 

 turbine against counter-projections in the interior of the casing, 

 tending to impress forward rotation on it, and in turn dashed 

 back from the projections in the casing against those in the 

 turbine, tending to resist its rotation. The important point is, 

 that the speed of the jets is intensified by the reactions to which 

 they are thus alternitely subjected ; and thus in virtue of this 

 circumstance a total resistance of very great magnitude is main- 

 tained within a casing of comparatively very limited dimensions. 



The nature of this arrangement will be gathered from the 

 accompanying figures. 



In Fig. I, A A represents the screw-end of the screw shaft ; 

 li li shows in section what has been termed " the turbine ; " it is 

 a disc or circular plate, with a central boss, keyed to the screw- 

 shif: in place of the screw, and revolving with the shaft. The 



disc is not flat throughout, its entire zone being shaped into a 

 channel of semi-oval section, which sweeps round the whole 

 circumference concentrically to the axis. To give definiteness 

 to the conception, imagine that, to deal with an engine of 2,000 

 Ind. horse-power, making 90 revolutions per minute, the diameter 

 of the turbine-disc to the outer border of the channel is five leet. 



In Fig. 2, Fig. i is repeated, and what has been called " the 

 casing" is added, being indicated by the letters c c, D D, the 

 former representing the front and the latter the back. The face 

 carries a channel, the counterpart of that carried by the disc, 

 which it also fronts precisely, so that the two semi-oval channels 

 in effect form one complete oval channel, though the two halves 

 are in substance separated by an imaginary plane of division. 

 The back of the casing embraces or incKides the disc entirely, 

 but without touching it ; the casing is also provided with a boss, 

 which is an easy fit over that of the disc or turbine, and thus the 

 disc carried by the shaft can revolve within the casing without 

 touching it, while the casing itself is stationary, and one half of 

 the oval channel is running round while the o her half is at rest. 



Thus far the two half channels have been regarded as open 

 and unobstructed ; they are, however, in fact each closed or cut 

 across by a series of fixed diaphragms, a single one of which is 

 shown in Fig. 3, as in its place in the disc-channel. The 

 diaphragms cut tne channel, not perpendicularly, but obliquely, 

 being semicircular in outline, so that when placed obliquely their 

 circular edges fit the oval bottom of the channel, while their dia- 

 meters span the major axis of the oval Fig. 4 shows one of the 

 diaphragms seen end on or edgeways, as it would appear in an 

 edgeways view of the turbine if this were transparent. 



Each half channel has twelve of these diaphragms, and is thus 

 divided into a series of cells, each of which, if viewed at right 

 angles to one of the diaphragms, or what is the same thing, if 

 shown in a section taken parallel to one of them, is semicircular 

 in outline ; and if thus viewed in connection with the cell which 

 is for the moment opposite to it in the counterpart half channel, 

 the two together make one complete cell with circular outline. 



Thus the whole oval channel may be regarded as a series of 

 obliquely placed circular cells, and as the function of the!turbine 

 is to lotate while the casing remains at rest, one half of each cell 

 is moving past the other half in such a manner that the moving 

 half, if viewed from its stationary counterpart, would by reason 

 of the oblique direction of the diaphra:.;ms which form the cell 

 sides, appear to be advancing antagonistically towards it ; mdeed 

 the motion virtually constitutes such an advance, because the 

 boLtoni of each moving half cell is continually growing nearer 

 to the boitom of the stationary half cell which it faces. The 

 eifectiveness of the combination to resist rotation will be seen to 

 depend essentially on this quasi-antagonistic virtual approach of 

 the moving to the stationary half cell. 



The channel and the whole casing is filled with water, and 

 the turbine is made to rotate as described. When the turbine 

 is thus put in motion, the water contained in each of its half 

 cells is urged outwards by centrifugal force ; and in obeying 

 this impulse it forces inwards the water contained in the 

 stationary casing half cells, and thus a continuous current is 

 established, outward in the turbine's half cells, inward in those 

 of the casing. 



Now the action of these cells on the water contained in them 

 may be rendered more clear by the following illustration : — Sup- 

 pose a person in a railway train moving at a certain velocity to 

 hold a racket fixedly in his hand and a ball thrown to him strikes 

 the racket ; also that there is provided a series of walls beside 

 the railway inclined at such an angle that the ball leaving the 

 racket and striking one of the walls will rebound to the racket. 

 Suppose, also, for the sake of simplicity, that the ball is perfectly 

 rigid while the walls and racket are perfectly elastic. On striking 

 the racket for the first time the ball will rebound with a velocity 

 equal to double that of the train added to the original velocity 

 of projection. In order to see this clearly we must look at what 



takes place in the racket. The ball meets the racket with a 

 velocity equal to that of projection added to that of the train, 

 llie strings of the racket stretch to such an extent that their 

 recoil would cause the ball to rebound with the sum of these 

 Velocities. At this instant suppose the train stopped, and we 

 should then see the ball projected through the air with the sum 

 of these velocities in consequence solely of the elastic reaction of 

 the racket. But in the experiment we are supposing the train is 

 not stopped but is at this instant capable of impressing on a ball 

 at rest the full velocity of the train ; this it does equally well on 

 the ball, which we imagine, for the instant, resting against the 

 stiained racket. Thus we see that the ball will be projected 

 through the air with a velocity equal to that of the train in 

 addition to that with which it would have been impressed on it 

 by the racket had the train been stopped at the moment indi. 

 cated, or, in other words, its velocity of projection will, after one 

 contact with the racket, have had added to it twice the velocity 

 of the train. Each time the ball, rebounding from the walls in 

 succession, meets the racket, an additional double train-velocity 

 will be impressed on it. Another impoitant point in this illus- 

 tration must not be passed over. The action of the ball on the 

 racket tends to retard the train, and that on the walls tends to 

 push them forward in the direction of train as well as away from 

 the line of railway. 



In an analogous manner the currents originated solely by cen- 

 trifugal tendency, in being unceasingly reversed in direction and 

 increased in velocity by the opposing cells, produce a resistance 

 to the rotation of the turbine which is measured by the torsion 

 it |iroduces on the casing. This torsion is most conveniently 

 measured by a spring balance attached to the end of a lever 

 made fast to the casing. 



The manner in which the currents, when established, produce 

 the dynamometric reaction, can be traced very easily. The 

 exiilanation already given of the internal form of the cells which 

 the current traverses, shows that the volume of water which 

 on stitutes it in each complete cell may be regarded as a circular 



