HYDRODYNAMICS. 



horse-power. 



the height of fall is sufficient to allow the water to 

 be let on above the centre of the wheel. When 

 the channel or trough is carried over the top, the 

 wheel is called an overshot wheel. This arrange- 

 ment is now seldom followed. It is found prefer- 

 able to make the diameter of the wheel a foot or 

 two greater than the fall, and to deliver the water 

 a little below the summit, as at the upper channel 

 in fig. 23. The water may enter the bucket with 

 greater impulse by the former arrangement, but 

 the dash makes the bucket overflow before it 

 is full, which more than counterbalances the 

 advantage. When sufficient fall is not available, 

 the water is sometimes laid on as low as the 

 level of the axis. The term rau/-wheel is gener- 

 ally applied where the water is delivered anywhere 

 above the level of the axis without being carried 

 over. 



The absolute working-power possessed by water 

 descending from a height depends upon the 

 quantity and the height of the fall. If the descent 

 is twelve feet, one pound of water is urged over 

 twelve feet by a pressure of a pound, which makes 

 twelve units of work, and is equivalent to the 

 work of lifting one pound to the height of twelve 

 feet. Therefore, the absolute work inherent in a 

 ton of water having a descent of twenty feet, is 

 2240 X 20 = 44,800 units. If the stream deliver 

 ten tons per minute, 44,800 X 10 = 448,000 is its 

 work per minute ; and as a rate of 33,000 units of 

 work in a minute is what is meant by a horse- 

 power, the power of such a stream is equal to 

 448,000 = 

 33,000 



But water acts fully on a bucket-wheel only 

 from the point where the bucket is filled to the 

 point where it begins to flow out, or for about 

 two-thirds of the fall. Owing to this and other 

 causes of loss, the proportion of the absolute power 

 of the fall rendered available seldom exceeds from 

 55 to 65 per cent. It is advantageous, with a 

 view to economising the power, to give a small 

 velocity to the circumference of the wheel, not 

 exceeding three or four feet per second. 



There are a great many kinds of horizontal 

 water-wheels. .In 

 the class known as 

 turbines (Ital. tur- 

 bino, a whirlwind), 

 the water is made 

 to escape horizon- 

 tally from a column 

 under pressure, in- 

 side the wheel ; and, 

 receiving a spiral 

 direction from 

 curved blades, 

 strikes on the ob- 

 lique palettes of the 

 wheel, and makes it 

 revolve. There is 

 another class called 

 reaction wheels, the 

 principle of which 

 is seen in the ma- 

 chines known as 

 Barker's or Segner's 

 mills. A drawing of 

 Fig. 24. a Barker's mill, in 



its simplest or typi- 

 cal form, is annexed (fig. 24). A is a wide metal 



pipe resting at its lower end by the spindle T, on 

 a metal block B, and kept in a vertical position by 

 the spindle S, at its upper end, which passes 

 through the frame of the machine, so that it can 

 easily revolve round its axis. Near its lower end, 

 two smaller pipes or arms, C, C, are inserted, 

 which project horizontally from it; and these 

 have, near their outer extremities, holes cut in 

 them, opening towards opposite sides. The water 

 is supplied by the pipe P. The reaction caused 

 by the water gushing from the arms, forces 

 them backwards, and gives to the whole machine 

 a rotatory motion. The action is similar to the 

 recoil of a musket. More closely analysed, it may 

 be thus explained. When an opening is made 

 in a vessel containing water, as at C in fig. 24, 

 the pressure is less on that side of the vessel than 

 on the other, and if it were standing on a piece of 

 wood floating on water, it would move in a direc- 

 tion opposite to the jet. 



Water-column Machines. We shall understand 

 the nature of those machines, if we conceive that 

 in a steam-engine, instead of steam, water, under 

 the pressure of a high column, were admitted 

 alternately above and below the piston. 



The Hydraulic-ram is a simple and conveniently 

 applied mechanism, by which the momentum or 

 weight of falling water can be made available for 

 raising a portion of itself to a considerable height. 

 In the annexed figure (fig. 25), which represents 

 a section of Montgolfier's hydraulic-ram, R is the 

 reservoir from 

 which the wa- 

 ter falls ; RS, 

 the height of 

 the fall ; and 

 ST, the horizon- 

 tal tube which 

 conducts the 

 water to the en- 

 gine, ABHTC 

 E and D are 

 two valves, the 

 former of which 

 closes its cavity 

 by ascending, 

 the latter by descending ; and FG is a pipe 

 reaching within a very little of the bottom CB. 

 The valves are such that the water at its normal 

 pressure cannot support their weight ; the valve 

 E is prevented from falling below a certain point 

 by a knob above mn. When the water is allowed 

 to descend from the reservoir, after filling the tube 

 BHS, it rushes out at the aperture mn, till its 

 velocity in descending RST becomes so great as 

 to force up the valve E, and close the means of 

 escape. The water being thus suddenly checked, 

 and unable to find a passage at mn, will produce 

 a great action on every part of the containing 

 vessels, and by its impact raise the valve D. A 

 portion of water being admitted into the vessel 

 ABC, the impulse of the column of fluid is ex- 

 pended, the valves D and E fall ; the opening at 

 D being thus closed, and that at mn opened. The 

 water now rushes out at mn as before, till its 

 motion is again stopped by its carrying up the 

 valve E, when the operation is repeated, the fluid 

 impulse opening the valve at D, through which a 

 portion of the water passes into ABC. The valves 

 at E and D thus alternately closing and opening, 

 and water at every opening of D making its way 



213 



Fig. 25. 



