208 



THE POPULAR EDUCATOR. 



falls of its own weight when the pipe is empty ; the pressure of 

 the water, however, closes it. Weights are now placed on the 

 upper end of the spindle, so that when the pipes are full of 

 water pressing upwards, the valve only just opens from its 

 weight. Another opening is made in the pipe at F, communi- 

 cating with a large reservoir, A, from the upper part of which 

 issues the pipe B, by which the water is to be raised. The 

 upper part of this reservoir is filled with air, and a small valve, 

 not shown in the figure, is so placed as to allow a small addi- 

 tional quantity to enter from time to time, and replace that 

 carried away by the water, which under pressure absorbs a small 

 amount of it. 



The opening between the pipe A B and the reservoir is closed 

 by the valve F, which rises by the inward pressure of the 

 water, and is closed by its own weight. 



We will now suppose the machine to be set in action. The 

 weight on the valve at c being more than sufficient to overcome 



Fig. 30. 



the pressure of the water, the valve opens, and the water escapes 

 and runs to waste. That in the pipe, however, acquires imme- 

 diately a small amount of momentum, which enables it to raise 

 the valve, and thus close the opening. The momentum thus 

 acquired by the water cannot be instantaneously destroyed, and 

 would burst open the end of the pipe were it not for the valve 

 at F. This provides an escape, and the water opens it, and 

 causes a certain amount to enter the reservoir, compressing the 

 air contained in it, and thereby forcing a fresh amount of water 

 up the tube B. 



The compressed air, however, acts as a spring, and thus the 

 momentum of the column of water is soon destroyed ; F then 

 closes of its own weight, and the water in the tube being now 

 at rest, c again opens and allows the water to escape as at first. 

 When the weight at c is carefully adjusted, this opening and 

 closing succeed one another rapidly, producing a series of stop- 

 pages, by each of which a small quantity of water is raised in 

 the pipe B. 



A larger amount, however, escapes at c than ascends in B, 

 and the amount raised diminishes, of course, with the height to 

 which it is raised ; still it is calculated that about 60 per cent. 

 of the power of the water may be utilised by the arrangement, 

 which certainly by its ingenuity reflects great credit on the 

 inventor. 



Occasionally, in mines, a stream of water is caused to move 

 an engine, constructed on exactly the same principle as the 

 steam-engine, the motive power being the pressure of the water 

 instead of the pressnre of steam. By an arrangement of valves 

 the water is made to press alternately on the upper and lower 

 sides of the piston, and the motion thus produced is by means 

 of a crank and fly-wheel communicated to the machinery. 



We have thus noticed all the mosb important machines 

 designed to derive motion from a fall of water, and now pass on 

 to the second class, or those which are intended to raise water 

 to any required elevation. 



Water is one of the prime necessaries of life, and as its ten- 

 dency is always to sink to the lowest level, various plans of 

 raising it have been tried from the very earliest ages. The 

 most primitive is by means of a bucket fastened to a rope ; 

 after a time, it was found more convenient, when the height to 

 which the water had to be raised was not great, to fix this rope 

 to one end of a lever supported near the middle on crossed 

 poles, and pull by means of a rope fastened to the other and 

 shorter end. A further improvement on this, which is at the 

 present day much used on the banks of the Nile, consisted in 

 fixing a weight at the other e::d of the lever, so as nearly to 



balance the bucket of water ; a man then alternately raises and 

 lowers it by pulling the rope. Much of the land in Egypt is 

 irrigated by this contrivance, which is known as the Shadoof. 

 The common windlass is used instead of this where the water 

 has to be raised from a great depth ; as, however, there are a 

 large number of machines in use, it will be best to make a 

 simple division of them, and perhaps the simplest we can make 

 is the following : 



1. Those which act mechanically ; 



2. Those which act by the pressure of the air ; 



3. Those which act by centrifugal force. 



The second of these divisions contains the common pump 

 and similar machines, which, strictly, ought not to be explained 

 till we come to treat of pneumatics, as they involve principles 

 which have not yet been explained ; but it is best to consider 

 all together, as in this way we can better understand their 

 differences in construction. 



First, then, we notice those which act mechanically. The plan 

 of raising water by means of a single bucket would naturally 

 suggest the idea of fixing several, one below the other, and thus 

 an endless chain of buckets passing over a wheel at the top was 

 constructed. 



The buckets are brought up full, and when they reach the 

 wheel strike against a support, and, being turned over, discharge 

 their contents into a channel prepared to receive them. The 

 wheel in this case may be turned by the foot, as is frequently 

 done, or the power of animals may be employed. 



The next modification of this arrangement is what is known 

 as the Persian Wheel, which is represented in Fig. 31. Floats 

 are fixed to one side of an undershot or tidal wheel, and in the 

 other side of the rim are fixed a number of pegs, from which 

 buckets are suspended. As the wheel is turned by the force of 

 the current, these successively dip into the water, and are 

 brought up nearly full. The weight of their contents keeps 



Fig. 31. 



them in a vertical position till they reach the top, where they 

 strike against a trough, and thus are emptied into it. The 

 water is conveyed from this by a channel not shown in the 

 figure. By this plan the water cannot well be lifted to any 

 great height, as the diameter of the wheel must be greater than 

 the height. This machine can be used in a tidal river, as it 

 will work in either direction. 



A further supply of water is, in this wheel, raised to the level 

 of the axis on a totally different principle. The spokes of the 

 wheel, instead of being made straight, as in the case of ordinary 

 wheels, are hollow and curve considerably. Openings will be 

 perceived on the rim, by which the water enters when they are 

 immersed, and from the shape of the spokes it cannot flow out 

 again, since the openings are higher than the bendsi The 

 water, therefore, travels along them towards the axis, and there 

 is discharged into a trough prepared far it. 



