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THE POPULAR EDUCATOR. 



curved piece forces them in. After passing this point they are 

 pressed out again. 



Now it will easily be seen, by reference to our illustration, 

 that as soon as one of these partitions has passed this point it 



leaves a vacuum behind it. 

 This is speedily filled by water 

 from the suction-pipe, which 

 is then, carried round in the 

 space B B by the next par- 

 tition. As the cylinder re- 

 L volves rapidly this water ac- 

 quires a considerable degree 

 of centrifugal force, which 

 tends to drive it np the exit- 

 pipe ; the partitions also, as 

 they move onwards, force the 

 water in the same direction. 

 Fig. 41. The machine, therefore, acts 



as a suction and a forcing- 

 pump, and has the advantage of maintaining a constant flow 

 of water. 



With this we conclude our description of machines for raising 

 water, having considered all the most important varieties. We 

 must now turn to the last of the three classes into which we 

 divided all hydraulic machines namely, those which are de- 

 signed to propel vessels through the water. Locomotion by 

 water has always been more or less employed, especially since 

 the invention of the mariner's compass ; but, till quite recently, 

 the only mode of propelling a vessel was either by human power, 

 employed in rowing or sculling, or else by the force of the winds 

 acting upon sails properly set. The former of these two modes 

 was found to be quite impracticable in the case of large vessels ; 

 and though the latter is still very largely used, being inexpen- 

 sive, there are great disadvantages in its use, arising from the 

 uncertainties of the wind, the ship being often detained, or 

 obliged to tack about frequently when the wind is unfavour- 

 able. 



Accordingly, very soon after the invention of the steam-engine, 

 an attempt was made to employ it in giving motion to vessels ; 

 and though great ridicule was at first thrown on the idea, as 

 happens in the case of most useful inventions, the experiments 

 were successful, and now steamboats are to be seen on almost 

 every river. 



The mode in which they are propelled is so familiar that but 

 little need be said here on the subject. There are two plans in 

 common use, the one being by means of paddle-wheels, the other 

 by a screw placed at the stern of the vessel. 



The ordinary paddle-wheel is simply a wheel with a number 

 of floats fixed round it, the axle turning on bearings fixed to the 

 boat, and being set in motion by a steam-engine. The principle 

 is exactly the same as that on which a rowing-boat is propelled, 

 the water serving the purpose of a fulcrum ; not that it remains 

 absolutely at rest, but the reaction it produces on the surface of 

 the paddles pressing against it is sufficient to propel the vessel 

 with considerable speed. With a well-built vessel an average 

 speed of upwards of twenty miles an hour is easily attained. 



There is one circumstance which was at first found to cause a 

 considerable loss of power, and that was that when the paddles 

 were thus fixed to the wheel there was a great loss of power 

 when they entered the water, and when they left it, from the 

 fact of their surfaces not being vertical, so that only a portion 

 of their force was utilised. To remedy this defect, the floats of 

 the best paddle-wheels are now fixed on pivots, and by means 

 of an eccentric are made to move in such a way that while im- 

 mersed their surfaces are always vertical, and thus a much 

 larger portion of the power is rendered available. The paddle- 

 wheels are usually placed at the side ; vessels have, however, 

 been constructed with them at the stern, and these occupy rather 

 less room, and are more available for river navigation. 



For a river, or in perfectly smooth weather, a paddle- steamer 

 is the best, but in rough weather it labours under the great dis- 

 advantage that if the vessel be inclined one paddle acts more 

 powerfully than the other, and thus tends to twist the vessel 

 out of her course. In the same way waves interfere with the 

 regularity of the motion. It is also found that there is one cer- 

 tain depth of immersion at which the paddles act beet ; and if 

 the vessel be loaded so as to sink deeper, or be lighter, in either 

 case there is a considerable loss of rjower. 



The screw is free from these disadvantages, and is therefore 

 frequently used for steamers intended for long sea-voyages. In 

 screw- vessels, instead of a shaft across the vessel, to which the 

 paddle-wheels are fastened, there is one which runs lengthways 

 from the engine room, and to the end of this the screw is fixed. 

 This consists of two or three large blades twisted somewhat 

 after the plan of a common screw, and as this turns rapidly the 

 water acts the part of a nut, and the vessel is driven forward. 

 Of course the water does not remain fixed, any more than in 

 the case of the paddle-wheel, but, as there, the reaction is suffi- 

 cient to propel the vessel. 



It is an important thing to have the blade inclined at the 

 right angle, and screws have been so contrived that this inclina- 

 tion can be altered at pleasure, but these have not been used in 

 practice. The plan of having two screws side by side is adopted 

 in some large vessels. 



Experiments have been made with a new kind of propeller, 

 which acts upon an entirely different principle. The paddle- 

 wheel and screw are altogether dispensed with, and in their 

 stead the engine works some very powerful force-pumps. The 

 water from these is conveyed by large pipes and discharged at 

 the side of the ship, very near to the water-line. Two sets of 

 pipes are fitted up, fo that the water may be discharged towards 

 the stern or the stem, according to the direction in which it is 

 required to move, and the reaction of the water as it issues 

 serves to propel the vessel. 



This principle has not been at all generally adopted, and it is 

 therefore unnecessary to enter into a long discussion as to its 

 merits. 



A model steamboat, which acts on a similar principle, is fre- 

 quently constructed as a scientific toy. A small brass cylinder 

 is closed at each end, a small hole being drilled in one end, near 

 the circumference, for the escape of the steam. This boiler is 

 filled with water, and placed over a lamp. As soon as the 

 water boils, the steam issues with considerable violence from the 

 small hole, and, striking against the air, causes, by its reaction, 

 the vessel to move rapidly along. 



These, then, are the methods of propelling vessels ; but there 

 is another question closely connected with this, and that is, What 

 shape should be given to the vessel in order for it to meet with 

 least resistance in passing through the water ? This question 

 has attracted much attention from naval architects, it being an 

 important matter to attain the greatest speed in a vessel from 

 a given power of engines. We cannot, however, in the space of 

 these articles, examine the matter. We may roughly state, 

 however, that it is best to let the vessel gradually taper off to 

 the front, and that the shape of the fore-part of a fish, or the 

 beak and head of a bird, approaches somewhat to the form in 

 question. Of course, in considering this, the pressure of the 

 water on the surface has to be resolved along the surface, and 

 at right angles to it. 



The same rule applies in the action of rudders. If, as the 

 vessel is going along, the rudder be inclined to either side, the 

 pressure of the water on it may be resolved into two parts one 

 acting parallel to it, and therefore producing no effect ; the other 

 acting at right angles, and forcing the rudder, and with it the 

 stern of the ship, towards the other side. The effect is thus the 

 same as if the bow were inclined towards the same side as the 

 rudder, and hence the vessel turns that way. The tails of fishes 

 and birds act just like a rudder, and serve to guide them in 

 their flight. 



The motions of rivers and the phenomenon of waves and 

 tides are closely connected with the science of hydrostatics, 

 though usually treated of separately. We will, however, just 

 notice the principal facts, leaving it to the student to pursue 

 this subject in books treating more specially of it. A river is 

 a body of fresh water flowing down an inclined channel towards 

 the sea. Now it is evident that the velocity with which it 

 flows will depend, in the first place, on the degree in which its 

 bed is inclined ; but the nature of it, whether it is rocky or 

 not, and whether or not it curves about much, will influence the 

 speed to a considerable extent ; and according to the speed with 

 which it travels will be the effects produced on its channel. It 

 is found that a velocity of about one-third of a mile per hour 

 will carry along with it fine sand ; from two-thirds of a mile to 

 a mile will carry gravel and small stones ; a little greater speed 

 will carry along shingle ; while a speed of two miles and up- 

 wards will roll along stones almost as large as the fist. 



