H \ DR08TATI08. 





(a.) The imlin-iit object precedes the direct object, when tbo 

 meaning would otherwise be doubtful : - 



/. do ruuiuuor par U 



douceur oes eaprits egarea. 



Try to brfef bod, by 

 tfeM tiring tpirit*. 



Any otlit-r rimwtruotiim would render the Mntenoe equivocal. 



HYDB08TATICS \l. 



FIBI-KNUINK CENTBIFUQAL PUMP8 MACHtHU FOB PEO- 

 PKLLINO VK88KL8 &IVIB8 POWCB 8TOHKD UP IN KA1N 

 s WAVB8. 



1 Uknuwn fire-engine is merely a combination of two tingle- 

 force- pumpa. Those are made of a large diameter, and 

 ;;u water into a (strong air-chamber, from which the hone 

 to convey tho water to the fire issues. A lever in supported by 

 its centre point juat above the machine, and the cross pole* by 

 which tho firemen work the engine are fastened to the end of 

 thin, tii,- piston-rods being attached about the middle of each 

 arm. Hence, since several men work at each aide, and there is 

 a gain from the leverage, the water is driven with great force, 

 . !i be thrown to a considerable height. The air-chamber 

 is mode very strong, and of large dimensions, so that the stream 

 of water flows at a constant rate ; and though the pumps are 

 only single action, yet as one piston is rising while the other is 

 falling the motion is nearly uniform. 



In mining operations pumps have to be employed on a very 

 large scale. The water finds its way through the cracks and 

 crevices, and would soon entirely flood the mine were not largo 

 and powerful pumps erected. In some 

 coses the weight of water raised is as 

 much as twenty times that of the coal. 

 The depth, too, is usually very great, and 

 thus it is quite impracticable to raise the 

 water at one lift, for the pressure on the 

 pipes would be so great as to burst them. 

 A number of small cisterns are therefore 

 placed at about equal distances above 

 one another. From each of these the 

 water is raised by a force-pump with a 

 solid plunger to the one above. All 

 these plungers are fixed to one pump- 

 rod, which passes from the top to the 

 bottom of the mine, usually in a part of 

 the shaft separated for the purpose. A 

 powerful engine at the top lifts this rod 

 with all the plungers, and they fall by 

 their own weight. The plans pursued 

 are, however, very various, and depend 

 very much upon the nature of the mine 

 and the special peculiarities of the case. 

 Sometimes, when the mine is sunk on the 

 side of a hill, the water can be discharged 

 by a side channel some way below tho 

 mouth, and where there is a stream of 

 water at the mouth of the pit this is 

 made to pass down a pipe to the level 

 of the side channel, and there by its 

 pressure to raise the water from the 

 bottom. The action of this apparatus 

 is very ingenious, but it can rarely be 

 applied, and need not, therefore, be ex- 

 plained here. 



There is one other apparatus, known 

 as Hero's Fountain, which raises water 

 by the compression of the air. This is represented in Fig. 39. 

 The tray at the top is filled with water, which rushes down the 

 pipe u, and thus compresses the air in the lower vessel, N ; this 

 compressed air escapes by the pipe A, and pressing on the liquid 

 in the upper reservoir, M, causes it to issue from the jet. This 

 apparatus ceases to act as soon as the lower reservoir is filled, 

 and the water must then be drawn off by the tap seen under w. 

 We have in our previous lessons described the construction 

 and mode of operation of those machines for raising water which 

 act mechanically, or by means of atmospheric pressure. These 

 embrace by far the greater portion, and we have now only to 

 look at those which act by centrifugal force. 



It may, perhaps, be thought ratlu 



construction of ao many BMhiBM.aU of which i 

 purpose; hot they each have their special peculiarities, which 

 render one or other of then the more td vantageou, according 

 to the special oiraamattBOM for which they are r*qired ; and 

 aa erne of them are in use in almost every hoaaa aad factory, 

 it Li aurely well to understand their mod* of actioa. 

 very good ml* in everything to try aad nilaMtanil the ream, 

 and not to be satisfied with the bare fact that the tfci&f 

 U ao. 



The plumber who understands the principle on which aay 

 pomp act* will be far more likely to anoeead in Us btuBiaess 

 than the one who merely work* by routine. The latter is 

 baffled by anything unusual or uncommon, while the other. 

 ainee he understands the principle on which he U working, tonai 

 master* the difficulty. The aame role applies hi all other 

 mattera. 



We have, then, to consider now those pomp* which Ml by 

 centrifugal force. In our lessons on Jir>k- t rt we gave aa 

 explanation of the action of this force, aad we eaw then that it 

 is merely a manifestation of the inertia of matter. A atone, for 

 instance, when whirled round by means of a string, tends at 

 every moment to fly off at a tangent, that is, to continue in the 

 line in which it was moving at the instant, instead of being bent 

 round in a curved path. 



In the aame way, if a tube be filled with water, aad swnag 

 round rapidly, the water will be thrown out < 



The apparatus usually employed to illustrate the 

 of this principle to pumpa is represented in Fig. 40. 



An upright apindle, c, is fixed ao that it can be tamed rapidly 

 by means of a multiplying wheel. To this apindle are fixed 

 two tubes, A A, open at each end, but at the top bent outwards 

 and downwards, BO that the water which issues from them is re- 

 ceived in the ring-shaped trough, B. As those tubes are rapidly 

 rotated with the axis, the water in the upper portion of them is 

 thrown off, by centrifugal force, into the trough. This i leetee 

 a vacuum in them, which is at once filled from the reservoir 

 into which they open, and thus a continual stream is produced. 

 The amount that could be raised 

 by these pipes is, however, far 

 too small to admit of the machine 

 being practically employed in this 

 form. 



One of the simplest forma of 

 a centrifugal pump consists of 

 a circular disc fixed on a shaft. 

 Attached to each side of this disc 

 are a number of partitions radi- 

 ating from the centre ; these are 

 made of the same height through- 

 out, so that the whole may revolve 

 between two fixed discs or cheeks 

 which the partitions nearly touch. 

 These cheeks form the ends of 

 the cylinder into which the exit- 

 pipe opens, and are cut away at 



the centre so that the water may Fig. 40. 



enter there ; and as the wheel re- 

 volves it is thrown off against the aides with such force that it 

 will rise in the exit-pipe to a considerable height. 



The size of the wheel is usually about a foot in diameter, that 

 being quite sufficient when it is rotated rapidly, and it is found 

 that when the partitions are made to curve to the right degree 

 nearly three-quarters of the power of the engine may be utilised. 

 If, however, the partitions are made straight, only about a 

 third of the power can be obtained. The great advantages of 

 this pump are the absence of valves, the small space in which 

 it may be made to work, and also the fact that it supplies a 

 continual stream. 



Another mode in which the pump is constructed will be under* 

 stood by reference to Fig. 41. The lower is the suction-pipe, 

 and the upper that into which the water is forced. These open 

 into the outer cylinder, which is fixed, and inside this there re- 

 volves another cylinder, A A, which baa longitudinal slits made 

 in it, in which are inserted the partitions o c. These partitions 

 can slide in and out, but are kept fully out by means of the 

 inner curved surface, except when they approach the dmsioa 

 which separates the suction and exit-pipes, when the other 



