

HYDRAULICS. 



THIS? science, which may be considered a branch of hydrostatics, treats of 

 liquids in motion. 



The particles of liquids flow over and amongst each other with less friction 

 than over solid substances, and all the substances gravitate independently. If a 

 hole be made in the bottom of a vessel the liquid will flow out ; those particles 

 directly over the orifice being discharged first, their motion causes a temporary 

 vacuum, into which the particles tend to flow from all directions ; and thus the 

 whole mass of the water is set in motion (fig. 1). As water flows through 

 bended pipes to the same level from whence it proceeds, it enables us to form 

 jets or fountains (fig. 2). If, for example, a body of water be collected in a 

 reservoir on the upper part of a house, and a tube descending from it to the 

 garden be made to turn upwards, having a very small bore, the water will spurt 

 up in a jet to nearly the same height as the surface of the water in the reservoir. 

 It will not rise quite so high on account of the resistance of the air, and the 

 effect of gravitation. Most of the ornamental fountains in public gardens are 

 formed upon this principle. 



Pumps and Machines for Raising Water- These may be divided 



into four classes. First, those machines in which water is lifted in vessels by the 

 application of some mechanical force to them. The earlier hydraulic machines 

 were constructed on this principle ; such as the Persian wheel (fig. 3), consisting 

 of upright buckets attached to the rim of a wheel moving in a reservoir of water ; 

 the buckets are filled at bottom as they pass through the water, and emptied at 

 top ; the water is thus raised to a height equal to the diameter of the wheel: 

 The Archimedean screw (fig. 4), and the chain pump (fig. 5), are modifications of 

 the same principle. 



The second class of machines are those in which the water is raised by the 

 pressure of the atmosphere, and comprises all those machines to which the name 

 of pump is more particularly applied. These act entirely by removing the pres- 

 sure of the atmosphere from the surface of the water, which may thus be raised 

 to any height not exceeding thirty-two feet. 



Fig. 6 represents the common pump, which consists of a cylinder, with an 

 air-tight piston, h'aving a valve, A, opening upwards. When the piston is raised 

 a vacuum is raised in the tube beneath, which is immediately occupied by the 

 water ; on depressing the piston, the water passes through the valve, -A, in its 

 centre, and on being raised, the water is lifted to the top of the barrel, and flows 

 through the spout. To prevent the water returning to the well when the 

 piston is depressed, a valve (B) opening upwards is placed near the bottom of the 

 pump. 



When it becomes necessary to raise water to a greater height than thirty-two 

 feet, the third class of machines, or those which act by compression on the water, 

 usually by the intervention of compressed air, are employed. All pumps of this 

 description are called forcing pumps, and by these water may be raised to any 

 height by applying sufficient force. 



Fig. 7 represents the forcing pump, consisting of two parts, or barrels, one 

 similar to the common pump, and the other rising by its side. The water is first 

 raised in the former part in the same manner as in the common pump, excepting 

 that the piston has no opening valve, but is solid, and the air is forced out 

 through the valve, A, into the adjoining barrel. Through this valve the water is 

 also forced, and the pressure of the descending piston causes it to flow up the 

 ascending pipe and issue out of the top. The vessel in which the lower end of 

 the ascending pipe is placed, encloses a volume of air ; when the water rises this 

 air is compressed, and being elastic it re-acts upon the water, thus causing it to 

 flow upwards with greater force. 



The fire-engine is an interesting example of the utility of this machine ; its 

 principle will be readily understood by reference to fig. 8. A is the suction-pipe 

 by which water is supplied from the street main ; B B are two valves opening 

 upwards into the barrels of two forcing pumps, containing solid pistons ; from 

 the lower sides of the pump-barrels proceed force-pipes, which communicate 

 with an air-chamber, c c, by valves, D D, opening upwards into it. Through the 

 top of the air-chamber descends nearly to its bottom a pipe. E E. vo tjie uppeij 

 part of which is attached the hose for directing a stream of watt-;i on thjp^re.. By 

 the alternate action of the pistons, water is drawn through the' valfes *B*B, *aad 



