THE PUMP.] 



MECHANICAL PHILOSOPHY. PNEUMATICS. 



773 





spheric pressure diminished by the weight of the column 

 L B ; the two pressures, therefore, balance. But as 

 soon as D is passed, and the column ill B C lengthens, 

 the equilibrium is destroyed, the downward pressure 

 prevails, and the liquid falls through C upon finding a 

 passage there : and the longer the part D C of the 

 syphon below the surface D of the liquid, the greater 

 will be the velocity with which it will issue, since the 

 greater will be the preponderating pressure downwards. 



In the second way of preparing the syphon, that is, by 

 first filling it witli the fluid, it is brought, in another 

 manner, into the same circumstances as when it is filled, 

 as above, through the pressure of the air on the surface 

 of the liquid. The liquid will evidently cease to flow as 

 scon as the surface of it descends to a level with the 

 rising surface in the vessel to which it is transferred ; at 

 this stage, however, the syphon will remain full, as there 

 will be a complete equilibrium. 



In the syphon just explained, it is necessary that the 

 legs be of unequal length ; but there is another kind of 

 syphon called the Wurtemburg syphon, in which the 

 two legs are of the same length. In this instrument (see 

 Fig. 195), the extremities of the equal legs are turned 

 rig. iw. upwards so that the two open 



ends D, E may be on the hori- 

 zontal level when the syphon 

 is held upright. The instru- 

 ment is kept constantly filled 

 with water, which remains sus- 

 pended in the two legs DAB, 

 ECU, because the equal atmo- 

 spheric pressures at D and E 

 place them in a state of equili. 

 brium ; but if the leg termi- 

 nating in D be immersed in 

 water, the end D will sustain 

 the additional pressure of the 

 water reaching from D to the 

 surface, and consequently E 

 being free from such addi- 

 tional pressure, the fluid is 

 When the surface of the water in 

 the vessel has descended to D, the stream from E stops, 

 but the syphon remains full, and, thus filled, is taken 

 out and hung up by a loop nt B till again wanted. 



INTERMITTING SPRINGS. The curious pheno- 

 mena of intermitting springs, are referable to the fore- 

 going principles. These springs, issuing from a fissure 

 in a mountain side, flow for a certain period and then 

 stop ; after awhile the water flows again, and so on. 

 These effects are produced through the operation of one 

 of Nature's syphons. Fig. 196 exhibits a section of the 

 mountain and the stream. A cavity exists in the former, 

 as here represented ; this is filled by gradual infiltrations, 

 or by slender fissures communicating with the upper 

 surface. 

 Suppose, now, that there is a syphon-like communi- 



Fig. 196. 



forced out at that end. 



cation between the reservoir of water, supplied by these 

 channels from above, and the mountain side ; as this 

 syphon is not artificially exhausted of air, it will not 

 deliver the water in the reservoir till the surface of that 

 water rises as high as the bend of the syphon B ; after 

 which it will begin to flow, pouring out a continued 

 stream till the reservoir is emptied, or at least till the 



Fig. 197. 



level reaches the immersed end of the syphon ; tho 

 stream will then stop, and will commence to How again only 

 when, by a fresh accumulation of water from, above, 

 the level of B is again reached. 



It is, of course, a condition necessary to the production 

 of these results that the surface of the reservoir be sub- 

 jected to the pressure of the atmosphere ; but through 

 the fissures which supply the cavity with water, air must 

 have previously found its way ; and, indeed, from 

 the principle of the equal transmission of fluid-pressure, 

 the weight of the atmosphere acting on the upper sur- 

 face of the water in a single downward column, as on 

 ihe water in the slender perforation to the right in the 

 foregoing figure, acts equally on every part of the sur- 

 face of the water in the reservoir. 



THE HOUSEHOLD PUMP. The common suction- 

 pump is another important contrivance, which owes its 

 usefulness entirely to the pressure of the atmosphere, 

 although its dependence upon this agency was little sus- 

 pected for many ages after its invention. As already 

 noticed (page 769), Torricelli, a pupil of Galileo, early 



in the seventeenth cen- 

 tury, was the first who 

 fully recognised the in- 

 fluence of atmospheric 

 pressure. He accounted 

 for the ascent of water 

 in the pump, and of 

 mercury in an exhausted 

 tube ; and the inven- 

 tion of the barometer 

 naturally followed. 



Fig. 197 represents a 

 sectiou of the suction- 

 pump ; A B is the tube, 

 or working pump-barrel, 

 communicating with the 

 water in the well. In 

 this barrel, an air-tight 

 piston C, moved by the 

 lever or pump-handle P, 

 freely p'ays. At tho 

 lower extremity of the 

 working barrel there is 

 a valve V opening up- 

 wards : this valve sepa- 

 rates the barrel from, 

 and completely covers, what is called the suction or 

 feeding-pipe, which is usually of smaller bore. In the 

 piston C there is also a valve similarly opening upwards. 

 The piston C is called the sucker. 



Now, imagine the piston C to be at first at the bottom 

 V of the barrel, and then to be raised by the action of 

 the pump-handle. As the piston is air-tight, the pres- 

 sure of the air in the barrel upon the ascending valve C 

 keeps it closed, so that that air, having no escape below, 

 is forced up and pumped out at the spout S. The 

 vacuum thus produced in the barrel is immediately 

 filled up by the ascent of the water through the feeding- 

 pipe, for the water in the well sustains the pressure of 

 the atmosphere on its surface all except that portion of 

 the surface which the feeding-pipe covers ; and from 

 this portion, as just explained, the air has been with- 

 drawn. The barrel is thus filled with the water forced 

 up by the pressure of the atmosphere on the exposed 

 surface of iliat in the well ; but no water escapes through 

 the spout S, since all that has been raised is below tha 

 closed valve C ; but it is retained suspended in the barrel, 

 though the valve V still remains open, the upper surface 

 of the water, immediately below the piston, being, of 

 course, not more than 32 or 33 feet above the surface of 

 the water in the well (page 769). 



Upon now lowering the piston, the val va C opens by 

 the resistance of the water confined in the barrel ; for the 

 downward pressure communicated to the confined water, 

 and exercised through the lever P, closes tho valve V, 

 and prevents the water from being, by this extra pres- 

 sure, forced back again into the well. Hence, upon 

 again raising the piston, the upward resistance of tha 



