HYDRAULICS. 



If the soil through which it passes be close 

 and retentive, the water will then be oc- 

 casionally raised, as well as lowered, in 

 proportion to the weight of the incum- 

 bent fluid, and will rise, if so guided by 

 the channel through which it passes, even 

 to the height of the source, as may be 

 proved by what has already been shewn 

 in fig. 2. Thus, after various changes of 

 altitude, the fluid may escape at any 

 height not above that source ; or it may 

 be carried away to any depth. The place 

 where it issues forth is called a spring. 

 Fig. f . exhibits such a current, which we 

 will suppose to have a perpetual supply. 



But the intermitting spring may also 

 have a regular supply. This is occasioned 

 by the existence of caverns connected by 

 syphons, as we may see by reference to 

 fig. 8, where A is the source, b b the chan- 

 nel : B is a cavern, which by means of the 

 arch, or rising channel, c c, becomes a 

 syphon leading into D. It is obvious that, 

 in the first instance, the water must, after 

 fillingB. rise in thejchannel, b 6, so as to be 

 above the greatest height of c c, to cause 

 its passing off into E, and thence ad libit- 

 um. Now the channel, c c, being of great- 

 er diameter than the channel, b b, when 

 the fdrmer commences its operation, it 

 will discharge more than the latter can 

 supply, so as to keep up the discharge 

 from c c; therefore, after B has been ex- 

 hausted so far as to allow air to pass from 

 it into c c,a certain quantity in that chan- 

 nel, which has not gained the summit, 

 will recede into B, and the water must 

 again rise to the height in b b, which 

 shall cause it to flow over the summit of 

 c c, before the spring can again appear 

 to be supplied. Yet the flow from the 

 source-was never diminished. 



The existence, or otherwise, of a va- 

 cuum, or void space, was long agitated, 

 and that too with no small degree of acri- 

 mony, among the philosophers of old ; and 

 we may say of a date by no means an- 

 cient. Common sense should have told 

 us, what experience so amply proves, that 

 where one body or element retires, an- 

 other must supply its place, else the 

 whole creation would inevitably be torn 

 asunder. It is, indeed, well known, that 

 the elasticity of the air, which could be 

 rarified ad infinitum, if we had the means 

 of effecting the process, enables it to oc- 

 cupy large spaces on emergency, or to 

 contract within the narrowest bounds. 

 See PJTEUWTATICS. Under ordinary cir- 

 cumstances, however, we consider the 

 air as being of a particular standard. 



namely, that a column ascending to the 

 summit of our atmosphere corresponds 

 in weight with a column of water of thir- 

 ty-three feet in height, allowing the bas- 

 es, i. e. of the air and of the water, to be 

 equal. Thus we find that where the air 

 is withdrawn, by means of suckers, pis- 

 tons, valves, &c. from within a pipe, of 

 wiiich the lowest part is immersed in the 

 water Contained in a well, &c. the fluid 

 will rise to the height of thirty -three feet 

 within the pipe, supplying the place of 

 the air thus withdrawn. This is effected 

 by the pressure of the atmosphere on the 

 surface of the water; whereby it is forc- 

 ed into the space formerly occupied by 

 the air. Generally speaking, it is not a 

 sudden operation ; for unless the well be 

 very shallow, it will require many strokes 

 of a pump to withdraw so much air as 

 may so far rarify the residue within the 

 pipe, as to allow the water to rise thirty- 

 three feet above its level. This is the 

 greatest height to which water can be in- 

 duced by a sucking pump. In this con- 

 trivance the piston, A, see fig. 9, has a 

 valve, which, as the rod draws up, is 

 closed by the pressure of the air above it ; 

 but in descending it opens, and allows the 

 water, which had flowed into the lower 

 part, whence the air was withdrawn, to 

 rush through; as the piston is raised 

 again, the weight of the water forcibly 

 oppresses the valve, until it finds a lateral 

 passage at B, whence it issues, and in 

 this manner any quantity may be raised. 

 If the water has a direct issue, as in the 

 common spouts of pumps, no further ap- 

 paratus is wanted; but if it is to be re- 

 tained, or pass through any other pipes 

 more elevated than the debouchure, B, 

 there must be a small angular projec- 

 tion, as shewn by the dotted lines, to 

 admit the valve C, also pointing up- 

 wards. In dry weather, or when the 

 pump is not much used, the leather bind- 

 ing of the piston, as also the valves* will 

 become dry ; therefore it is necessary, on 

 such occasions, to throw in a pail-full or 

 two of water, to moisten them ; else the 

 air will pass downwards as the piston ris- 

 es, and prevent that exhaustion on which 

 the ascent of the water depends. It is ge- 

 nerally necessary to have a valve at the 

 bottom of the pipe, to keep in the water 

 drawn into it, in order that the labour 

 may be decreased ; and that, if the pump- 

 ing be intermitted, there may be less 

 trouble in bringing up the water within 

 reach of the piston. 

 Where the water lays near the surface, 



