HYDRAULICS. 



the air-vessel through the pipe x, a va- 

 cuum will be formed in the upper part 



fiff- 12. 



of the working-barrel, and this is sup- 

 plied by water through a second feeding- 

 pipe c also descending into the well, and 

 having a stop-valve d applied to it in a 

 chamber or cavity e formed for that pur- 

 pose ; the upper part of this second 

 suction-pipe opens into the top of the 

 working-barrel above the greatest height 

 to which the piston can ascend, and thus 

 by its descent is that part of the barrel 

 which is above the piston completely 

 filled with water, while the lower part of 

 it is emptying ; and when the piston as- 

 cends again, all the water that has been 

 so deposited above it, is forced up the 

 pipe/ into the same air-vessel y. The 

 pipe / is likewise closed at its upper 

 end by a valve opening upwards to pre- 

 vent the return of the water when the 

 piston descends ; and thus by the alter- 

 nate action of one piston ' moving in 

 one barrel is all the beneficial effects of 

 two pumps produced with the friction 

 of only one. 



Since it is impossible, when a pump 

 is well made and is in good order, that 

 its piston can move without displacing 

 the water that is above or below it, ac- 

 cording to the circumstances of its con- 

 struction, so in all pumps that consist 



of cylindrical working-barrels and pis- 

 tons, nothing more is necessary to as- 

 certain the quantity of water they will 

 deliver, than to calculate the solid or 

 cubical contents of that part of the bar- 

 rel in which the vacuum is produced, 

 and to reduce it to some standard 

 measure, and then to multiply this by 

 the number of strokes made in a given 

 time : thus if a pump is nine inches 

 diameter, and makes an effective stroke 

 of about eighteen inches, such a cylin- 

 der will be found to contain about 1 134 

 cubic inches, and as 27 7^ cubic inches 

 make an imperial gallon, so four gal- 

 lons will be equal to 1109 cubic inches ; 

 consequently such a barrel will contain 

 and throw out rather more than four 

 gallons at every stroke, and supposing 

 this pump to make ten strokes in a 

 minute, it would yield above forty gal- 

 lons in a minute, or sixty times that 

 quantity in an hour, and so on. This 

 rule applies in every case, whether the 

 water is sent to a small or great eleva- 

 tion, because the piston cannot move 

 without displacing the water in the bar- 

 rel ; but a small allowance must be made 

 for leakage or waste, because some 

 water will constantly pass the piston 

 and escape, or be otherwise lost and 

 wasted. 



This mode of calculation, as before 

 observed, only applies to such pumps 

 as have cylindrical working-barrels and 

 pistons, but sometimes pumps are other- 

 wise constructed, of which the fire-en- 

 gine of the late Mr. Bramah, and the 

 excentric pump are instances. In the 

 former of these contrivances, the work- 

 ing-barrel, instead of being an entire 

 cylinder, is a semi-cylinder, and lies ho- 

 rizontally, while the place of a piston is 

 supplied by a parallelogram of the same 

 radius and length as the semi-cylinder 

 moving by an iron bar passing through 

 its axis, and properly packed at its ex- 

 terior edges. This parallelogram is 

 made to vibrate through about 170 de- 

 grees by its handles, while its outer edges 

 keep in contact with the interior surface 

 and ends of the semi-cylinder, and two 

 feeding and two delivering valves are 

 placed upon the flat top or covering of 

 the whole. This pump, therefore, in 

 effect is the same as that of M. de la 

 Hire last described, though quite dif- 

 ferent in form, arid its mode of opera- 

 tion is nearly allied to The Excentric 

 Pump, a section of which is shewn at 

 fig. 13. It consists of a hollow drum 

 or cylinder of metal a d, in the interior 



