14 Mr. Seaward's Description of a Hydro-pneumatic Pump 



pressure of 30 atmospheres, which cannot be forced out, and 

 which, as the piston recedes for the return stroke, will expand 

 in the barrel, and occupy a great part of the space ; thereby 

 preventing the admission of another full charge. 



And this is one of the great defects of this sort of pump ; 

 for, allowing the space to be one quarter of an inch, it will 

 be just one forty-eighth of the whole capacity of the pump ; 

 and, adding to this the space left by the rising of the eduction 

 valve d, which will remain open until the piston has receded 

 a little in the return stroke, we may doubtless presume that 

 a portion of compressed gas, equal in volume to one-fortieth 

 of the whole stroke of the pump, remains behind every time 

 in the barrel : therefore, when the pump commences working, 

 and the gas in the receiver arrives at a pressure of 10 atmo- 

 spheres, only three-fourths of the gas is forced out of the 

 barrel ; — at 20 atmospheres, one half; — at 30 atmospheres, 

 one quarter; — and when it arrives at 40 atmospheres, the 

 pump will cease to act, as the compressed gas which remains 

 will expand itself, and fill the whole barrel : therefore no 

 more gas can then be admitted from the gasometer. More- 

 over, there is an actual loss of gas occasioned by the leaking 

 of the piston, which is a failing that these pumps are all more 

 or less liable to ; for, whether they be packed with metallic 

 rings, cupped leathers, or hemp packing, still there will be some 

 escape under this great pressure : and if the leathers &c. are 

 screwed up so hard as totally to prevent the escape of the gas, 

 the friction will become immense, consequently one-half the 

 power will be absorbed, and thus very little advantage would 

 be gained by the remedy. 



These observations will, it is presumed, place the defective 

 operation of the common forcing pump in a clear point of 

 view, and will naturally lead us to comprehend the advantages 

 of the hydro-pneumatic pump. It will have been observed, 

 that the great evil in the common pump is the space or cavity 

 that is left when the piston is down at the bottom of the 

 stroke. Now the remedying of such evil is the primary object 

 sought for in this improvement; for this purpose a quantity 

 of non -elastic fluid is introduced into the chamber of the 

 pump, which, filling up the whole of the cavity when the pis- 

 ton is down, necessarily forces out every particle of the com- 

 pressed gas ; the method of accomplishing which will be 

 readily understood in the description of the new pump. 



Fig. 2 is an elevation, and fig. 3 a plan of the hydro-pneu- 

 matic pump. A A is a frame for supporting the machine : the 

 pump consists of two chambers B and D ; in the chamber B 

 works the solid plunger C, through a cupped leather (r>), by 



