Air- Pump in England, 341 



leaning against it (for the description is not precise on this 

 particular), moved his feet alternately up and down, as a 

 handloom- weaver does, or a culprit on the tread»-mill. The 

 pistons or suckers GG had valves ^ probably of bladder), 

 opening upwards like that of an ordinary water-pump, and 

 similar valves were placed at the bottom of the cylinders, 

 which were filled with water to a certain height, that the pis- 

 tons might move air-tight in them. From the cylinders, tubes 

 H H passed to a common canal I, terminating in the air-pump 

 plate K, on which receivers L to be exhausted were laid, 

 as in Boyle's second engine. 



The advantages of Papin's arrangement were very great. 

 When a single pump is used it becomes increasingly difficult, 

 as the exhaustion proceeds, to draw out the piston against 

 the pressure of the external air, which comes towards the 

 end to oppose an unresisted force equal to nearly fifteen 

 pounds on each square inch, to the extrusion of the piston. 

 When the piston, on the other hand, is pushed home, it is 

 driven into the barrel with the same force which resists its 

 withdrawal, and is liable to break the valves or injure the 

 bottom of the cylinder. But if the piston-rods are balanced 

 against each other as they were in Papin's machines, so that 

 the one ascends as the other descends, the evils described are 

 obviated. The resistance which the air offers to the ascent 

 of the one piston, is balanced, or nearly so, by the force with 

 which it compels the other to descend, so that the two hang 

 against each other almost in equilibrio. A very slight expen- 

 diture of force, accordingly, little more than is requisite to 

 overcome the friction of the moving parts, suffices for the 

 working of the pump. A double-barrelled air-pump not only 

 exhausts twice as expeditiously as a single-barrelled one, but 

 does double work for nearly the same expenditure of force. 



It may almost be regretted that Papin's and other similar 

 two-barrelled pumps, had not been designated by another 

 name. The title given to them is apt to convey the idea that 

 they excel single-barrelled air-pumps, in the same way that a 

 double-barrelled gun does a single-barrelled gun. In the latter 

 case, however, I need not say, a double eff^ect is gained only 

 at the expense of a double expenditure of time and force in 



VOL. XLVl. NO. XCII. — APRIL 1849. Z 



