352
Pressure Engines.
[Book III.
CHAPTER IX.
Pressure engines: Öf limited application—Are modifications of gaining and losing buckets andpumps—Two kinds of pressure engines—Piston pressure engine described by Fludd—Pressure enginefrom Belidor—Another by Westgarth—Motive pressure engines—These exhibit anovelmode of employ-inff water as a motive agent—Variety of applications of a piston and cylinder—Causes of the ancientsbeing ignorant of the steam engine—Secret of making improvements in the arts—Fulton, Eli Whitney and Arkwright—Pressure engines might have been anticipated, and valnable lessons in Science maybederived from a disordered pump—Archimedes—Heron’s Fountain—Poidable ones recommended inFlowerGardens and Drawing-rooms, in hot weather—Their invention gave rise to a new dass of bydrau-lic engines—Pressure engine at Chemnitz —Another modification of Heron’s fountain—Spiral pumpof Wirtz.
Pressure engines, named by the French Machines ä colonne d’eau,form an interesting variety of hydraulic devices belonging to the presentdivision of the subject. They consist of working cylinders with valvesand pistons, and resemble forcing pumps in their construction, but differfrom them in their Operation; the pistons not being moved by any exter-nal force applied to them through cranks, levers, &c. but by the weightor pressure of a column of water acting directly upon or against them.Pressure engines are not very common, because they are only appli-cable to. particular loc.ations—such as afford a suitable supply of water forthe motive column; but wherever refuse, impure, salt or other water canbe obtained from a sufficient elevation, such water may be used to raise aquantity of fresh by these machines.
In some forms pressure engines appear rather complicated, but whenUnalyzed, the principle of their action and mode of Operation will be fotmdextremely simple :—If two buckets, partly filled with water, be suspend-ed and balanced at the ends of a scale beam, and a stream be directed intoone of them, that one will preponderate, and consequently the other withits contents will be raised, and to a height equal to the descent of theformer; but when it is required to raise water in this mar.ner to an eleva-tion that exceeds the distance through which the descending vessel canfall; then the capacity of the latter is enlarged, and it is suspended nearerto the fulcrum or centre on which the beam turns, as in the gaining andlosing bücket, page 66 :—It is virtually the same principle that is em-ployed in pressure engines ; the dilference is principally in the manner ofperforming the Operation. Instead of vessels suspended as above, twosolid pistons, moving in cylinders, are attached by rods or chains to theends of a beam, or to the ends of a cord passed over a pulley, so that thepressure of a longer or heavier column of water resting upon one pistonforces it down, and thereby raises the other and with it the lighter orshorter column reposing upon it.
By referring to the 16th illustration on page 64, it will be apparent thatif a cylinder extended from B to the top of the cistern Z, and a hollowpiston like the upper box of an atmospheric pump fitted to work in itwere substituted for the bücket B, the effect produced would be much.the same as with the two buckets, for the same quantity of water could beraised through the cylinder into the cistern Z, if allowance were made for