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so high as the close air-vessel; it is sufficient if it reachesa little more than four feet above the level of the dash-board, the water, which it is designed to receive, notrising higher than this. In other respects, the structureof these machines agrees entirely with that of the singleones already described. It must be observed only that thecullenders of the lower machines should be, as nearly aspossible, of the fame dimensions with those of the upperones. For if they are of smaller bores, they will not admitof all the water which passes through the upper ones, sothat part of it must run to waste: if they are larger, thewater will pass off too fast, without producing its dueeffect. The regulators, described in page 312, are hereparticularly useful, affording ready means of increasing ordiminishing the apertures occasionally while the machinesare at work.

Of blowing machines with low falls of water.

The dimensions hitherto given are such as appear themost advantageous. Much lower falls, however, thanthose which the foregoing machines are calculated for, asten, eight, or perhaps seven feet, may be made to afford astrong blast. To produce such a compressure of the air inthe air-vessel, as to raise the gage four feet, a fall of aboutsix feet is necessary for the lower pipe. If the upper pipeis only about a foot and a half or two feet, the water,when divided by means of the cullender, will carry downa certain quantity of air j and though the quantity, froman equal stream of water, will not be so great as when thefall is higher, yet, as there are in many parts of the king-dom, large bodies of water running with such a descent,the deficiency may be compensated, as already takennotice, by enlarging or multiplying the machines.

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