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For many purposes still less falls will suffice. Thesmiths bellows, as we have formerly seen, raises the gageonly about fourteen inches; and such a compressure, it ispresumed, may be gained from a fall of five feet or less.Small falls may be applied also to another purpose, of nolittle importance, the ventilation of mines and coal-pits, orthe driving in of fresh air, in the room of that, which themineral vapours have rendered unwholesome or pernicious.

In all these machines it must be observed, that theheight of the column of water, falling through the pipe,determines, not the actual force of the blast, but thegreatest force which can be given it in that machine; thatthe height of the gage is always the measure of the actualforce; that this force depends on the width of the pipethrough which the air is discharged from the air-vessel,and may be diminished, or increased in any degree up tothe greatest that the column of water can resist, by widen-ing or narrowing the aperture of the pipe; that differentmachines will give- blasts of equal force through pipes ofgreater or less width, according to the greater or less quan-tity of air which the water carries down with it; and thattherefore the size of the blast-pipe must be adjusted by trialfor each particular machine.

The distance of the dash-board under the pipe maylikewise admit of some variation, and require to be re-gulated according to the size of the pipe. In some of thecommon machines, this distance is three or four feet ormore; but so large a space is apparently a disadvantage;for .so much of it, as is more than sufficient for the freepassing off of the water, is entirely useless, being, in effect,so much taken off from the height of the fall. Thedistance of six inches, laid down in the foregoing machines,,is designed for a circular pipe of twelve inches diameter;,in which cafe, the area, by which the water is discharged

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