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effectual in cases where the quantity or fall of water wouldotherwise be insufficient.

Of cons rufting blowing machines with falls of water of

great height.

Where the height of the fall is great, the quantity ofwater is usually small; and in all the ways of applicationthat have hitherto been contrived, the height will by nomeans make amends for the deficiency in quantity.

In the common construction of these machines, wherethe upper pipe or funnel is no more than three, four, orfive feet high; though the fall ffiould be such as to admitof the lower pipe being thirty or forty feet or more, it doesnot appear that any material advantage could result fromsuch a height. For, as the air is admitted into the wateronly at the top of this long pipe, it cannot, I think, be sup-posed, that the quantity admitted will be the greater forthe length of the passage under the place of its admission.Water indeed has been found by Mariotte to run faster,through an upright long pipe, than through a short one:^quantity of water which was forty-five seconds in runningthrough a pipe three feet long, was discharged in thirty-feven seconds, or near a lixth part less time, through a pipeof the fame bore and a double length; so that as morewater passes successively through the long pipe thanthrough a short one, in equal times, more air also must becarried down by it. But in the cafe which we are hereconsidering, no benefit can be expected on this principle,*for as the supply of water is supposed to be limited, thebore of the pipe must necessarily be made less, in pro-portion to the increase which its length may produce inthe velocity. If the lower pipe is of such height, that thewatery column it contains may sufficiently resist the forceof the compressed air in the air-vessel, it should seem that

any