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outer box, and in great measure prevents the air fromescaping between them.

Though the wooden bellows have an advantage abovethe leather ones of being less expensive and more durable,they have considerable defects ; for it is scarcely possibleto make the junctures-so tight as to allow no exit to thecompressed air, and the friction must necessarily be verygreat. Some have therefore had recourse to water, fordoing the office of the under board of the bellows. Abellows on this principle is described by Mr. Triewaldin the Philosophical Transactions, and I have beenfavoured with descriptions and drawings of two singularones, now used at some of the iron works in this king-dom, one for the finery, the other for a large iron furnace,in which the fuel is coak, and which requires the greatestforce of air of any known kind of furnace. An account ofthese will be given hereafter.

There is another method of applying water, so as toproduce a strong blast, by means more simple than anyof the foregoing, and at little expence. A stream ofwater, falling through a pipe, in certain circumstances,carries air down with it ; and this air, afterwards disen-gaged from the water at the bottom, may be so collected,as to have no other vent than a pipe which shall carry itto the furnace.

Machines, constructed on this foundation, though littleknown among us, are used in different countries, insteadof bellows, for animating the large furnaces. But theirstructure and principles of action have hitherto so littleundergone a scientific examination, that those, whichhave been sound to answer the best, may be presumedto owe their excellence merely to chance, and that theworkmen have often laid the greatest stress upon the pro-portions of parts which are inessential. These machines4 are