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double, which is the most valuable Consideration in an Engine, where the Pipes will sustain such Force.
Secondly, The Bores that cart off the Water from the Forcers are too small, there being (nearly) always two Pillars of 7 Inches Diameter, forcing into one Pipe of the same Diameter, and 7 x 7 = 49 + 49 = 98.
Therefore those Pipes of Conveyance should be near nine Inches Diameter.
The Perfections of the Machine.
The Timber-work is all admirably well performed, and the Composition and Contrivance, for Strength and Usefulness, not exceeded by any I have seen.
The cast Iron Cranks are better than wrought ones, by reason they are very stiff, and will not be strained, but sooner break; but then they are cheap, and new ones easily put in.
The Wedge for putting on or releasing the Crank and Forcers, is better than the sliding Sockets commonly used.
The forcing Barrels, Trunks, and all their Apparatus, are very curiously contrived for putting together, mending, altering or cleansing, and subject to as little Friction as possible in that Part.
The Machine for raising and falling the Wheels is very good, though but seldom used, as they tell me; for they will go at almost any Depth of Water, and as the Tide turns, the Wheels go the same Way with it.
These Machines at London-Bridge are far superior to those so much famed at Marly in France, in regard the latter are very ill designed in their Cranks, and some other Parts.
III. Epistola
