"At the time I was informed of this plan, a thought struck me that it might be accomplished by another mode preferable to this: by a steam-engine to force water in pipes round the dock, to say 30 or 40 lbs. to the inch, more or less, and to have a worm-shaft, working in a worm-wheel, the same as a common roasting-jack, and apply to the worm-shaft a spouting arm like Barker's mill; the worm-shaft standing perpendicular would work the worm-wheel fixed in the chain-barrel shaft of the crane. "This would make a very simple and cheap machine, and produce a circular motion at once, instead of a piston alternating motion to drive a rotary motion. My report had some weight with them; inquiry is to be made into the plan proposed by me, so as to remunerate me, provided my plan is considered good. Mr. Linthorn wishes an investigation before scientific and able judges, and requested me to name some one. I must again make free in asking the favour of your advice (which you have so ably given me for thirty years) on this plan. Mr. Linthorn intends to request Dr. Wollaston to accompany you, any day convenient to you. In the meantime, should you see him, it might not be amiss to mention it to him; and should you be able to attend for an hour or two to this business, I would thank you to drop me a note, saying when it may be convenient. There is a memorandum of an agreement between Mr. Linthorn and me; but the plan I suggest is only at present made public to him and yourself. "Your most obedient servant, "RICHARD TREVITHICK." The reduction of friction by the use of an air-vacuum engine for working cranes, as designed by Mr. Linthorn, in lieu of an air-pressure engine, was doubted by Trevithick. The Mont Cenis pneumatic-pressure machines which the writer saw at work lost much power by friction before experience had taught remedies. The pneumatic vacuum tubes which propelled the trains on the South Devon Railway, failed to give the power that was expected. Sir William Armstrong's hydraulic cranes, brought into use not many years after the date of Trevithick's letter, have been found effective. The writer, not knowing that Trevithick had before recommended hydraulic cranes for warehouses, accompanied Sir William over his works, then being erected near Newcastle-on-Tyne, and talked with him on the detail of his crane designs. Trevithick thought of giving circular motion to the crane chain-barrel by the attachment of a screw-propeller, acted on by the force of a current of water at a pressure of 30 or 40 lbs. to the inch. Sir William Armstrong's arrangement was quite different; the merit due to Trevithick was for having pointed out the suitability of water as a means of conveying power through warehouses where fire was inadmissible. "MR. GILBERT, "LONDON, 42, ST. MARY AXE, _June 29th, 1828_.