"piston B B. But there it was mentioned, that the desired "effect could not be sufficiently attained; but that, notwith"standing all the precautions there specified, there always "remained in the tube about a fifth part of the air usually "contained in it. Whence a twofold inconvenience arises: “ viz. 1st, that in this way we lose half of the desired effect, so that scarcely can 150 lbs. weight be raised to the height "of one foot, when otherwise 300 lbs. ought to have been raised, if the tube A A had been perfectly emptied; and 2ndly, that as the piston gradually descends, the force "which makes it descend is itself diminished, as was also "observed in the passage already referred to. Thus we

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"have to provide, by some contrivance, that, as the moving "force decreases, the resistance may in like manner decrease, so that it may be overcome by the aforesaid moving "force until the end: just as in watches the unequal force "of the main-spring, which moves the whole machine, is "so regulated by art, that through the whole of its running "down it overcomes the resistance of the wheels with equal 66 ease. But it would be far more convenient if the moving "force were to remain always the same, from the beginning "to the end. Some attempts have, therefore, already been "made to ascertain how a perfect vacuum could be obtained by the help of gunpowder; for in this way, were there no "air to offer resistance beneath the piston, the whole column "of the atmosphere pressing upon the said piston would press "it with an equable force down to the bottom. But hitherto "such attempts have been in vain; and always, after the “flame of the gunpowder is extinguished, about a fifth part "of the air remains in the tube A A.

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"By another way, therefore, I endeavoured to attain the same end; and, since it is a property of water that a small quantity of it, converted into steam by the force of heat, has an elastic force like that of the air, but, when cold supervenes, is again resolved into water, so that no trace "of the said elastic force remains; I felt confident that "machines might be constructed, wherein water, by means "of no very intense heat, and at small cost, might produce

"that perfect vacuum which had failed to be obtained by "aid of gunpowder. But of the various constructions which " can be contrived for this purpose, the following seemed to 66 me to be the most suitable.

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"AA is a tube of uniform diameter throughout, close shut " at the bottom; BB is a piston fitted to the tube; DD a "handle fixed to the piston; EE an iron rod moveable “round an axis in F; G a spring, pressing the cross rod E E, so that the said rod must be forced into the groove H as soon as the piston with the handle has arrived at such a height as that the said groove H appears above the lid II; "L is a little hole in the piston, through which the air can escape from the bottom of the tube A A, when first the "piston is forced into it. The use of this instrument is as “follows:—A small quantity of water is poured into the tube "A A, to the depth of 3 or 4 lines; then the piston is inserted, and forced down to the bottom, till a portion of the "water previously poured in comes through the hole L; then "the said hole is closed by the rod M M. Next the lid II, pierced with the apertures requisite for that purpose, is put on, and a moderate fire being applied, the tube A A soon grows warm, (being made of thin metal), and the water "within it, being turned into steam, exerts a pressure so 'powerful as to overcome the weight of the atmosphere and "force up the piston B B, till the groove H of the handle "DD appears above the lid II, and the rod EE is forced, "with some noise, into the said groove by the spring G. "Then forthwith the fire is to be removed, and the steam in "the thin metal tube is soon resolved into water, and leaves "the tube entirely void of air. Next, the rod EE being "turned round so far as to come out of the groove H, and "allow the handle DD to descend, the piston BB is forth"with pressed down by the whole weight of the atmosphere, "and causes the intended movement; which is of an energy "great in proportion to the size of the tube. Nor is it to be " doubted that the whole weight of the atmosphere exerts its "force in tubes so constructed; for I have established by experiment, that a piston, raised to the top of the tube by

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"the force of heat, shortly afterwards descends again to the "bottom, and so on alternately for a number of times, so "that no suspicion can arise of air pressing beneath. Now my tube, the diameter of which does not exceed 2 inches, "yet raises sixty lbs. aloft with the same velocity as the piston is forced down into the tube, and the tube itself "scarcely weighs five ounces.

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"I therefore have little doubt but that tubes may be manu"factured, the weight of each of which would scarcely amount "to 40 lbs., and yet which could raise, at each operation, "two thousand lbs. to a height of four feet. Moreover, I "ascertained that one minute's time is sufficient for a mode"rate fire to drive the piston in my tube up to the top; but,

as the fire ought to be proportionate to the size of the "tubes, large tubes could be heated almost as soon as small "ones: whence it is clear what vast moving powers may be "obtained by help of this most simple contrivance, and at "how small a cost. For it is known that the column of air "pressing on a tube whose diameter is a foot, counterbalances "nearly two thousand lbs.; but if the diameter be two feet, "the weight would be nearly eight thousand lbs. ; and that, "in other dimensions, the pressure increases in like manner "in the triplicate ratio of the diameters. Hence it follows, "that the fire in a grate whose diameter scarcely exceeds "two feet, might suffice to raise 8000 lbs. each minute to "a height of 4 feet, if tubes were provided of such a height; "for a fire might be made in a grate of thin iron, to be "easily moved from one tube to another, and so the same "fire might continually be preparing that most efficacious vacuum in one tube or another.

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“If any one now will consider the magnitude of the forces "to be obtained in this way, and the trifling expense at "which a sufficient quantity of fuel can be procured, he will

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certainly admit that this my method is far preferable to the "use of gunpowder above spoken of, especially as in this way "a perfect vacuum is obtained, and so the inconveniences "above recounted are avoided.

"In what manner that power can be applied to draw water

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"or ore from mines, to discharge iron bullets to a great dis"tance, to propel ships against the wind, and to a multitude " of other similar purposes, it would be too long here to "detail; but each individual, according to the particular "occasion, must select the construction of machinery appropriate to his purpose. Here, however, I will in passing "remark, how greatly such a power is to be preferred to common rowers for moving vessels in the sea; for, 1st, the weight of the common rowers loads the vessel, and retards "its progress; 2ndly, they require much room, and so are a "great hindrance in the ship; 3dly, it is not always possible "to find the requisite number of men; 4thly and lastly, the rowers, whether they are toiling on the deep, or resting “in harbour, must always be supplied with needful food, "whereby the expenses are greatly increased. But my tubes would, as has been already observed, have a very small weight to retard the ship; would also take up little room; might also be readily prepared in sufficient numbers, if "once a manufactory were built and fitted up for that purpose; and, lastly, no fuel would be consumed for the said tubes, excepting at the time of the operation; while in "harbour they would require no expenditure.

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"But since common oars could not easily be moved by "such tubes, paddle-wheels would have to be employed, such "as I remember having seen in a machine constructed at "London by command of the Most Serene Rupert Prince Palatine, which was put in motion by horses, by aid of oars "of that sort, and which left a long way behind it the Royal "barge manned by sixteen rowers. So, no doubt, oars fixed "into an axis could be most conveniently driven round by my tubes, by having the rods of the pistons fitted with "teeth, which would force round small wheels, toothed in “like manner, fastened to the axis of the paddles. It would "only be requisite that three or four tubes should be applied "to the same axis, by which means its motion could be continued without interruption; for, while any one piston might "be touching the bottom of its tube, so that it could drive the "axis round no further, before being again propelled by the

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