The Life of James Watt

steam, a Florence flask, capable of containing about a pound "of water, had about one ounce of distilled water put into "it; a glass tube was fitted into its mouth, and the joining "made tight by lapping that part of the tube with pack"thread, covered with glazier's putty. When the flask was "set upright, the tube reached down near to the surface of "the water, and in that position the whole was placed in a "tin reflecting oven before a fire, until the water was wholly “evaporated, which happened in about an hour, and might "have been done sooner had I not wished the heat not much "to exceed that of boiling water. As the air in the flask was heavier than the steam, the latter ascended to the top, "and expelled the air through the tube. When the water was all evaporated, the oven and flask were removed from "the fire, and a blast of cold air was directed against one "side of the flask, to collect the condensed steam in one "place. When all was cold, the tube was removed; the flask " and its contents were weighed with care; and the flask “being made hot, it was dried by blowing into it by bellows, "and when weighed again, was found to have lost rather more than four grains, estimated at 4 grains. When the "flask was filled with water, it was found to contain about 174 ounces avoirdupois of that fluid, which gave about 1800 "for the expansion of water converted into steam of the heat " of boiling water.

"This experiment was repeated with nearly the same result; and in order to ascertain whether the flask had "been wholly filled with steam, a similar quantity of water "was for the third time evaporated; and, while the flask was "still cold, it was placed inverted, with its mouth (contracted 'by the tube) immersed in a vessel of water, which it sucked "in as it cooled, until in the temperature of the atmosphere "it was filled to within half an ounce measure of water. In "the contrivance of this experiment I was assisted by Dr. "Black. In Dr. Robison's edition of Dr. Black's Lectures, "vol. i., p. 147, the latter hints at some experiments upon "this subject, as made by him; but I have no knowledge of "any except those which I made myself.

"In repetitions of this experiment at a later date, I sim"plified the apparatus by omitting the tube and laying the "flask upon its side in the oven, partly closing its mouth by a cork, having a notch on one side, and otherwise pro"ceeding as has been mentioned.

"I do not consider these experiments as extremely accu"rate, the only scale-beam of a proper size which I had then "at my command not being very sensible, and the bulk of “the steam being liable to be influenced by the heat to which "it is exposed, which, in the way described, is not easily "regulated or ascertained; but, from my experience in actual practice, I esteem the expansion to be rather more than I ❝ have computed.

"A boiler was constructed which showed, by inspection, "the quantity of water evaporated in any given time, and thereby ascertained the quantity of steam used in every "stroke by the engine, which I found to be several times the "full of the cylinder. Astonished at the quantity of water required for the injection, and the great heat it had acquired "from the small quantity of water in the form of steam which "had been used in filling the cylinder, and thinking I had "made some mistake, the following experiment was tried :— "A glass tube was bent at right angles; one end was inserted horizontally into the spout of a tea-kettle, and the other part was immersed perpendicularly in well-water contained "in a cylindric glass vessel, and steam was made to pass through it until it ceased to be condensed, and the water in "the glass vessel was become nearly boiling hot. The water "in the glass vessel was then found to have gained an addi"tion of about one-sixth part from the condensed steam. "Consequently, water converted into steam can heat about "six times its own weight of well-water to 212°, or till it can "condense no more steam. Being struck with this remark"able fact, and not understanding the reason of it, I men"tioned it to my friend Dr. Black, who then explained to me "his doctrine of latent heat, which he had taught for some "time before this period, (summer 1764); but having myself "been occupied with the pursuits of business, if I had heard

" of it, I had not attended to it, when I thus stumbled upon one of the material facts by which that beautiful theory is "supported.

"On reflecting further, I perceived that, in order to make "the best use of steam, it was necessary-first, that the "cylinder should be maintained always as hot as the steam "which entered it; and, secondly, that when the steam was "condensed, the water of which it was composed, and the injection itself, should be cooled down to 100°, or lower, "where that was possible. The means of accomplishing these "points did not immediately present themselves; but early "in 1765 it occurred to me, that if a communication were "opened between a cylinder containing steam, and another "vessel which was exhausted of air and other fluids, the steam, as an elastic fluid, would immediately rush into the "empty vessel, and continue so to do until it had established "an equilibrium; and if that vessel were kept very cool by "an injection, or otherwise, more steam would continue to "enter until the whole was condensed. But both the vessels being exhausted, or nearly so, how were the injection-water, "the air which would enter with it, and the condensed steam, "to be got out? This I proposed, in my own mind, to per"form in two ways. One was, by adapting to the second “vessel a pipe, reaching downwards more than 34 feet, by "which the water would descend, (a column of that length "overbalancing the atmosphere), and by extracting the air "by means of a pump.

"The second method was by employing a pump, or pumps, "to extract both the air and the water, which would be "applicable in all places, and essential in those cases where "there was no well or pit.

"This latter method was the one I then preferred, and is "the only one I afterwards continued to use.

“In Newcomen's engine, the piston is kept tight by water, "which could not be applicable in this new method; as, if any of it entered into a partially-exhausted and hot cylin"der, it would boil, and prevent the production of a vacuum, " and would also cool the cylinder by its evaporation during

"the descent of the piston. I proposed to remedy this defect 'by employing wax, tallow, or other grease, to lubricate and keep the piston tight. It next occurred to me, that the "mouth of the cylinder being open, the air which entered to act on the piston would cool the cylinder, and condense "some steam on again filling it. I therefore proposed to put an air-tight cover upon the cylinder, with a hole and stuffing"box for the piston-rod to slide through, and to admit steam "above the piston to act upon it, instead of the atmosphere. "The piston-rod sliding through a stuffing-box was new in steam-engines; it was not necessary in Newcomen's engine, as the mouth of the cylinder was open, and the piston-rod "was square and very clumsy. The fitting the piston-rod "to the piston by a cone was an after-improvement of mine, "(about 1774). There still remained another source of the "destruction of steam, the cooling of the cylinder by the "external air, which would produce an internal condensation "whenever steam entered it, and which would be repeated "every stroke; this I proposed to remedy by an external "cylinder, containing steam, surrounded by another of wood, 66 or of some other substance which would conduct heat slowly.

"When once the idea of the separate condensation was started, all these improvements followed as corollaries in quick succession, so that in the course of one or two days "the invention was thus far complete in my mind, and I immediately set about an experiment to verify it practically. "I took a large brass syringe, 1 inches diameter and 10 "inches long, made a cover and bottom to it of tin-plate, "with a pipe to convey steam to both ends of the cylinder "from the boiler; another pipe to convey steam from the

upper end to the condenser, (for, to save apparatus, I in"verted the cylinder); I drilled a hole longitudinally through "the axis of the stem of the piston, and fixed a valve at its "lower end, to permit the water, which was produced by the "condensed steam on first filling the cylinder, to issue. The "condenser used upon this occasion consisted of two pipes of "thin tin-plate, ten or twelve inches long, and about one-sixth

"inch diameter, standing perpendicular, and communicating "at top with a short horizontal pipe of large diameter, having an aperture on its upper side, which was shut by a valve opening upwards. These pipes were joined at bottom to "another perpendicular pipe of about an inch diameter, "which served for the air and water-pump; and both the " condensing pipes and the air-pump were placed in a small "cistern filled with cold water. This construction of the con"denser was employed from knowing that heat penetrated "thin plates of metal very quickly, and considering that if "no injection was thrown into an exhausted vessel, there "would be only the water of which the steam had been composed, and the air which entered with the steam, or through the leaks, to extract.

"The steam-pipe was adjusted to a small boiler. When "steam was produced, it was admitted into the cylinder, and "soon issued through the perforation of the rod, and at the "valve of the condenser. When it was judged that the air "was expelled, the steam-cock was shut, and the air-pump "piston-rod was drawn up, which leaving the small pipes of "the condenser in a state of vacuum, the steam entered them " and was condensed. The piston of the cylinder immediately "rose, and lifted a weight of about 18 lbs., which was hung "to the lower end of the piston-rod. The exhaustion-cock "was shut, the steam was readmitted into the cylinder, and "the operation was repeated; the quantity of steam con“sumed, and the weights it could raise, were observed; and, excepting the non-application of the steam-case and external "covering, the invention was complete, in so far as regarded "the savings of steam and fuel. A large model, with an "outer cylinder and wooden case, was immediately constructed, and the experiments made with it served to verify "the expectations I had formed, and to place the advantage "of the invention beyond the reach of doubt. It was found "convenient afterwards to change the pipe-condenser for an "empty vessel, generally of a cylindrical form, into which an " injection played, and, in consequence of there being more "water and air to extract, to enlarge the air-pump.

« Anterior Continuar »

Libros

The Life of James Watt: With Selections from His Correspondence