Page images
PDF
EPUB

CHAPTER VII.

MR. WATT'S NARRATIVE OF THE INVENTIONS DESCRIBED IN HIS SPECIFICA

TION OF 1769, GIVEN IN HIS NOTES ON ROBISON FURTHER ANECDOTES OF HIS INVENTION OF THE SEPARATE CONDENSER HIS NARRATIVE ENTITLED A PLAIN STORY."

THE account given by Mr. Watt himself, in his Notes on Professor Robison's Dissertation on Steam-engines, * is as follows:

• My attention was first directed, in the year 1759, to the subject of steam-engines, by the late Dr. Robison, then a “ student in the University of Glasgow, and nearly of my own

age. He at that time threw out an idea of applying the

power of the steam-engine to the moving of wheel-carriages, “and to other purposes; but the scheme was not matured, “and was soon abandoned on his going abroad.

“ About the year 1761, or 1762, I tried some experiments “ on the force of steam in a Papin's digester, and formed a

species of steam-engine by fixing upon it a syringe, one-third “ of an inch diameter, with a solid piston, and furnished also “ with a cock to admit the steam from the digester, or shut “ it off at pleasure, as well as to open a communication from “ the inside of the syringe to the open air, by which the steam "contained in the syringe might escape. When the commu“ nication between the digester and syringe was opened, the “ steam entered the syringe, and by its action upon the piston “ raised a considerable weight (15 lbs.) with which it was “ loaded. When this was raised as high as was thought

proper, the communication with the digester was shut, and " that with the atmosphere opened; the steam then made its

escape, and the weight descended. The operations were repeated, and, though in this experiment the cock was "turned by hand, it was easy to see how it could be done by “ the machine itself, and to make it work with perfect regu

* • Robison's Mechanical Works,' edited by Sir David Brewster.

larity. But I soon relinquished the idea of constructing an engine upon its principle, from being sensible it would be “ liable to some of the objections against Savery's engine, "viz., the danger of bursting the boiler, and the difficulty of

making the joints tight, and also that a great part of the power of the steam would be lost, because no vacuum was “ formed to assist the descent of the piston. I, however, “ described this engine in the fourth article of the specifica" tion of my patent of 1769; and again in the specification " of another patent in the year 1784, together with a mode " of applying it to the moving of wheel-carriages.

“ The attention necessary to the avocations of business pre"vented me from then prosecuting the subject further; but " in the winter of 1763-4, having occasion to repair a model " of Newcomen's engine belonging to the Natural Philosophy " class of the University of Glasgow, my mind was again " directed to it. At that period my knowledge was derived "principally from Desaguliers, and partly from Belidor. I "set about repairing it as a mere mechanician; and when " that was done, and it was set to work, I was surprised to “ find that its boiler could not supply it with steam, though

apparently quite large enough, (the cylinder of the model “ being two inches in diameter, and six inches stroke, and “the boiler about nine inches diameter). By blowing the “ fire it was made to take a few strokes, but required an

enormous quantity of injection water, though it was very

lightly loaded by the column of water in the pump. It “soon occurred that this was caused by the little cylinder

exposing a greater surface to condense the steam, than " the cylinders of larger engines did in proportion to their

respective contents. It was found that by shortening the “column of water in the pump, the boiler could supply the “ cylinder with steam, and that the engine would work regu“ larly with a moderate quantity of injection.

It now " appeared that the cylinder of the model, being of brass, “ would conduct heat much better than the cast-iron cylinders

66

“ of larger engines, (generally covered on the inside with a “stony crust), and that considerable advantage could be

gained by making the cylinders of some substance that “ would receive and give out heat slowly. Of these, wood “ seemed to be the most likely, provided it should prove

sufficiently durable. A small engine was, therefore, con

structed, with a cylinder six inches diameter, and twelve “ inches stroke, made of wood, soaked in linseed oil, and “ baked to dryness. With this engine many experiments “ were made; but it was soon found that the wooden cylinder

was not likely to prove durable, and that the steam con“ densed in filling it still exceeded the proportion of that

required for large engines, according to the statements of Desaguliers. It was also found that all attempts to produce a better exhaustion by throwing in more injection, caused a

disproportionate waste of steam. On reflection, the cause “ of this seemed to be the boiling of water in vacuo at low “ heats, a discovery lately made by Dr. Cullen and some “ other philosophers, (below 100°, as I was then informed); “ and consequently, at greater heats, the water in the cylinder “would produce a steam which would, in part, resist the “pressure of the atmosphere.

By experiments which I then tried upon the heats at “ which water boils under several pressures greater than that “ of the atmosphere, it appeared that when the heats pro“ ceeded in an arithmetical, the elasticities proceeded in some "geometrical ratio; and, by laying down a curve from my

data, I ascertained the particular one near enough for my "purpose.

also appeared, that any approach to a vacuum “ could only be obtained by throwing in large quantities of

injection, which would cool the cylinder so much as to

require quantities of steam to heat it again, out of propor“tion to the power gained by the more perfect vacuum ; and " that the old engineers had acted wisely in contenting them“ selves with loading the engine with only six or seven pounds

on each square inch of the area of the piston. It being “ evident that there was a great error in Dr. Desaguliers' “ calculations of Mr. Beighton's experiments on the bulk of “ 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 5 flask was filled with water, it was found to contain about

17} 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 simplified the apparatus by omitting the tube and laying the “ flask upon its side in the oven, partly closing its mouth by “a cork, baving a notch on one side, and otherwise proceeding as has been mentioned.

“I do not consider these experiments as extremely accurate, 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 u 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

« PreviousContinue »