STEAM-ENGINE. The few following corrections of these articles in the Encycl. were communicated by the author.
Page 745, col. 1.—It was not at the York Building waterworks in London that the boiler built, but in the country in an engine erected by Dr Deaguiriers. See his Experimental Philosophy, Vol. II. p. 489.
Page 746, col. 2.—The condensation requires more cold water than is here allowed, as will appear by and bye; and we also suspect that the rapidity is overrated with which a great volume of steam is condensed by by the cold surface of a vessel. We are well informed that Mr Watt was much disappointed in his expectations from a construction in which this mode of condensation was adopted. The condenser employed by Mr Cartwright (see Phil. Mag.) was one of the very first thought of and tried for this purpose, and was given up, as well as all others on the same principle; and the immediate contact of cold water was preferred as incomparably more effective. The great superiority of the capacity of water for heat is now well known. It is true, that when we employ an extensive cold surface of the condenser, this surface is kept cold by the water round it; and therefore we still avail ourselves of this great avidity of water for heat. But this water must act through the intervention of the vessel; and the substance of the vessel does not convey heat to the surrounding water in an instant.
Page 749, col. 2.—No distinct experiment shews so great an expansion of water, when converted into steam at the temperature $212^\circ$; and under the pressure of the air Mr Watt never found it more than 1800 times rather than water.
Page 753, col. 1.—The heat expended in boiling off a cubic foot of water is about six times as much as would bring it to a boiling heat from the medium temperature ($55^\circ$) in this climate.
Page 758, col. 2.—The quantity of water necessary for injection may be determined on principle, at least for an engine having a separate condenser. Every cubic foot of common steam produces about an inch of water when condensed, and contains about as much latent heat as would raise 1100 inches of water one degree. This steam must not only be condensed, but must be cooled to the temperature of the hot well; therefore as many inches of cold water must be employed as will require all this heat to raise it to the temperature of the hot well. Therefore let $x$ be the cubic feet of steam, or capacity of the cylinder, and let $y$ be the inches of cold water expended in condensing it. Let $a$ be the difference between $212^\circ$ and the temperature of the hot well, and $b$ the difference between the temperature of the well and the injection cistern. We have
$$y = \frac{1100 + a}{b} x.$$
Thus, if the temperature of the hot well be $100^\circ$ (and it should never be higher, if we would have a tolerable vacuum in the cylinder), and that of the injection cistern be $50^\circ$, we have $a = 112$, and $b = 50$, and
$$y = \frac{1212}{50} x = 24.24 x,$$ or $24\frac{1}{2} x$; that is, every foot of the capacity of the cylinder, or every inch of water evaporated from the boiler, requires more than 24 inches of water to condense the steam. A wine pint for every inch of water boiled off, or every cubic foot of capacity of the cylinder, may be kept in mind, as a large allowance. Or, more exactly, if the engine be in good order, and the injection water as low as $50^\circ$, and the hot well not above $100^\circ$, we may allow 25 gallons of injection for one gallon of water boiled off. This greatly exceeds the quantity mentioned in the case of a good Newcomen's engine, the cylinder of which contained almost 30 cubic feet of steam. And this circumstance shews the superiority of the engine with a separate condenser. The injection of Newcomen's engine had been adjusted by experience, so as to make the best compensation for the unavoidable waste in the cylinder. We presume that this machine was not loaded above eight pounds per inch, more likely with seven; whereas Watt's engine, working in the condition now described, bears a load not much below twelve, making at least twelve strokes per minute.
This is not a matter of mere curiosity; it affords a very exact rule for judging of the good working order of the engine. We can measure with accuracy the water admitted into the boiler during an hour, without allowing its surface to rise or fall, and the water employed for injection. If the last be below the proportion now given (adapted to the temperatures $50^\circ$ and $100^\circ$), we are certain that steam is wasted by leaks, or by condensation in some improper place. The rule is not strictly conformable to the latent heat of steam which balances the atmosphere, $1100^\circ$ being somewhat too great a value. It is accommodated to the actual performance of Watt's engines, when in their best working condition.
It is evident that it is of great importance to have the temperature of the hot well as low as possible; because there always remains a steam in the cylinder, of the same, or rather higher temperature, possessing an elasticity which balances part of the pressure on the other side of the piston, and thus diminishes the power of the engine. This is clearly seen by the barometer, which Mr Watt applies to many of his best engines, and is a most useful addition for the proprietor. It shews him, in every moment, the state of the vacuum, and the real power of his engine, and tells him when there are leaks by which air gets in.
Page 762, cols. 1, 2.—Mr Watt's first experiment was not exactly as here related, but much more analogous to the present form of his engine. The condenser was a cylinder of tinplate, fitted with a piston, which was drawn up from the bottom to the top, before the reduction cock was opened. Without this previous radiation in the condenser, there was no inducement for the steam to take this course, unless it were made much stronger than that of ordinary boiling water.
The description of the first form of the engine is also faulty, by the omission of a valve immediately below the reduction pipe. This valve is shut along with the valve I, to prevent the steam, which should then go into the lower part of the cylinder, from also going down into the condenser. This is not absolutely necessary, but its advantage is evident.
Page 765, col. 1.—This form of the engine was very early put in practice by Mr Watt—about the year 1775. The small engine at Mr Boulton's works at Soho was erected in 1776; and the engine at Shadwell waterworks, one of the best yet erected, had been working some time when we saw it in 1778. We mention this, because we have been told that Mr Hornblower puts in some claim to priority in this invention. We do not think that Mr Hornblower erected any of his engines before 1782; and as Mr Hornblower was, we believe, working with Boulton and Watt before that time, we think it fully more probable that he has in this respect profited by the instruction of such intelligent employers. We may also observe, that Mr Watt employed the same contrivance which we have described with much approbation in p. 772. Encycl. for keeping the collar round the piston rods steam and air tight. He found them effectual, but that they required more attention for keeping them in fit condition than the usual mode of packing. He made a similar packing for the piston, and with a similar result.
Page 769, cols. 1, 2.—Mr Boulton estimates the performance of the engines in the following manner. Seeing that the great expense of the engine is the consumption of fuel, he makes this the standard of computation, and estimates the performance by the work which he engages to perform by the consumption of one bushel of good Newcastle coal, London measure, or containing 84 lbs. without regard to the time in which this bushel is expended. This depends on the size of the engine.
The burning one bushel of coal will,
1. Raise 30 million pounds one foot high. 2. It will grind and dress 11 bushels of wheat. 3. It will lift and draw into nail rods 5 cwt. of iron. 4. It will drive 1000 cotton spindles, with all the preparation machinery, with the proper velocity. 5. It is equivalent to the work of ten horses.
The general performance of the double stroke expansive engines is somewhat beyond this; and their performance in cotton spinning, or as compared with horse work, is much under rated. The first estimation is without ambiguity. Suppose the engine of such a size as to consume a bushel of coals per hour. This will be found equivalent to raising 97 wine hogsheads of water ten feet high in a minute, which ten stout draught horses cannot do for a quarter of an hour together. They can raise 60 in that time, and work at this rate eight or perhaps ten hours from day to day.
Mr Watt finds that, with the most judiciously constructed furnaces, it requires eight feet of surface of the boiler to be exposed to the action of fire and flame to boil off a cubic foot of water in an hour, and that a bushel of coals so applied will boil off from eight to twelve cubic feet.
Boulton and Watt now make steam-engines equivalent in power to one or two horses. The cylinder and whole machinery does not occupy more room than a fine lady's working table, standing in a square of about 2½ feet, and about 5 feet high.