Page images
PDF
EPUB

phons, existing in the sides of mountains, &c. at various depths. Some springs, situate on the tops of hills, near to larger ones, supply water all the year, others only periodically.

3. A column of air ascending to the top of our atmosphere, under ordinary circumstances, corresponds in weight with a column of water 33 feet in height, allowing the bases of the air and of the water to be equal. Where the air is withdrawn, by means of suckers, pistons, and valves, from within a pipe, of which the lowest part is immersed in the water contained in a well, the fluid will rise 10 the height of thirty-three feet within the pipe, supplying the place of the air thus withdrawn. This is effected by the pressure of the atmosphere on the surface of the water. The water in a common or sucking pump is raised by this means, and rises to the height of 33 feet. The lifting and forcing pumps are unlimited in regard to the height to which water may be raised, provided the machinery be sufficiently sirong and powerful. On this principle, fire-engines and those for watering gardens are formed.

The Steam Engine. The-steam engine was originally contrived for the purpose of raisinig water by means of the expansive force of steam or vapour produced from water or other liquids in a state of ebullition. In consequence of its late improvements, it is now made to serve as the first mover in all kinds of mill-work, in planing, sawing, boring, &c.

There have been steam engines of various kinds, to describe all which, at large, would require a volume. We shall here avail ourselves of the general description of the principles and manner of operation of the steam engines of Savery, Newcomen, and Cawley, and of Watt, as given by Dr. Gregory, in the 2nd volume of his Mechanics.

1. Let there be a sucking pipe with a valve opening upwards at the top; communicating with a close vessel of water, not more than thirty feet above the level of the reservoir; and if the steam of boiling water be thrown on the surface of the water, in the vessel, it will force it to a height as much greater than thirty three feet, as the elas

[ocr errors]
[ocr errors]

tic force of the steam is greater than that of air; and if the steam be condensed by the injection of cold water, and a vacuuni thus formed, the vessel will be filled from the reservoir by the pressure of the atmosphere; and the steam being admitted as before, this water will also be forced up; and so on, successively.

Such is the principle of the first steam engine, said, by the English, to be invented by the Marquis of Worcester, while he French ascribe it to Papin : though, we believe, the fact is, that Brancas, an Italian, applied the force of steam ejected from a large clopile, as an inpelling power for a stamping engine, so early as 1629. The hint so obscurely exhibited in the Marquis of Worcester's century of inventions, was carried into effect by Captain Savery.

2. If the steam be adinitted into the bottom of a hollow cylinder, to which a solid piston is adapted, the piston will be forced upwards by the difference between the elastic forces of sieain and common air; and the steam being then condensed, the piston will descend by the pressure of the atmosphere, and so on successively.

This is the principle of the steam engine first contrived by Messrs. Newcomen and Cawley of Dartmouth. . This. is sometimes called the atmospherical engine, and is commonly a forcing pump, having its rod fixed to one end of a lever, which is worked by the weight of the atmosphere upon a piston at the other end, a temporary vacuuin being made helow it by suddenly condensing the steam, that had been admitted into the cylinder in which this piston works, by a jet of cold water thrown into it. A partial vacuum being thus made, the weight of the atmosphere presses down the piston, and raises the other end of the straight lever, together with the water from the well., Then immediately a hole is uncovered in the bottom of the cylinder, by which a fresh quantity of hot steam rusbes in from a boiler of water below it, which proving a counterbalance for the atmosphere above the piston, the weight of the pump rods, at the other end of the lever, carries that end down, and raises the piston of the steam cylinder. The steam hole is then immediately shut, and sa cock opened for injecting the cold water into the cylinder of steam, which condenses it to water again, and, thus making a vacuum below the piston, the atmosphere again presses it down and raises the pump rods, as before; and so on continually.

3. The great features of improvement made by Mr. Watt upon the engine of Newcomen and Cawley, are, as Mr. Nicholson remarks, first, that the elasticity of the steam itself is used as the active power in this engine; and secondly, that besides various other judicious arrangements for the economy of heat, he condenses the steam, not in the cylinder, but in a separate vessel.

In the cylinder or syringe, concerning which we have spoken, in mentioning the engine of Newcomen, let us suppose

the upper part to be closed, and the piston-rod to slide air-tight through a collar of leathers. In this situation, it is evident, that the piston might be depressed by throwing the steam upon its upper surface, through an aperture at the superior end of the cylinder. But if we suppose the external air to have access to the lower surface of the piston, we shall find, that steam no stronger in its elasticity than to equal the weight of the atmosphere would not move the piston at all; and consequently that this new engine would require much denser steam, and consume much more fuel than the old engine. The remedy for this evil is to maintain a constant vacuum beneath the piston. If such a vacuum were originally produced by steam, it is certain that its permanency could not be depended on, unless the engine contained a provision for constantly keeping it up. Mr. Watt's contrivance in his simplest engine is as follows : The steam is conveyed from the boiler to the upper part of the cylinder through a pipe, which also communicates occasionally with the lower part, and beyond that space with a vessel immersed in a trough of water; in which vessel the condensation is performed by an injected stream of cold water. This water is drawn off, not by an eduction-pipe but by a pump, of which the stroke is sufficiently capacious to leave room for the elastic fluid, separated during the injection, to follow and be carried out with the injection water. Suppose now the piston to be at its greatest elevation, and the communication from the boiler to the upper as well as to the lower parts of the cylinder to be opened. The steam will then pass into the whole internal part of the engine,

and will drive the air downwards into the condenser, and thence through the valves of the air-pump. In this situation, if the communication from the boiler to the lower part of the cylinder be stopped, and an injection be made into the condenser, a vacuum will be produced in that vessel, and the steam contained in the lower part of the cylinder and communication pipe will expand itself, with wonderful rapidity, towards the condenser, so that in a period of tinie too minute to be appreciated, the whole of the steam beneath the piston will be practically condensed, The steain which continues to act above the piston will immediately depress'it into the vacuum beneath ; at the same time that by connection with the external apparatus the piston of the air-puinp also descends in its barrel. When the stroke is nearly completed downwards, the requisite part of the apparatus shuts the communication with the boiler; opens that between the upper and lower parts of the cylinder and condensing vessel ; and turns the injection-cock. At this very instant the piston loses its tendency to descend, because the steam presses equally on both surfaces, and continues its equality of pressure while the condensation is performed. It therefore rises; the injection is stopped ; and the air-pump making its stroke suffers the injection water and a considerable part of the elastic fluid to pass through its lower valve. The vacuum is thus kept up through the whole internal capacity of the engine. As soon as the piston has reached the upper part of the cylinder, the communication to the under part of the cylinder is stopped, and that with the boiler opened, as before; the consequence of which is, that the piston again descends; and in this manner the alternations res peatedly take place.

The principal augmentation of power in this engine, compared with that of Newcomen, arises from the cylinder not being cooled by the injection water, from its being practicable to use steam which is more powerful than the pressure of the atmosphere, and from the employing of this steain both to elevate and to depress the piston. In general, these engines are worked by steam which would support a column of four or five inches of mercury besides the pressure of the atmosphere, and sometimes more; for Mr.

1

Nicholson says, he has sometimes seen the gage as high as eight inches.

Mr. Watt has made several successive modifications and additions to the engine just described.

CHAP. IV.-PNEUMATICS. 1. PNEUMATICS treats of the weight, density, compressibility, and elasticity of air and all aeriform fluids. Air differs from all other fluids in ihe following particulars : (1.) It can be compressed into a much less space than it naturally possesses, the spaces into which a given quantity is compressed being always reciprocally as the compressing forces. (2.) It cannot be congealed or tixed as other duids may. (3.) It is of a different density in every part upward from the earth's surface; the inferior strata being more and more compressed by the weight of the superincumbent atmosphere. (4.) It is of an elastic or springy nature, and the force of its spring is equal to its weight. (5.) It is necessary to the life of animals, and the growih' of plants. The invisibility of air, is only the consequence of its transparency; but it is possessed of all the common properties of matter. When a vessel is empty, in the ordinary way of speaking, it is, in fact, still filled with air. But it is possible to empty a vessel even of the air which it contains. The instrument or machine, by which this operation is performed, is called an air-panip. By this means, the mechanical properties of air are clearly demonstrated. By the air-pump, we are taught what ihe earth would be without an atmosphere, and how much all vital, generative, and nutritive power depends ou this circumambient and all-penetrative Auid." The air, which we breathe, surrounds the earth on every side, and together with the clouds and vapours, floating in it, is called the atmosphere.

2. That air may be rarefied or extended in bulk, without the accession of any new matter, is thus simply proved. If a bladder, apparently empty, and its neck tied with a string, be placed before the fire, the heat will so rarefy the air inclosed in it, as to extend the bladder to its utmost

« PreviousContinue »