George. In preparing for this lesson, I learned that one cubic inch of water will form about a cubic foot of steam, which will be condensed into a cubic inch of water again when cooled below the boiling-point. 5. Mr. M. You have learned in that fact the great principle on which the steam-engine acts. The instrument represented in the figure gives a clear idea of the elementary parts of what is called the low-pressure steam-engine.* It consists of a cylindrical glass tube, B, terminating in a bulb, A. In the tube a piston moves up and down, air-tight, and a little water having been placed in the bulb, it is brought to the boilingpoint by the application of a lamp. As the steam forms, it presses the piston upward by reason of its elastic force; but on dipping the bulb into cold water, the steam condenses, and produces a partial vacuum, and the piston is then driven downward by the pressure of the external air. A B Fig. 34. 6. John. And if the rod attached to the piston were made to turn a crank, or work a lever, it would very well illustrate the working of a steam-engine. *Low-pressure Engine, Fig. 35. C B The chief parts of this engine are the boiler, A, the cylinder, C, the piston-rod, I J, the condenser, L, and the air-pump, M. B is the steam-pipe, branching into two arms, communicating respectively with the top and bottom of the cylinder, and K is the eduction-pipe, formed of the two branches which proceed from the top and bottom of the cylinder, and communicating with the cylinder and the condenser. N is a cistern, or well of cold water, in which the condenser is immersed. Each branch of pipe has its own valve, as F, G, P, Q, which may be opened or closed as the occasion requires. Suppose, first, that all the valves are open, while steam is issuing freely from the boiler. It is easy to see that the steam would circulate freely throughout all parts of the machine, expelling the air, which would escape through the valve in the piston of the air-pump, and thus the interior spaces would all be filled with steam. This process is called blowing through; it is heard when a steam-boat is about setting off. Next, the valves F and Q are closed, G and P remaining open. pressing the cylinder forces it down, and the instant when it begins to descend the stopcock O is opened, admitting cold water, which meets the steam as it rushes from the cylinder, and effectually condenses it, leaving no force below the piston to oppose its descent. Lastly, G and P being closed, F and Q are opened, the steam flows in below the piston, and rushes from above it into the condenser, by which means the piston is forced up again with the same power as that with which it descended. Meanwhile the air-pump is playing, and removing the water and air from the condenser. N The steam now Mr. M. Can George now tell in what respect the highpressure steam-engine differs from the low-pressure? George. In the high-pressure engine, the steam, being admitted first on one side of the piston and then on the other, is pushed out against the atmosphere; but in the low-pressure, a partial vacuum is produced alternately on each side of the piston by allowing the steam to escape into a fountain of cold water, which condenses it. 7. Mr. M. I have here a very interesting description of the steam-engine, and of its wonderful power and multiplied uses, by Dr. Arnott, and I will read the closing part of it to you. In the view here taken of it, you see the steam-engine is not only a wonderful instrument in itself, but one of the most effective instruments of human progress and civilization ever invented. 8. "It regulates with perfect accuracy and uniformity the number of its strokes in a given time, counting or recording them, moreover, to tell how much work it has done, as a clock records the beats of its pendulum; it regulates the quantity of steam admitted to work, the briskness of the fire, the supply of water to the boiler, the supply of coals to the fire; it opens and shuts its valves with absolute precision as to time and manner; it oils its joints; it takes out any air which may accidentally enter into parts which should be vacuous; and when any thing goes wrong which it can not of itself rectify, it warns its attendants by ringing a bell. 9. "Yet with all these talents and qualities, and even when exerting the power of six hundred horses, it is obedient to the hand of a child. Its aliment is coal, wood, charcoal, or other combustibles; it consumes none while idle; it never tires, and wants no sleep; it is not subject to malady when originally well made, and only refuses to work when worn out with age; it is equally active in all climates, and will do work of any kind; it is a water-pumper, a miner, a sailor, a cotton-spinner, a weaver, a blacksmith, a miller, etc., etc.; and a small engine, in the character of a steam pony, may be seen dragging after it on a rail-road a hundred tons of merchandise, or a regiment of soldiers, with greater speed than that of our fleetest coaches. It is the king of machines, and a permanent realization of the genii of Eastern fable, whose supernatural powers were occasionally at the command of man." 10. Frank. Dr. Arnott speaks of a steam pony which is used on a rail-road. This must be the very pony which gave the poet Saxe such a pleasant "ride on the rail,” "Singing through the forests, Rattling over ridges, Shooting under arches, Rumbling over bridges; Whizzing through the mountains, Buzzing o'er the vale, Bless me! this is pleasant, Riding on the rail!" 11. Mr. M. It would have been fortunate if poets had written more on scientific and philosophical subjects, as the lan guage of poetry is so well calculated to impress truths on the mind. In addition to the extract from Dr. Arnott, I have one from Lord Jeffrey on the same subject, the steam-engine, which I will read to you. 12. The Steam-engine.-"It has become a thing stupendous alike for its force' and its flexibility'; for the prodigious power which it can exert', and the ease, precision, and ductility with which it can be varied, distributed, and applied'. The trunk of an elephant, that can pick up a pin' or rend an oak', is as nothing to it. It can engrave a seal', and crush masses of obdurate metal before it'; draw out, without breaking, a thread as fine as gossamer', and lift up a ship of war like a bauble in the air'. It can embroider muslin' and forge anchors'; cut steel into ribbons', and impel loaded vessels against the fury of the winds and waves'. But I perceive, George, that you also have something which you wish to read. If it has any connection with this subject, we will hear it if you please, and let it conclude this lesson. entitled George. It is a few verses from a little poem 13. 14. 15. 16. 17. THE SONG OF STEAM, by G. W. CUTLER. Harness me down with your iron bands, Be sure of your curb and rein, For I scorn the power of your puny hands How I laughed, as I lay concealed from sight At the childish boast of human might, And the pride of human power. Ha ha ha! they found me at last, They invited me forth at length, And I rushed to my throne with a thunder-blast, And laughed in my iron strength. Oh then ye saw a wondus change Where now my fiery armies range, Hurra! hurra! the waters o'er The mountain's steep decline; I blow the bellows, I forge the steel, I hammer the ore, and turn the wheel I manage the furnace, the mill, the mint, And all my doings I put in print On every Saturday eve. I've no muscle to weary, no breast to decay, And soon I intend you may go and play, But, harness me down with your iron bands, For I scorn the strength of your puny hands, LESSON I.-BLESSINGS ON CHILDREN. 1. BLESSINGS on the blessing children, sweetest gifts of Heaven to earth, In the daylight, in the starlight, still with thought that freely flies, What trim fancies bring them flowers; what rare spirits walk their wood, W. G. SIMMS. |