« ПретходнаНастави »
193. The triumph of mechanics is the steam engine. The inventor observed the excessive force of steam in lifting up the stiff lid of a kettle as he sat at breakfast, and he and others have since applied this resistless power to produce a motion applicable to all kinds of machinery.
194. In constructing these engines, steam from a copper is thrown into a hollow iron cylinder, with a close lid or stopper, which rises as the steam rushes into the cylinder, and falls when the steam is condensed by cold water thrown in for the purpose.
An upright iron rod is fixed to that lid, and to one end of a large beam ; which, in consequence, has an action communicated to it similar to that of a see-saw, and is lifted up and pulled down with wonderful prefision and force.
THE STEAM ENGINE.
Obs.- A regular and powerful motion being thus produced, the mechanic seizes upon it, and applies it with ease to all kinds of machinery. The apparatus itself has been slightiy varied by different persons, and for different objects: but the principle remains the same, and it is, perhaps, the greatest discovery that was ever made in mechanics.
2. Mr. Watt, of Birmingham, has made many improvements in the steam engine; and, among others, he fastens the top of the cylinder, working the rod through it, and injects steam above as well as below, so that the motion down
wards is produced by steam, as well as that upward; he also condenses the steam in an adjoining vessel. One borse can, by common machinery, raise 25,000 pounds one foot high in a minute; but some steam machines perform the labcur of 60 or 80 horses ! A small one of a ten horse power, with the steam produced by a single bushel of coals, will raise thirty million of pounds one foot high ; or it will grind and dress three sacks of wheat, slit and draw into nails five cwt. of iron, and drive at the same time 1,000 cotton spindles.
3. Robert Fulton, Esq. of New York, has since applied the steam engine with success, to impel boats and ships.
4. Mr Blenkinsop, of Leeds, has, with great success, lately applied steam to move coal waggons on a rail-way, instead of drawing them with the power of horses. Here is represented his machine, to which any carriage may be annexed.
195. The pump for raising water is a very useful machine; and its principle, which is founded on the elasticity or pressure of the air, should be understood.
It can raise water, if required, to the height of thirtythree feet, by the pressure of the air on the water.
Obs. If a long glass tube, closed at one end, were deprived of air, and its open end immersed in quicksilver, the quicksilver will rise in it about 29 inches ; or, if placed in water, 33 feet of water will rise in it; the weight of 33 feet of water being equal to 29 inches of quicksilver. The rise of those fluids in such a tube, is caused by the pressure of-the air on the surface of the external mercury or water; hence it is inferred, and with reason, that the elasticity of the air wbich we breathe, is in all places equal in force to the weight of about 29 inches of mercury, or 33 feet of water.
196. To raise water 33 feet high, nothing more then is requisite than to put one end of a pipe in it, and to draw the air out of that pipe, when the water will instantly ascend in the pipe.
Such is the purpose and effect of a pump; and all that is to be done is by proper contrivances to draw out the air above, and keep up a supply of the water below.
197. A pump consists of a wooden or copper pipe, with a long iron rod to work up and down within it, by means of a handle.
At the lower end of the iron rod is fixed a metallic hoop, provided with leather to fit the pipe : in the centre of the hoop is a little trap-door or valve, which opens only upwards, and when down, shuts very close.
At the bottom of the pipe, near the water, another such valve also opening upwards, is fixed tight within the pipe itself.
198. The handle of the pump being raised, the iron rod, (called the piston), with its valve at the bottom of it is forced down the pipe.
As the valve opens upwards, the air in the pipes passes up through the valve.
On pulling down the handle, the piston is raised, and with it the valve, leaving a vacuum or vacuity between it and the lower fixed valve.
To fill up the vacuum, the water rushes up through the lower valve.
On again raising the handle, the piston again descends; and the water now rushes through its valve, and, on pulling down the handle again, the piston and its closed valve rise, bringing up the water.
Its ascent creates a new vacuum, and more water rushes through the lower valve; the upper valve is made to descend again, to rise again, closed, and bring up water.
199. Fire engines, and other forcing engines, have no valve or flap fixed to the piston; but a solid plate is moved up and down by it, and the rising water is thus violently driven into an adjoining air-tight vessel.
Through the top of that vessel, the playing pipe is so inserted, that its mouth may lie below the water, leaving the upper part of the vessel filled with air.
Then the elastic power of that portion of air, forces the driven water up the playing pipe.
The energy of the stream, will of course depend on the power applied to force down the piston, and drive the water into the air vessel.
Obs.-An inspection of a pump, or fire engine, will teach more in ten minutes, than mere description in as many hours.
XI. Trade and Commerce. 200. The barter of commodities is necessarily coeval with the first formation of society. One man might have too much corn; and another too much wool ; and each would be willing to give what he had to spare of his own superfluity, for what he might want of the superfluity of the other.
201. In time, such barter would become a system; otherwise, every family would have to grow every article it consumed, and to manufacture every commodity it wanted. The tailor would make clothes for the farmer, and take provisions for his labour. The carpenter would build on the same principle of reward ;
and hence would arise all the distinct trades which we now see exercised.
202. One farmer too would cnltivate wheat; and another would make cheese and butter, according to the nature of their respective soils. They would either exchange on the spot, or each
his peculiar produce to a common market, and exchange it for gold or silver, articles of universal currency, which he could exchange at any time, for whatever else he wanted.
203. The application of labour to particular or individual objects, has also tended greatly to improve every manufacture.
A man who is nothing but a tailor, is far more expert at making clothes, than if he were also, a shoemaker, carpenter, and blacksmith; and still more so, if, instead of making all kinds of clothes, he work at particular parts of garments.
This is called division of labour.
204. The utility of dividing labour is exemplified in making pins.
Were a piece of metal given to a man to make one pin, he could scarcely do it in a day.
In pin manufactories, however, each pin passes through twenty-five hands : one draws out the wire, another straightens it, another cuts it, another points it, three or four prepare the head, two or three put it on, and others finish them, and put them on a paper.
Twenty-five persons, thus make one hundred and twenty-five thousand pins in a day; or five thousand to each person !
205. Labour likewise subdivides itself numerously in every branch of the elegant and useful arts.
Thus, in building, there are, the brickmaker, the stone-mason, the architect, the surveyor,
the bricklaya er, the sawyer, the carpenter, for rough work, the joinor for fine work; the slater, the plasterer, the plumber, the glazier, the ironmonger, and the painter; all necessary in their several departments.