The cylinder being again filled with steam as before, the stopcock (R) is turned, and the water spouts in; the steam is condensed; the piston descends; the pump-rod rises; and so on, stroke after stroke. The use of the small tube (T), proceeding from the cistern, is to pour a little water above the piston, to keep it air-tight.

As may be supposed, much care and attention was at first required in Newcomen's engine on the part of the person whose work it was to keep incessantly turning the stopcocks (V and R); the first for the admission of steam from the boiler, the second for the admission of the cold water for the condensation of the steam. The whole action of the machine depended on the attention of the person who watched these two cocks. A curious accident, however, remedied this inconvenience. A boy of the name of Humphrey Potter, being employed to tend one of Newcomen's engines, found the constant watching so troublesome, that he set himself to contrive a way by which the cocks might be turned at the right time, and yet he might enjoy himself for an hour or so at a time with the boys in the street. Observing that the particular moment at which the valve (V) required to be opened for the admission of the steam was that at which the pump-rod end of the beam was raised to its highest, and that the moment at which the other cock (R) required to be opened was when the piston-rod end was at its highest, he saw that, by attaching strings to the stopcocks, and connecting them with various parts of the beam, the rising and falling of the two ends would turn the cocks regularly as was necessary. Such was the Scogging or skulking gear of the boy Potter; so called because it enabled him to scog or play truant from his work, and afterwards improved by the substitution of rods for strings. The steam-engine was now entirely self-working; the only attendant necessary was the fireman to tend the furnace.

Such was the atmospheric engine of Newcomen, used to a considerable extent for the purpose of draining mines, and upon which various engineers employed their skill during the first half of the eighteenth century, with a view to render it applicable to other mechanical purposes, such as driving mills, &c. Among those who thus directed their attention to the steam-engine was the celebrated Smeaton; and some of the finest specimens of Newcomen's engine were of his construction. No improvement of essential consequence, however, was effected in the steam-engine until it came into the hands of Watt, whose successive contrivances to render it perfect we now proceed to describe. WATT'S IMPROVEMENTS ON THE STEAM-ENGINE AS A DRAINING AND PUMPING MACHINE.

Watt was a man with whom, to repeat the words of Professor Robison, 66 everything became the beginning of a new and serious study;" accordingly, not content with merely repairing Professor

upper part will, if the proper quantity of water be in the boiler, force the water up the longer pipe, while only steam issues from the shorter. Should both pipes emit water, then it is known that the boiler is too full; should both emit steam, that it is not full enough; and the supply can be regulated accordingly. Besides these gauge pipes, there is in the boiler a safety valve (S V), loaded so as to lie tight until the steam in the boiler accumulates to a degree sufficient to force it up. From the boiler the steam passes through the connecting tube, guarded by the regulating valve (V), made so as to open and shut easily, into the cylinder (C). Up and down in this cylinder, which is open at the top, moves the piston (P), attached by means of the piston-rod (M) to a flexible chain, which is fastened to the top of the arch at the end of a beam, moving on the pivot (I). The end of the beam to which the pistonrod is attached is made lighter than the other end, so that when the engine is at rest, it ascends and pulls up the piston to the top of the cylinder. The piston thus lying at the top of the cylinder, lets the steam from the boiler be admitted through the regulating valve (V). The steam rushing in expels the air which was in the cylinder through the snifting valve (H), which is at the bottom of the cylinder, and so constructed, that although it permits the escape of the air, it allows none to enter. The whole space of the cylinder underneath the piston being now filled with steam, the next operation is to condense it. This is done by turning a cock (R) in the tube (A), which descends from a cistern kept constantly full of cold water. The water, tending to rise to the height from which it has fallen, spouts into the cylinder, striking against the bottom of the piston, and falling down in a shower of drops, which cool the cylinder and condense the steam. This condensation of the steam produces a vacuum in the cylinder; and the piston, pressed down by the weight of the atmosphere outside, rapidly descends-the water which was thrown into the cylinder being carried off by the long eduction pipe which, having a valve at its extremity opening only outwards, leads to a cistern (S), whence the boiler is supplied. The descent of the piston pulls down the piston-rod and chain, and the end of the beam to which they are attached. The other end of the beam accordingly rises, pulling up a chain which is attached to the pump-rod (N), working the pump by which the mine is to be drained. The purpose of the smaller pump-rod working parallel to N, is, by the action of the engine, to raise a portion of the water through the tube (E E) to the cistern from which the water is sent into the cylinder. The piston is now at the bottom of the cylinder, and would remain there by the pressure of the atmosphere on its upper surface; but by opening the valve (V), the steam from the boiler is admitted under it, and the pressure of the atmosphere being thus counterbalanced, the superior weight of the pump-rod end of the beam causes it to descend, elevating the other end with the piston attached to it.

The cylinder being again filled with steam as before, the stopcock (R) is turned, and the water spouts in; the steam is condensed; the piston descends; the pump-rod rises; and so on, stroke after stroke. The use of the small tube (T), proceeding from the cistern, is to pour a little water above the piston, to keep it air-tight.

As may be supposed, much care and attention was at first required in Newcomen's engine on the part of the person whose work it was to keep incessantly turning the stopcocks (V and R); the first for the admission of steam from the boiler, the second for the admission of the cold water for the condensation of the steam. The whole action of the machine depended on the attention of the person who watched these two cocks. A curious accident, however, remedied this inconvenience. A boy of the name of Humphrey Potter, being employed to tend one of Newcomen's engines, found the constant watching so troublesome, that he set himself to contrive a way by which the cocks might be turned at the right time, and yet he might enjoy himself for an hour or so at a time with the boys in the street. Observing that the particular moment at which the valve (V) required to be opened for the admission of the steam was that at which the pump-rod end of the beam was raised to its highest, and that the moment at which the other cock (R) required to be opened was when the piston-rod end was at its highest, he saw that, by attaching strings to the stopcocks, and connecting them with various parts of the beam, the rising and falling of the two ends would turn the cocks regularly as was necessary. Such was the scogging or skulking gear of the boy Potter; so called because it enabled him to scog or play truant from his work, and afterwards improved by the substitution of rods for strings. The steam-engine was now entirely self-working; the only attendant necessary was the fireman to tend the furnace.

Such was the atmospheric engine of Newcomen, used to a considerable extent for the purpose of draining mines, and upon which various engineers employed their skill during the first half of the eighteenth century, with a view to render it applicable to other mechanical purposes, such as driving mills, &c. Among those who thus directed their attention to the steam-engine was the celebrated Smeaton; and some of the finest specimens of Newcomen's engine were of his construction. No improvement of essential consequence, however, was effected in the steam-engine until it came into the hands of Watt, whose successive contrivances to render it perfect we now proceed to describe. WATT'S IMPROVEMENTS ON THE STEAM-ENGINE AS A DRAINING AND PUMPING MACHINE.

Watt was a man with whom, to repeat the words of Professor Robison, 66 everything became the beginning of a new and serious study;" accordingly, not content with merely repairing Professor

Anderson's model, so that it should work as before, in presence of the students in the class-room, he devoted himself to the thorough investigation of all parts of the machine, and of the theory of its action. Directing his attention first, with all his profound physical and mathematical knowledge, to the various theoretical points involved in the working of the machine, "he determined," says M. Arago, "the extent to which the water dilated in passing from its liquid state into that of steam. He calculated the quantity of water which a given weight of coal could vaporise-the quantity of steam, in weight, which each stroke of one of Newcomen's machines of known dimensions expended-the quantity of cold water which required to be injected into the cylinder, to give the descending stroke of the piston a certain force-and finally, the elasticity of steam at different temperatures. these investigations would have occupied the lifetime of a laborious philosopher; whilst Watt brought all his numerous and difficult researches to a conclusion, without allowing them to interfere with the labours of his workshop."

All

Leaving Watt's theoretical researches into the mode and power of action by steam, let us attend to the practical improvements which he made in the construction of the engine itself. Newcomen's machine laboured under very great defects. In the first place, the jet of cold water into the cylinder was a very imperfect means of condensing the steam. The cylinder, heated before, not being thoroughly cooled by it, a quantity of steam remained uncondensed, and, by its elasticity, impeded the descent of the piston, lessening the power of the stroke. Again, when the steam rushed into the cylinder from the boiler, it found the cylinder cold, in consequence of the water which had recently been thrown in; and thus a considerable quantity of steam was immediately condensed and wasted, while the rest did not attain its full elasticity till the cylinder became again heated up to 212 degrees. These two defects—the imperfection of the vacuum created in the cylinder when hot, and the loss of steam in rushing into the cylinder when cold-were sources of great expense. Both defects, it will be observed, had their origin in the alternate heating and cooling of the cylinder; and yet, according to Newcomen's plan, this alternate heating and cooling was inevitable.

Watt remedied the evil by a simple but beautiful contrivancehis SEPARATE CONDENSER. The whole efficacy of this contrivance consisted in his making the condensation of the steam take place, not in the cylinder, but in a separate vessel communicating with the cylinder by a tube provided with a stopcock. This vessel being exhausted of air, it is evident that, on the turning of the stopcock in the tube connecting it with the cylinder, the steam from the cylinder will rush into it so as to fill the vacuum; and that this will continue until the steam be equally distributed through both vessels-the cylinder and the other. But if, in addition to being free from air, the separate vessel be kept con

stantly cool by an injection of cold water, or other means, so as to condense the steam as fast as it rushes in from the cylinder, it is evident that all the steam will quit the cylinder and enter the separate vessel to be condensed there. The cylinder will be thus left a perfect vacuum, without having lost any of its heat by the process; the piston will descend with full force; and when the new steam rushes in from the boiler, no portion of it will be wasted in reheating the cylinder.

So far the invention was all that could be desired; an additional contrivance was necessary, however, to render it complete. The steam in the act of being condensed in the separate vessel would give out its latent heat; this would raise the temperature of the condensing water;* from the heated water vapour would rise; and this vapour, in addition to the atmospheric air which would be disengaged from the injected water by the heat, would accumulate in the condenser, and spoil its efficiency. In order to overcome this defect, Watt attached to the bottom of the condenser a common air pump, called the condenser pump, worked by a piston attached to the beam, and which, at every stroke of the engine, withdrew the accumulated water, air, and vapour. This was a slight tax upon the power of the machine, but the total gain was enormous-equivalent to making one pound of coal do as much work as had been done by five pounds in Newcomen's engine.

This, certainly, was a triumph; but Watt's improvements did not stop here. In the old engine, the cylinder was open at the top, and the descent of the piston was caused solely by the pressure of the atmosphere on its upper surface. Hence the name of Atmospheric Engine, which was always applied to Newcomen's machine, the real moving power being not the steam, which served no purpose except to produce the necessary vacuum, but the atmosphere pressing on the piston with the force (supposing the vacuum to be complete) of about fifteen pounds to a square inch. This was attended with the inconvenience, that the atmosphere being cold, tended to cool the inside of the cylinder in pushing down the piston, which of course caused a waste of steam at every stroke. The inconvenience was avoided, and the whole engine improved, by entirely shutting out the atmospheric action, and employing the steam itself to force down the piston. This was accomplished in the following way. Instead of a cylinder open at the top, Watt used one with a close metallic cover, with a nicely-fitted hole in it, through which the greased piston-rod could move freely, while it did not allow the passage of air or steam. Thus the cylin

*The effect of the latent heat of the steam in heating the water in the condenser may be judged of from the fact, that if two pounds of steam be condensed by ten pounds of freezing water, the result will be twelve pounds of water at the boiling point; in other words, two pounds of steam at 212 degrees contain latent heat sufficient to boil ten pounds of freezing

water.