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History of the Steam Engine.

CHAPTER III.

CONTENTS.-DEFECTS OF BOLTON AND WATT'S ENGINE.-FRICTION-RECIPROCATION-POWER OF THE CRANK-NO POWER LOST THEREBY—IRREGULAR WEAR OF THE PISTON ROD AND CYLINDER.-DISADVANTAGES ATTENDING THE USE OF TWO ENGINES WITH THE CRANES AT RIGHT ANGLES WITH BAGH OTHER.

We now come to speak of the imperfections attendant on the Bolton and Watt engine: these are, friction from the rubbing of the moving parts against each other-the reciprocation of the machinery, and the irregularity of the motion: we shall notice them successively. 1st, The rubbing of the parts against each other.-This evil must exist in every conceivable form into which the steam engine may be modified, but no doubt the quantity may be considerably reduced. The steam, in order that we may have its full effect, acts against a moveable piston in a cylinder from which it cannot escape. This piston is, as has been explained, packed or stuffed on its edges, which prevents the steam from escaping past it; and from the nature of the material used for packing, and the tightness with which it is pressed against the cylinder the friction arises. This is sometimes so great that we have seen engines where the whole force of the steam could not give them motion. It is usually estimated at one third of the power of the steam-that is to say, if the steam acted upon a piston with a force of 1500 lbs., the effect produced would not exceed 1000 lbs., a power of 500 lbs. having been absorbed by the movement of the machinery alone.

The next objection is the reciprocation of the parts. This is an evil of considerable magnitude. It originates from an inherent law in matter by which all bodies have a tendency to continue in the motion communicated to them, or remain in their natural state of rest. If a cannon ball be discharged from the mouth of a cannon, it requires an exertion of force to give it an impetus greater than would be required to continue its motion. If its progress be arrested whilst in motion a shock will be experienced by the body which impedes it, the force of which shock will vary as the velocity of the ball. When this ball ceases to move without any visible impediment it is not that its original impetus is exhausted or spent, (though the latter term is frequently used) but that it is gradually overcome by the particles of air which form a succession of points of resistance upon which its force is nearly destroyed, and it is then drawn down to the earth by the superior attraction of gravitation. If we could destroy the intermediate resistance of the air the ball would continue in motion for ever, because nothing would intervene to destroy the primary impetus. This property of matter occasions a considerable destruction of power in the steam engine. The motion of a massive beam, and its necessary appendages of machinery, a piston, connecting rod, parallel motion, and pump buckets, have to be reversed at each stroke of the engine, and that too when the speed is very great. The natural state of rest, or visinertia, i. e.the force of inactivity, has to be overcome at the commencement of each stroke, and when a great velocity is acquired

it is as suddenly checked to prepare for the returning one. This necessarily produces a great strain upon the machinery, which must be made proportionably more massive: and it requires likewise great skill in the construction of the engine house to prevent its being ultimately destroyed by the alternate push and pull which it experiences at each reversion of the beam. We have repeatedly noticed the best constructed engine houses shaken, and almost falling to pieces from this cause.

Various schemes have been proposed to remedy one of the evils of reciprocation. We mean the shock experienced by the reversion of the matter. It is not expected to prevent the loss of power sustained thereby; for that must remain as long as the law of which we have just spoken exists. Where a crank and fly wheel are used to obtain a rotatory motion a shock is prevented by the velocity being gradually retarded, the crank having to perform a greater portion of its revolution with only the same surface of steam at the commencement and termination of each stroke of the piston: we explain our meaning by reference to the marginal diagram. a b is the crank of a steam engine, of which the semi-circle, d, c, a, c, f, represents the motion communicated by one stroke of the piston; when, therefore, the crank in its present position is moved from a to c, the piston is at its greatest speed, and travels nearly at the same velocity as the point a of the crank. But when moving from c to d, an equal piston of a revolution, the piston only moves a distance equal to gd, in the same space of time, as it had previously moved a distance equal to 6 g, almost double of d g. Hence it appears that the crank, by gradually

h

decreasing the speed, is admirably adapted for preventing the violent shock which would otherwise be experienced by the piston striking the top and bottom of the cylinder, and by changing the motion of the beam too suddenly, but it does nothing towards reducing the power lost by reciprocation. In pumping engines, where a fly wheel and crank are not used, other means are adopted to check the force of the piston, or guard against the shock of suddenly changing the motion of the beam. In the coal districts the usual way is to shut off the steam when about two thirds of a stroke has been performed; the expansive force of that already in the cylinder, together with the impetus of the piston sufficing to barely carry it to the termination without violence. In such engines springs are sometimes fixed above and below the beam, so as to check its progress should the steam possess more force than may be expected. "It once happened," says Mr. Farey, "that the valve of the pump bucket breaking, the engine suddenly lost its load or resistance, which occasioned the piston to descend and strike on the spring beams for two or three successive strokes with such violence as to break one of the beams, and at last the piston striking the bottom of the cylinder, the momentum of the beam forced down upon the rod so violently as to bend the great piston rod quite

crooked. To prevent similar accidents, a smaller steam pipe wasadded to the side of the vertical steam pipe communicating with the passage into the bottom of the cylinder. This pipe is kept closed by a valve; but if the engine descends so low as to strike on the spring beam, a catch pin on the beam strikes a small lever, and by a wire of communication opens the valve and lets the steam into the lower part of the cylinder beneath the piston and thus destroys the vacuum, so. as to prevent the further descent of the piston."

This addition, it will be understood, applied only to the single acting engine, but it serves to shew that the objections we have given. arising from momentum are not merely theoretical.

The beautiful addition of the crank to the steam engine, although the means of extending its utility tenfold has been the subject of much objection. Engineers and others possessing considerable claims to the character of scientific men, have not unfrequently maintained that there is a considerable loss of power by the change in the length of the lever as the crank revolves. We shall endeavour to show the error into which such persons have fallen.

The principle of the lever is so well known, that it is scarcely necessary to explain it: lest, however, it should not present itself to all our readers, we shall give a short description. "In all levers the universal property is, that the effect of either the weight or the power, to turn the lever about the fulcrum, is directly as its intensity and its distance from the prop; whence it is deduced, that if parallel forces acting perpendicularly upon a straight lever keep it in equilibrio, they will be to each other reciprocally, as the distances from the fulcrum upon which they act." Thus, supposing a bar of four feet in length be fixed upon a fulcrum exactly in the middle, and an ounce weight be suspended at each end, the two ends will be in equilibrio, because the force of gravitation is equal, neither possessing it in a greater degree; but if the fulcrum be shifted and placed three feet from one end, then it will require three ounces at the shorter end to balance one ounce at the other. If motion be given to the shorter end whilst the fulcrum remains the same, the end of the longer lever will traverse three times the space of the shorter.

The crank of a steam engine is a lever whose facrum is at a. It is the nature of the crank that its power or leverage varies with its position. Let ab represent the crank, the point b is moved by the connecting rod, and revolves round the centre a. Supposing the resistance be equal to 100 lbs. or that 100 lbs. have to be raised 3.1416 feet for every revolution of the crank it is evident if a force or weight exceeding 100 lbs. be applied at b, whilst the crank is horizontal, it will be sufficient to raise the weight. But when the point 6 has descended to c, the length of the lever being described by its sine, the vertical line, e c, drawn through

* Good and Gregory.

ab, shews ca to be the length of the lever, which is only one half of ba. It would, therefore, require a weight double of the former to continue the motion. And if the crank descend to f, the vertical line, df, shews da to be the length of the lever, and to be only one fourth of what it was when horizontal. When it reaches g no power on earth applied through the medium of the connecting rod, would further continue the motion.

To equalise this irregularity, and in some degree to compensate this great variation, the cylinder is of such dimensions as to give out a considerably greater power when the crank is horizontal than is then necessary. This extra power is employed to give motion to the fly wheel, which is of sufficient dimensions to retain the impetus until it is past the point d, whan the steam begins to act with effect upon the lower side of the piston.

Let a represent a cylinder, the length of the stroke of the piston being two feet. de is the crank, the length of which is one foot; b.c, the beam, the fulcrum of which is exactly in the middle. If the piston be put in motion the extreme end of the crank will describe a semi-circle of 3.1416 feet. Now let us suppose that a drum be fixed upon the axle e, whose circumference is four feet, equal to one ascending and one descending stroke of the piston. If a weight be suspended by a rope to this drum, as at h; the power of the engine at that point will exceed the power necessary to raise the weight as much as de exceeds ie. This extra power is communicated to the fly wheel, which faithfully gives it out when required. When the crank has descended so as to decrease the length of the lever, that it is shorter than i e, then a portion of the extra power in the fly wheel is destroyed in aiding the decreased leverage of the crank. And although the power gradually decreases, yet the speed of the piston gradually decreases also, so that if the power of the crank be only one half in a certain position, yet the quantity of steam used is only one half, and thus the effect of no part of the steam is wasted,

the effect being in every point equal to the steam expended. It is true that if we could have applied the power at a point equidistant from the centre in every part of the revolution, we should have obtained much greater leverage, but then the expenditure of the steam would have been proportionably greater.

We will further explain this theory by referring to another diagram. gd represents a lever like the crank:

g being the axle. ab are two vessels fitted with pistons, and in every respect resembling cylinders, excepting that they are curved so as to describe portions of circles formed from the point g. We will suppose that the piston in a acts upon the extremity of the crank, and that in b, at half the distance from the centre. The

vessels are of the same area, so that if steam were introduced from a boiler, it would press with equal force upon each piston, and consequently the rods would each press with an equal force upon the points, e d. Now it would be maintained that, because at e there is only half the leverage, therefore half the effect of the steam in bis lost; but it will be found, that if that lever, gd, be moved any given distance round its centre, that the piston, b, only moves half the distance of the piston, a; and consequently, the areas being equal, and the distance but one half, only half the steam is expended. Hence it is clear, that the consumption of the steam in every point of the lever is only equal to the effect produced.

There are minor objections against Watt's engine which, nevertheless, should be noticed. One is the waste of steam at the reversion of the motion of the piston. First, from the pipes between the valve and the cylinder. In filling the cylinder these must be filled, and in discharging, these must likewise be emptied; so that they are filled and emptied at each change of the motion. But in the cylinder every particle of the steam produces an effect: whilst here the steam used produces no effect, and is therefore wasted. Secondly, From the changing of the valves themselves at the improper time. It may indeed be said, there is no proper time to change the valves, because there is no time at which they can be changed without disadvantage by loss of steam; and the difficulty of determining the precise time frequently occasions their being changed at such a time as to waste more steam than is unavoidable. The necessary waste arises from the change of the valves being a work of time, whilst the reversion of the stroke is instantaneous: therefore, either the change of valves begins too soon, and admits steam into the vacuum before the stroke is completed, or ends too late, and admits steam into that part of the cylinder when a vacuum is forming, thereby preventing its formation; or otherwise it is attended with both these disadvantages. The improvements in the valves, we are sorry to say, have but increased this difficulty, and which we shall notice in their proper place.