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the rod of the air-pump, but is received from the axis of the fly-wheel.

Plate I. STEAM-ENGINE contains a general view of the double-acting engine. The cylinder and piston have already been described, and as such will want no further illustration. The beam A H is in this case formed of cast-iron, and furnished with circular. projections for connecting on the various parts. The piston-rod, instead of being connected with a chain and arch-head, is now attached by a very beautiful arrangement, called the parallel motion. The beam moving on its axis C, every point in its arm moves in the arc of a circle of which C is the centre. Let B be the point which divides the arm A C into equal parts, A B and B C: and let D E be a straight rod equal in length to C B, and playing on the fixed centre or pivot D. The end E of this rod is connected by a straight bar E B with the point B, by pivots at B and E, on which the rod B E turns freely. If the beam be supposed to move alternately on its axis C,the point B will move up and down in a circular arc, of which C is the centre; and at the same time the point E will move in an equal circular arc round the point D as a centre. According to what we have just explained, the middle point F of the point B E will move up and down in a straight line.

Also let a rod A G, equal in length to B E, be attached to the end A of the beam by a pivot on which it moves freely; and let its extremity G be connected with E, by a rod G E, equal in length to A B, and playing on pivots at G and E. The point G will move in a straight line parallel to that in which F moves. The piston-rod of the steam-piston is attached to the point F, so that they are thus moved in parallel straight lines, one having double the motion of the other, being double the distance from the centre C.

The opposite extremity of the great beam is attached by the rod H I to the fly-wheel crank K.

U, plate I., represents the condenser, in which the vacuum is formed by the cold water, and W the airpump. In many cases it is of importance to ascertain the degree of exhaustion of the condenser, or rather the elasticity of the vapour remaining in it; this is now accomplished by adapting a barometer tube filled with mercury (open at both ends) to the condenser, and about thirty inches long. Its action is nearly the same with the common barometer. Were the vacuum in the condenser perfect, the mercury would be raised in the tube to the barometric height of the atmosphere; but, as this is never attained in common The fly-wheel has justly been considered one of instances in the condensers of steam-engines, the the most important and valuable parts of the steam-mercury in the tube is raised to a height correspondengine; when combined with the crank it is employed ing to the degree of its elasticity or exhaustion. A to convert a reciprocating into a rotatory motion. similar tube, called the steam-gauge, is fixed on the If of a moderate size, it should be cast of one piece boiler, which indicates, by the rise and fall of the of metal; this, however, cannot often be accom-mercury, the elasticity of the steam, that on the plished from its great weight, the fly-wheel of a large condenser showing the force with which the atmoengine frequently exceeding ten tons. When of this sphere pressed on the mercury to enter the condenser, size the ring is usually cast in six pieces of about a while the rise of the mercury in the tube placed on ton each, and connected by wrought-iron bolts; but the boiler shows the force with which the steam a method has recently been introduced in large en- presses it to enter the atmosphere, at all temperatures gines of substituting the dove-tail for that mode of above 212°: below this its action is the same as the connecting the parts. In this case the arms are condenser gauge. The pump which carries the hot fastened into the ring, and the segments of the ring water to the boiler is shown at X, and Y is the fastened together by a system of dove-tails which cold water pump. admit of being put together only in one direction, which is contrary to that in which the centrifugal force acts. It is a great object in constructing flywheels to choose that form which offers the least possible resistance to the medium through which it revolves, and on this account the ring should be smooth and truly circular, the radii being made with a thin edge to the air. It is also necessary that the various pieces connected with the fly should be cast in the most solid manner, as the centrifugal force of so large a mass frequently moving at the rate of more than 300 feet per second would, in the event of any part flying off, be productive of the most fatal

consequences.

In some of the early engines a sun and planet wheel

We have next to describe the governor. This is one of the most beautiful applications of theoretical to practical science on record. Its action will however be best understood by a reference to plate I., where it is shown in connection with the engine. L is a perpendicular shaft or axle, to which a wheel M with a groove is attached, and which turns with the shaft in pivots at the end of the shaft L. A strap or rope which is rolled upon the axle of the fly-wheel is passed round the groove in the wheel M, in the same manner as the strap acts in a turning lathe. means of this strap the rotation of the fly-wheel will produce a rotation of the wheel M and the shaft L, and the speed of the one will always increase or aiminish in the same proportion as the speed of the

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engines, is thus explained by Davies Gilbert, esq., the late learned president of the Royal Society, in his paper read before the Society, January 25, 1827: he says that "the criterion of the efficiency of ordinary machines is force multiplied by the space through which it acts (ƒ × 8); and the effect which they produce, measured in the same way, has been denominated duty, a term first introduced by Mr. Watt, in ascertaining the comparative merit of steamengines, when he assumed one pound, raised one foot

other. N,N, are two heavy balls of metal placed at the ends of rods, which play on an axis fixed on the revolving shaft at O, and extend beyond the axis to QQ. Connected with these by joints at QQ are two other rods RR, which are attached to a broad ring of metal, moving freely up and down the revolving shaft. This ring is attached to a lever whose centre is S, and is connected by a series of levers with the throttle-valve T. When the speed of the fly-wheel is much increased, the spindle L is whirled round with considerable rapidity, and by their na-high, for what has since been called in other countural tendency (the centrifugal force) the balls N, N, fly from the centre. The levers which play on the axis O, by this motion, diverge from each other, and thereby depress the joints Q, Q, and by this draw down the joints R, and with them the ring of metal which slides upon the spindle. By these means the end of the lever playing on S is depressed, and the end V raised, and the motion is transmitted to the throttle-valve, which is thereby partially closed, and the supply of steam to the cylinder checked.

We have shown in the plate the mode of procuring the hot water for the supply of the boiler, and the process by which that supply is regulated is beautifully simple. The apparatus called the stone-floatvalve is represented beneath.

E D

C

If we suppose the pipe G to serve as a channel of communication between the hot-water pump and the boiler, the box C will form a sort of reservoir for supplying the pipe V. The lever DE is provided with a balance-weight A, and the stonefloat F. Now,

if we suppose the float and balance-weight to be in the state of equilibrium, as soon as the water sinks in

the boiler the float F will have a tendency to follow it, and by that means open the value V, allowing a portion of water to descend, by which the float will be brought to its original situation.

tries the dynamic unit, and by this criterion one bushel of coal weighing eighty-four pounds has been found to perform a duty of thirty, forty, and even fifty millions, augmenting with improvements chiefly in the fire-place, which produce a more rapid combustion, with consequently increased temperature and a more complete absorption of the generated heat, in addition to expansive working, and to the use of steam raised considerably above atmospheric pressure."

Mr. Gilbert goes on to propose that, as a machine is efficient in producing duty or effect in proportion to the force applied multiplied into the space through which it acts, this function (ƒ × 8) should be denominated efficiency, retaining the word duty for a similar function indicative of the work performed: and, he adds, it is obvious that by a comparison of these two quantities, the efficiency expended on any machine and the duty performed by it, an exact measure will be ascertained of its intrinsic worth.

Having thus pointed out the general arrangement of the parts which constitute this extraordinary machine, it may now be advisable to furnish an accurate idea of its powers. These, in many cases, exceed those of the fabled genii of the eastern world, but the actual amount of power put in operation with a given quantity of fuel will be best understood by turning to the steam-engine as it is employed in the mining districts. This part of the subject has been very fully illustrated by Mr. Taylor in the Records of Mining.

Mr. Taylor says, "I believe that considerable misapprehension exists as to the means employed in Cornwall for measuring the duty of steam-engines, or, in other words, of determining, according to the preceding explanation, the number of millions of pounds of water lifted one foot high by a bushel of coal. It has been often objected that the account is taken inaccurately, or by interested persons, by such as wish to make engines appear better than they are; further, that the water discharged is not actually measured or weighed, and that calculating its quantity by the length and bore of the pumps is fallacious, as the valves are leaky, and the pumps sometimes draw air. Now, as to the impartiality and care with which the account is taken, I can only say it is done at the expense and under the direction of those whom, of all others, it most concerns to see that a correct estimate of the merit of the engines is produced,-I mean the proprietors of the mines, who employ these vast machines, and who look to these accounts to govern them in the choice of engines, and in their judgment of those who erect or superintend them. If they neglect the means which may render this information accurate, they must be very inattentive to their own concerns, and they misapply a liberal sum which is allowed by them for the service. It has also been said that the duty is reported by engineers or persons concerned in the manufacture of engines. I need hardly contradict this after the observations previously made, but shall only add that every one The history of mining in England is intimately acquainted with the mines of Cornwall is well aware connected with that of the steam-engine; and, if we that this assumption is not true, and that the had time to examine both, we should find that, as monthly reports are, in fact, the test by which enthe mines were the scene of all the early efforts of gineers must submit to be judged, and that, therefore, inventors or improvers of this machine, so they also of course, many such may find in them matter for have not only benefited by it, and in a great mea- dissatisfaction, although they seldom venture to quessure become dependent upon it, but have gone on tion their accuracy. Where doubt of this kind has contributing to the latest period towards its perfection been started, it has frequently been met by proper and economy. The term duty, as applied to steam-experiment and examination, and the correctness of

the statements have thus been very generally esta- I would do double as much as his own, which, blished. With regard to the second part of the as we see above, was 9.45 millions, consequently objection, we have seen that this mode of ascertaining 18,900,000. the comparative merit of steam-engines was introduced by Mr. Watt, and it is almost enough to remark that as he commenced so it continues to be practised. He did not pretend to measure or weigh the water discharged, but he ascertained it by the capacity of the pumps, as we now do: he knew 1778. Boulton and Watt erected an engine at that there were cavils to be raised as to valves, and Hawkesbury Colliery, near Coventry, cylinder fiftyother imperfections, but he saw that it was, notwith-eight inches diameter, stroke eight feet,load 26,064 lbs., standing, a good practical method, and was, there- which was found to do nearly 19,000,000. fore, a fair standard of comparison for steam-engines working pumps, which would, after all, be much alike in those respects; and that thus the objection, as between one and another, would have but little weight.

Note. The best was 7,440,000, the worst was 3,220,000. 1772. Smeaton began his alterations in the steam-tion of the adventurers in the mines was more disengine, and succeeded in performing 9,450,000.