can be ascertained with great accuracy, by gauges and safetyvalves; but the resulting disposable power is not so easily discovered, as the friction of the various parts vary greatly according to the state which they are in. The state of the condensation, in condensing engines, gives a more or less perfect vacuum, which will vary notwithstanding the utmost vigilance. It has been generally set down, among engineers, that nearly one half of the power of steam must be deducted from the disposable force; therefore, suppose an engine of 24 inch piston, the area of which will be 452 square inches, has a perfect vacuum, as exhibited by the barometer of the condenser, and the weight of the atmosphere, denoted by the weather-barometer, be about fourteen pounds, and the steamgauge on the boiler stands at about two inches, which is an indication of two pounds pressure, we may estimate that there is 17 lbs. per square inch pressing upon the piston; therefore 17 x 4527684 lbs. on the piston, half of which being deducted for allowance of friction, leaves a disposable force of 3842 lbs. moving through the distance at the same rate in which the piston moves; which force being divided by Messrs. Bolton and Watt's estimate of a horse-power, will give the nominal power of such engine. In high-pressure engines, where the steam is not condensed, what is indicated by the steam-gauge of the boiler only, must be estimated as the power acting upon the piston.

That the increments of power take place in a quicker ratio than those of the temperature, has been long known; and an ingenious mechanic of the present day has attempted to use steam at very high pressures. Without entering into a description of the obstacles he met with, we will briefly observe, that the requisite strength of the parts to withstand the pressure, conjoined to the excessive heat, present obstacles not easily to be overcome.

The reciprocating motion in steam-engines is a loss of power which cannot be denied. For the momentum of the beam and other parts passing in one direction have suddenly to be arrested and moved in the opposite direction, which produces a loss of power. Rotatory action has been sought, therefore, with propriety, but has not yet been obtained with advantage.

Messrs. Boulton and Watt, in the introduction of the steam-engine into many works where the power of horses was used, were obliged to take into consideration the number of horses used for any particular purpose, in order to ascertain the amount of force wanted. Upon the conclusion of a

numerous set of experiments they decided, that a horse, working eight hours a day, was capable of raising 33,000 lbs. one foot high, in a minute. Therefore, by dividing the nuinber of pounds an engine can lift one foot high in a minute, it will give the amount of horses' power to which that engine is equal.

An entire view of an engine of the construction termed portable, is represented at fig. 207. A is the cylinder, B the air-pump, C the cola-water pump, D the hot-water pump, E the beam, F the connecting rod, G the fly-wheel, H the eccentric shaft, and I the governor.

It would occupy many volumes to describe the various forms of construction of engines which have, since the knowledge of the power of steam, been contrived; and the information such descriptions would convey would be, comparatively speaking, of very little value, as the majority of them have arisen from men ignorant of the principles of the action of the machine, and whose productions should be classed as futile alterations.

In attempting any improvements, the principles of action should first be taken into consideration. In condensing engines the movement is effected by the alternate increase and decrease of temperature, the perfection of both of which is of great importance. The primary point to be aimed at, therefore, is the maintaining of high temperature whilst the steam is forcing, and reducing it suddenly when the condensation is to be effected. This was taken into consideration in the construction of Newcomen's engine, and was most effectually attained by Mr. Watt.

The other parts of the engine may be examined with a view of improvement, by considering their weight and friction, and by the substituting of a rotatory instead of a reciprocating motion.

Simplicity of construction cannot be too strongly recommended in all mechanical combinations; for there are many contrivances which would certainly be deserving of the name of improvements, were they not inapplicable on account of their intricacy.

Attempts have frequently been made to avoid the use of the air-pump, which takes up a considerable portion of the power of an engine. A water barometer, adapted to the condenser, has been sometimes adopted; and a fall of water has been made to pass over the upper edges and down the orifice of a tube, forcing the air before it. The upper end of this tube communicates with the eduction pipe, and is said to support a yacuum of considerable rarity. Exposing the steam which is to be condensed to an increased surface by passing it along

tubes surrounded by water, or amongst tubes containing water, has likewise been frequently adopted. Indeed the exposure of considerable surface to receive heat in the generation of steam, and the same to abstract it in condensation, have been subjected to frequent trial. That an advantage is to be procured by the adoption of such plans is undoubted; but to attain such increased surface an intricacy in the parts, we fear, must be adopted, which will more than counterbalance the advantages gained.

The valves, or those parts of an engine which direct the distribution of the steam, have always had the attention of engineers, and, as we have shown, many elegant combinations have resulted from their ingenuity.

In running a steam-engine, attention should be given to the working parts. The cylinder should be packed with clean hemp and the best tallow, and frequently examined to see that the packing is in order. The steps of the fly-wheel, shaft, and of the crank, beam, &c. should be frequently examined, and kept well oiled with sperm oil, which is the best for all machinery. These parts must be kept from dust, and if dry grindstones are driven in the mill, the dust must be carefully boxed off from the engine. The use of sand on the floor of an engine-house should, for the same reason, be dispensed with.

The method of starting an engine is, first to shut the condensing cock, then to open all the valves to let the steam pass into the jacket, into the cylinder, through the eduction pipe into the condenser, and out at the blow-valve, in order to expel the air from all the parts, and get them to a proper temperature, which will be shown by the steam issuing from the blow-valve; for previously to the parts being sufficiently warmed, the steam in its progress becomes condensed.

When all the parts are heated, the injection water may be let on, and a vacuum procured on one side of the piston, which produces instant action.

The lever of the throttle-valve, which is ultimately to be attached to the governor, should, on starting the engine, be held by the hand of the attendant until the work is thrown on, and the engine has acquired regular motion.

BROWN'S VACUUM, OR PNEUMATIC ENGINE.

HAVING Concluded our account of the steam-engine, we shall now proceed to give a description of the engine abovementioned, which has recently claimed much attention from the mechanical part of society. It is represented in fig. 208. AA a beam, capable of vibrating upon a centre at B.

C and C two chambers, formed of metal, of sufficient strength to resist the pressure of the atmosphere (about 14 lbs. to the square inch) upon its external surface, and having the caps C2 C2 suspended, one at each end of the beam, capable of closing each of these chambers in an air-tight manner. The chamber C1 is shown in section.

EE and E1 E1 are two pipes, containing valves opening upwards, leading and affording a communication from the vessels F and F with each of the chambers C and C'. These vessels, F and F1, contain floats, F2 F2, attached to the beam AA, by rods which receive motion from the floats; to these rods are attached the slides tt, to close alternately, at each vibration of the beam, the apertures h h. The pins pp, attached to one of the rods from the floats, give motion to the small vibrating tube R, which, by the rods R1 R1 attached to the cranks in the chamber S, alternately opens and closes the pipes S1 S1, communicating with the vessels F and Fi,

DD is a pipe leading from the gasometer, branching off at D' D1 into the two chambers C and C1, for the purpose of supplying the gas that is to be consumed in effecting the vacuum. This supply can be admitted or shut off by means of the cocks D2 D2, which open and close by cranks, worked by the movement of the beam.

GG two other branch-pipes, supplied with gas from the gasometer, and ending in a jet at each end. By the slanting direction of the ends, it is evident, that the flames from these jets will, when their respective orifices hh be open, protrude into the chambers C and C1.

K and K1 are two pipes, affording a communication from the outer air to the interior of each of the chambers C and C1; their outer ends are capable of being closed by means of the cranks n n, which are attached by chains to the floats Fa F2.

The mode of operation consists in allowing the gas to pass from the gasometer along one of the branches of the pipe D D, and thence into one of the chambers C or C', (suppose C',) where, by the jet of ignited gas playing in the orifice h, it becomes ignited, and by its combustion rarefies and expels a considerable portion of the atmospheric air from that chamber. Suppose now the cap of the chamber be put down, and by the movement of the rod attached to the float the orifice h and gas-pipe D be closed, the combustion will immediately cease, and leave therein a partial vacuum. The atmosphere beginning now to press upon the vessel F, will cause so much of the water to pass from it into the chamber C1 as will nearly compensate the vacuum, when the valve through which the water passed being closed, and communication between the interior of the chamber and the open air effected by the opening of K1, the water contained in the chamber flows from thence through the aperture u, and affords power, by its fall and weight, to the overshot water-wheel W. From hence it passes into the vessel S, and finally is admitted by S! S' into F or F1, leaving the engine in a condition to renew the operation.

By inspecting the plate it will be seen, that when the cap of one chamber croses, the several openings to the same chamber close with it; and by the rising of the other end of the beam the similar openings to the other chamber are opened, and prepared for a like operation. It will also be seen that the production of this motion is attained by the rising of the two floats in the chambers F and F1.

The advantages to be derived from this engine, as detailed in the descriptive outline of the inventor, are,

First, "The quantity of gas consumed being very small, the expense of working the engine is moderate. In its application on land the saving will be extremely great, the cost of the coal gas (deducting the value of the coke) being inconsiderable, The expense of working a marine engine will certainly be greater, as the gas used for that purpose must be extracted from oil, pitch, tar, or some other substance equally portable, yet even in this case, it will not equal the cost of the fuel required to propel a steam-boat; and as a few butts of oil will be sufficient for a long voyage, vessels of the largest tonnage may be propelled to the most distant parts of the world.

Secondly, "The engine is light and portable in its construction, the average weight being less than one-fifth the weight of a steam-engine (and boiler) of the same power. It also occupies a much smaller space, and does not require the erection of so strong a building, nor of a lofty chimney. In vessels, the saving of tonnage will be highly advantageous, both in the smaller comparative weight and size of the engine, and in the very reduced space required for fuel.

Thirdly, "This engine is entirely free from danger. No boiler being used, explosions cannot take place, and as the quantity of gas consumed is so small, and the only pressure that of the atmosphere, it is impossible that the cylinder can burst, or the accidents incidental to steam-boats occur.

"The power of the engine (being derived from the atmospheric pressure of ten pounds and upwards upon the square inch) may be increased with the dimensions of the cylinders, to any extent, and always ascertained by a mercurial gauge.

"It is scarcely necessary to allude to the well-known fact, that, after deducting the friction arising from the use of the air and cold-water pumps, &c. &c. the general available power of the condensing steam-engine is from seven to eight pounds per square inch.

"The cost of the machine will be more, particularly as constructed for raising water; it is therefore peculiarly adapted for draining fens, &c. or supplying reservoirs. The expense of wear and tear will also be considerably less than that of the steam-engine, and when occasionally out of order,