stated. In order to raise 0 345 kil. of steam from 143° to 230°, the temperature to which superheating was carried, we have, on the other hand, 27 8 calories.

The amount of caloric necessary to dilate steam in the ratio of 0.845

is then scarcely a 0.426th part of the quantity required to 0.451 furnish directly from water a volume equal to the excess which is due to expansion. If it now be remembered that the superheating coil acts as a supplementary boiler, it will readily be understood that the dilatation due to superheating is readily convertible into economy of fuel.

of

It has been shown that Woolf's engine afforded an economy 16 per cent. by the employment of steam superheated to 210°. The same calculations and reasonings are equally applicable in this as in the preceding case, and it consequently follows, that this machine should produce an economy of 16 per cent. on the fuel, caused by the excess of volume which the steam has been made to occupy. This machine not only gave, under these circumstances, an economy on the fuel of 16 per cent. but also showed an increase of 5 horse-power.

Neither the evaporation of the vesicular water, nor the expansion which the steam is made to undergo, are, however, sufficient to effect this increase of power, and it is, consequently, necessary to investigate still further the various phenomena relating to superheated steam.

3.-Modifications of the laws relating to the expansion of steam. It is well known that when steam is made to expand, without at the same time receiving an accession of heat from the surfaces of the vessel in which it is enclosed, a certain amount of condensation is the consequence, arising from the fall of temperature consequent on the increase of volume assumed. By obviating this condensation, and the consequent rapid diminution of pressure, the steam-jacket of Watt produces its beneficial effect. This arrangement communicates caloric to the steam, and thus prevents condensation; for, although the heat supplied is originally derived from the boiler, the loss of power economised amply compensates for the loss sustained. The addition of caloric to steam, whether previous to its admission into the cylinder or at the moment of its expansion, will afford results of a somewhat similar character, and it may consequently be anticipated, that the effect produced would in a great measure resemble that obtained from the employment of the ordinary steam-jacket.

The permanent gases expand according to two principal laws, whilst steam is affected by a third, dependant on its calorific state before and after expansion.

1st.-When the temperature of a gas is maintained constant, the pressure decreases in the inverse ratio of the volumes which it is successively made to occupy.

2nd. When an increase of caloric is not imparted to gases during their expansion, they necessarily become cooled, and the relation which exists between their successive tensions and volumes differs essentially from those they should possess in accordance with the law of Mariotte.

3rd.-When no addition of caloric is made to expanding gases, partial condensation takes place, and the relations which exist between the successive volumes and pressures is in this case not known. All that has been ascertained is, that the pressure decreases much more rapidly in relation to the volumes than when permanent gases are concerned.

It follows from the above that steam may be expanded under three different conditions.

First. When heat is not applied.

Secondly. When only a sufficient amount is added to prevent condensation.

Thirdly. When a sufficient addition is made to maintain uniformity of temperature.

During the time that steam is flowing from the boilers into the cylinder, the whole of the surfaces with which it comes in contact must, at least, be elevated to the temperature which corresponds to the point of saturation; since condensation will not be arrested until that degree has been attained. The moment the valve is closed, and expansion, consequently, begins, the surfaces of the cylinder and piston are placed in contact with steam of which the temperature is continuously decreasing, and it consequently follows that their acquired caloric is partially given up to the expanding steam. It ensues from this that in no form of engine the steam expands without at the same time receiving a certain addition of caloric, and, consequently, that even if the laws relative to the expansion of saturated and superheated steam had been perfectly determined by experiments, they could not be applied with any degree of accuracy for the purpose of calculating the advantages to be derived from working steam expansively in the cylinder.

On the other hand, it is quite impossible to so arrange the surfaces of a cylinder in which steam is expanded as to retain a constantly equal temperature.

Since the cylinder of a steam-engine must be regarded both as a source and reservoir of heat, and not merely as an envelope to retain the steam, it naturally follows that its action on the enclosed vapours will be more or less modified in accordance with

the circumstances under which it is admitted from the boiler. In fact, the influence of the surfaces of a cylinder on the vapour it encloses are such as to entirely modify the properties of steam, abstractedly considered, and to render any calculations founded on the properties of watery vapour in its isolated state in a great measure inapplicable to an analysis of its action on the pistons of steam machinery.

The foregoing, although a mere abstract of the mémoire of M.Hirn on this important subject, may serve to explain both the arrangement of the apparatus he employs, and the results obtained therefrom, as well as to show the character of the deductions to which he has arrived, and the extremely complicated nature of the various elements necessary to a theoretical solution of the problem.

RESUMÉ.

The various causes which effect the economy arising from the employment of superheated steam are recapitulated by M. Hirn as follows::

the

Superheating entirely obviates the inconveniences arising from presence of vesicular water, and consequently a greater amount of steam is obtained from a given quantity of fuel.

The vesicular water thus introduced evaporates during expansion, and, consequently, operates prejudicially by cooling the surfaces of the piston and cylinder, which, having to be again reheated by the steam admitted when the valve is next opened, causes a considerable loss of caloric. When, on the contrary, superheated steam is employed, no traces of vesicular water can enter the cylinder, and, consequently, this source of loss is avoided. The loss of heat caused by the cooling of the metallic surfaces is much greater than is generally supposed, and was first pointed out by M. Combes.

Saturated steam is, under ordinary circumstances, partially condensed during its expansion; and as this phenomenon causes a great diminution of pressure, a corresponding loss of power is naturally the consequence.

Since the power of steam depends (all other things being equal) on the volume employed, and not on its weight, it follows that superheating, by which aqueous vapour may be expanded at least one-fourth of its original bulk, yields a corresponding increase of power; and as the expenditure of caloric necessary to produce this effect is extremely small, an equivalent saving of fuel must be the result.

This economy of one-fourth must obviously be a minimum, since three other causes constantly contribute to produce the same effect.

117

CRUSHING AND DRESSING MACHINERY.

METALLIC ores are usually concentrated by the agency of water, in accordance with the following laws.-If bodies of various sizes, forms and densities, be let fall into a liquid in a state of rest, it is evident that the amount of resistance which they experience will be very unequal, and that they will consequently not arrive at the bottom at the same time. This necessarily produces a sort of classification of the fragments, which becomes apparent on examining the order in which they have been deposited.

If it be supposed that the substances have similar forms and dimensions, and differ from each other in density only, and it is known that the resistance which a body will experience in moving through a liquid medium depends solely on its form and extent of surfaces, and not on its specific gravity, it follows that all substances will lose, under similar circumstances, an equal amount of moving force.

This loss is, however, most sensible in substances possessing this power of movement in a less degree; or, in other words, it will be proportionally greater in light bodies than in those having a more considerable density. The former, for this reason, fall through the liquid with less rapidity than the denser fragments, and must therefore arrive later at the bottom; so that the deposit will be constituted of different strata, arranged in direct relation to their various densities, the heaviest being at the bottom and the lightest at the top of the series.

Supposing, on the contrary, that all the bodies which fall through the liquid possess similar forms and equal specific gravities, and that they only differ from each other in point of volume; it is evident that the rapidity of motion will be in proportion to their sizes, and the larger fragments will be deposited at the bottom of the vessel.

As we have supposed them on starting to have the same forms and densities, it follows that the resistance they experience whilst descending through the water will be in proportion to the surface exposed: and as the volumes of bodies vary according to the cubes of their corresponding dimensions, whilst the surfaces only vary

in accordance with the squares of the same measurements, it follows that the force of movement animating them is regulated by their cubes, whilst their resistance is in proportion to their squares.

If, lastly, we imagine that all the fragments have the same volume and density, but are of various forms; it follows that those which possess the largest amount of surface will arrive at the bottom last, and, consequently, the upper part of the deposit will consist of the thinnest fragments.

It is evidently, then, of the greatest importance that the grains of ore which are to be concentrated by washing should be as nearly as possible of the same size, as otherwise the smaller surface of one fragment, in proportion to its weight, will in a measure compensate for the greater density of another, and thus cause it to assume a position in the series to which by its constitution it is not entitled. This difficulty is constantly found to occur in practice; and in order, as much as possible, to obviate it, care is taken to separate, by the use of sieves, into distinct parcels, the fragments which have nearly the same size. Although, however, the grains of ore may by this means be, to a certain extent, classified according to their respective dimensions, it is impossible by any mechanical contrivance to regulate their forms, which must greatly depend on the natural cleavages of the substances operated on, and therefore this circumstance must always in some drgree affect the results obtained.

Each of the broken fragments of ore must necessarily belong to one of the three following classes:-The first class consists of those which are composed of the mineral sought, without any admixture of earthy matter. The second will comprehend the fragments which are made up of mixture of mineral ore and earthy matters; whilst the third division may be entirely composed of earthy gangue, without any admixture of metallic ore. By a successful washing these three classes should be separated from each other. The first will form the lowest stratum, and the mixed fragments follow next in succession, whilst the unproductive portion is deposited upon the two other layers.

CRUSHING APPARATUS.

CRUSHING MILLS.-Among the various modern improvements introduced into mining is the crushing mill. This machine, which has been brought to a great degree of excellence, is become of much value for the treatment of many varieties of ore, and is more especially adapted for mines yielding large quantities of dredgy or disseminated mineral.