5 ft. COUPLED GOODS ENGINE.

TABLE showing the Working of the Slide Valves of the "Jason" Engine, fitted with Expansion Gear, 16 in. Cylinders, 24 in. Stroke, Feb. 1852.

N.B. The Front Stroke is the Piston moving from the Buffer Bar towards the

Fire Box.

5 ft. COUPLED GOODS ENGINE.

TABLE showing the Working of the Slide Valves of the "Giaour" Engine, fitted with Expansion Gear, Cylinders, 24 in. Stroke, May, 1852.

17 in.

Outside Lap of Slide, 1 in. | Dimen. of Steam Ports,

Steam cut

Exhaust

opens.

Compression begins.

Slide opens.

Remarks.

Front

Stroke.

Back

Stroke.

Front

Stroke.

Back

Stroke.

Front

Stroke.
Back

Stroke.

N.B. The Front Stroke is the Piston moving from the Buffer Bar towards the Fire Box.

The physical condition of bodies is determined by the action of two forces acting in opposite directions; viz. the pressure exerted upon them, and heat communicated to them. We may admit, as a general principle, that all bodies exist in three different states-solid, liquid, and gaseous; under the influence of the varied action of the two forces above-mentioned, increase of pressure tending to induce the solid state, and that of temperature inducing the gaseous state.

Water, which is liquid under the ordinary pressure and temperature of the atmosphere, assumes the gaseous state either by diminishing the pressure or by increasing the temperature in a certain ratio. Under the atmospheric pressure of 14.8 lbs. per square inch, and at a temperature of 212° Fahr., it becomes gaseous, or evaporates; under a higher pressure it will evaporate only at a higher temperature, and with a less pressure the evaporation will take place at a less temperature. According to the law by which these phenomena are governed, the increments of pressure being supposed regular, the respective increments of temperature at which evaporation takes place will not be regular.

If, for instance, the boiling points be denoted by T and T'at 30 and 35 lbs. per square inch, and if at 25 and 30 lbs. by T" and T, then, although the differences 3025 and 35-30 be equal, yet the differences T-T" and TT will not be equal.

The law of the increments of temperature corresponding to equal increments of pressure cannot be expressed by a simple formula; the following Table will be found very useful.

In this Table the first and second columns contain the pressures expressed in atmospheres and in pounds per square inch, the third indicates the corresponding temperature of the steam; the other columns show other properties of steam calculated partly from the preceding columns and partly from the density of the steam at a certain pressure ascertained by experiment.

A quantity of steam being inclosed in a vessel where water is also present, the temperature will bear some relation to the pressure, and the temperature, although it may be above what is indicated by that relation, cannot fall below it. Now let a part of that steam be put into a separate vessel, and let the vessel be increased in volume, there being no escape of steam or temperature from it, then the pressure will become less, and as the specific heat of gases increases as the pressure diminishes, there must

also be a fall of temperature. It is upon this principle of the difference of specific heat of gaseous bodies at different pressures that their temperature is raised by reducing their bulk.

Let us take a certain volume of steam at 212° and at one atmosphere, and compress it till it has a pressure of three atmospheres; supposing that there is no loss of heat in the experiment, the elevation of temperature which follows will be greater than, equal to, or less than the difference of 212° and 275°, the latter being the temperature given in the Table opposite to three atmospheres.

If it be equal, the fall of temperature which results from the increase of volume which we have alluded to above, will be such, that when the pressure diminished by the expansion is one atmosphere, the temperature of the steam is exactly 212°. If it be less, the temperature of the steam after expansion will be between 275° and 212°. If it be greater, the final temperature will be less than 212°, if the steam could remain in the gaseous form; but since this cannot take place, except by raising the temperature to balance the pressure tending to reduce the steam to water, it follows, in the last case, that when a fall of temperature takes place without a corresponding diminution of pressure, condensation must ensue.

The latent heat evolved from the condensed vapour will be immediately absorbed by that portion which remains as steam; the process of condensation will cease immediately, and will be resumed only when a repeated increment of volume causes a further decrement of temperature.

Supposing this process to be repeated, it is clear that when the steam comes to 212° there will be produced a certain quantity of water.

In order to produce these phenomena, as we have before observed, it is necessary that the heat evolved by the steam passing from 275° to 212° be insufficient to supply the additional absorption arising from the difference of the specific heats. Now that this is the case is evident from the known laws of the specific heat of gases.

F

Although we are ignorant of the specific heat of steam at different pressures, yet we know the laws for other gases; we may therefore infer that, for an increase of pressure of two atmospheres, which we have supposed, the increase of temperature will undoubtedly be much greater than that of 212° to 275°. *

The law being general for specific heats of gases, which have been proved by experiment within small limits, we may consider ourselves warranted in deducing the existence of the law of partial condensation, otherwise the vapour of water is an anomaly among the other gases; it would therefore violate the law of continuity.

If the condensation were of great importance it would enter as an element into the calculation of the effect of engines working by expansion, as its effect is to accelerate the reduction of pressure arising from increase of volume, just as that effect would take place if the water of condensation were continually drawn from the cylinder as it was formed during the stroke. There are, besides the reduction of temperature, which is not taken into account in calculating the expansive force of steam, and from which there must necessarily follow an expansive force inferior to what is supposed, other causes of reduction in the effect of steam acting expansively. The object which we have had in view in what we have stated, has been simply to show that, in every case where the pressure of the steam is diminished, the temperatures corresponding to the successive pressures are always those which correspond to its formation under the given pressures as put down in the Table.

We shall enter more fully into this subject afterwards.

* A writer in the Mechanics' Magazine gives an investigation which shows that the "specific heats are inversely as the atomic weights," which is the celebrated result of the experiments of Dulong and Petit. A Table will afterwards be given which contains the results above-mentioned.