most inconceivably luxuriant vegetation, there was stored up for the benefit of the human race, then uncreated, an inconceivably great treasure of potential energy, which we are now just beginning to utilize. This potential energy becomes kinetic and available wherever and whenever the powerful chemical affinity of oxygen for carbon is permitted to come into play; and the fossil fuel stored in our coalbeds or the wood of existing forests is, by the familiar process of combustion, permitted to return to the state of combination with oxygen in which it existed in the earliest geological periods.

The philosophy of the steam-engine, therefore, traces the changes which occur from this first step, by which, in the furnace of the steam-boiler, this potential energy which exists in the tendency of carbon and oxygen to combine to form carbonic acid is taken advantage of, and the utilizable kinetic energy of heat is produced in equivalent amount, to the final application of resulting mechanical energy to machinery of transmission, through which it is usefully applied to the elevation of water, to the driving of mills and machinery of all kinds, or to the hauling of “lightning" trains on our railways, or to the propulsion of the Great Eastern.

The kinetic heat-energy developed in the furnace of the steam-boiler is partly transmitted through the metallic walls which inclose the steam and water within the boiler, there to evaporate water, and to assume that form of energy which exists in steam confined under pressure, and is partly carried away into the atmosphere in the discharged gaseous products of combustion, serving, however, a useful purpose, en route, by producing the draught needed to keep up combustion.

The steam, with its store of heat-energy, passes through tortuous pipes and passages to the steam-cylinder of the engine, losing more or less heat on the way, and there expands, driving the piston before it, and losing heat by the

transformation of that form of energy while doing mechanical work of equivalent amount. But this steam-cylinder is made of metal, a material which is one of the best conductors of heat, and therefore one of the very worst possible substances with which to inclose anything as subtile and difficult of control as the heat pervading a condensible vapor like steam. The process of internal condensation and reëvaporation, which is the great enemy of economical working, thus has full play, and is only partly checked by the heat from the steam-jacket, which, penetrating the cylinder, assists by keeping up the temperature of the internal surface and checking the first step, condensation, which is an essential preliminary to the final waste by reëvaporation. The piston, too, is of metal, and affords a most excellent way of exit for the heat escaping to the exhaust side.

Finally, all unutilized heat rejected from the steam-cylinder is carried away from the machine, either by the water of condensation, or, in the non-condensing engine, by the atmosphere into which it is discharged.

Having traced the method of operation of the steamengine, it is easy to discover what principles are comprehended in its philosophy, to learn what are known facts bearing upon its operation, and to determine what are the directions in which improvement must take place, what are the limits beyond which improvement cannot possibly be carried, and, in some directions, to determine what is the proper course to pursue in effecting improvements. The general direction of change in the past, as well as at present, is easily seen, and it may usually be assumed that there will be no immediate change of direction in a course which has long been preserved, and which is well defined. We may, therefore, form an idea of the probable direction in which to look for improvement in the near future.

Reviewing the operations which go on in this machine during the process of transformation of energy which has been outlined, and studying it more in detail, we may de

duce the principles which govern its design and construction, guide us in its management, and determine its efficiency.

In the furnace of the boiler, the quantity of heat developed in available form is proportional to the amount of fuel burned. It is available in proportion to the temperature attained by the products of combustion; were this temperature no higher than that of the boiler, the heat would all pass off unutilized. But the temperature produced by a given quantity of heat, measured in heat-units, is greater as the volume of gas heated is less. It follows that, at this point, therefore, the fuel should be perfectly consumed with the least possible air-supply, and the least possible abзtraction of heat before combustion is complete. High temperature of furnace, also, favors complete combustion. We hence conclude that, in the steam-boiler furnace, fuel should be burned completely in a chamber having nonconducting walls, and with the smallest air-supply compatible with thorough combustion; and, further, that the air should be free from moisture, that greatest of all absorbents of heat, and that the products of combustion should be removed from the furnace before beginning to drain their heat into the boiler. A fire-brick furnace, a large combustion-chamber with thorough intermixture of gases within it, good fuel, and a restricted and carefully-distributed supply of air, seem to be the conditions which meet these requisites best.

The heat generated by combustion traverses the walls which separate the gases of the furnace from the steam and water confined within the boiler, and is then taken up by those fluids, raising their temperature from that of the entering "feed-water" to that due the steam-pressure, and expanding the liquid into steam occupying a greatly-increased volume, thus doing a certain amount of work, besides increasing temperature. The extent to which heat may thus be usefully withdrawn from the furnace-gases depends upon the conductivity of the metallic wall, the

rate at which the water will take heat from the metal, and the difference of temperature on the two sides of the metal. Extended "heating-surface," therefore, a metal of high conducting power, and a maximum difference of temperature on the two sides of the separating wall of metal, are the essential conditions of economy here. The heating-surface is sometimes made of so great an area that the temperature of the escaping gases is too low to give good chimneydraught, and a "mechanical draught" is resorted to, revolving "fan-blowers" being ordinarily used for its production. It is most economical to adopt this method. The steam-boiler is generally constructed of iron-sometimes, but rarely, of cast-iron, although "steel," where not hard enough to harden or temper, is better in consequence of its greater strength and homogeneousness of structure, and its better conductivity. The maximum conductivity of flow of heat for any given material is secured by so designing the boiler as to secure rapid, steady, and complete circulation of the water within it. The maximum rapidity of transfer throughout the whole area of heating-surface is secured, usually, by taking the feed-water into the boiler as nearly as possible at the point where the gases are discharged into the chimney-flue, withdrawing the steam nearer the point of maximum temperature of flues, and securing opposite directions of flow for the gases on the one side and the water on the other. Losses of heat from the boiler, by conduction and radiation to surrounding bodies, are checked as far as possible by non-conducting coverings.

The mechanical equivalent of the heat generated in the boiler is easily calculated when the conditions of working : are known. A pound of pure carbon has been found to be capable of liberating by its perfect combustion, resulting in the formation of carbonic acid, 14,500 British thermal units, equivalent to 14,500 × 772 = 11,194,000 foot-pounds of work, and, if burned in one hour, to 11194000 = 5.6 horse-power. In other words, with perfect utilization, but = 0.177, or

1980000

about one-sixth, of a pound of carbon would be needed per hour for each horse-power of work done. But even good coal is not nearly all carbon, and has but about ninetenths this heat-producing power, and it is usually rated as yielding about 10,000,000 foot-pounds of work per pound. The evaporative power of pure carbon being rated at 15 pounds of water, that of good coal may be stated at 13. In metric measures, one gramme of good coal should evaporate about 13 grammes of water from the boiling-point, producing the equivalent of about 3,000,000 kilogrammetres of work from the 7,272 calories of heat thus generated. A gramme of pure carbon generates in its combustion 8,080 calories of heat. Per hour and per horse-power, 0.08, or less than one-twelfth, of a kilogramme of carbon burned per hour evolves heat-energy equal to one horse-power.

Of the coal burned in a steam-boiler, it rarely happens that more than three-fourths is utilized in making steam; 7,500,000 foot-pounds (1,036,898 kilogrammetres) is, therefore, as much energy as is usually sent to the engine per pound of good coal burned in the steam-boiler. The "efficiency" of a good steam-boiler is therefore usually not far from 0.75 as a maximum. Rankine estimates this quantity for ordinary boilers of good design and with chimney-draught at

F

S

in which is the ratio of weight of fuel burned per square foot of grate to the ratio of heating to grate surface; this is a formula of fairly close approximation for general practice.

The steam in the engine first drives the piston some distance before the induction or steam valve is closed, and it then expands, doing work, and condensing in proportion to work done as the expansion proceeds, until it is finally released by the opening of the exhaust or eduction valve. Saturated steam is modified in its action by a process which