ing new pleasures into existence, but by so cheapening former enjoyments as to render them attainable by those who before could never have hoped to share them. The surface of the land and the face of the waters are traversed with equal facility by its power; and by thus stimulating and facilitating the intercourse of nation with nation, and the commerce of people with people, it has knit together remote countries by bonds of amity not likely to be broken. Streams of knowledge and information are kept flowing between distant centers of population; those more advanced diffusing civilization and improvement among those that are more backward. The press itself, to which mankind owes in so large a degree the rapidity of their improvement, has had its power increased in a manifold ratio by its union with the steam-engine. The steam-engine is a piece of mechanism by which fuel is rendered capable of executing any kind of labor. By it coals are made to spin, weave, dye, print, and dress silks, cottons, woolens, and other cloths; to make paper, and print books on it when made; to convert corn into flour; to press oil from the olive, and wine from the grape; to draw up metal from the bowels of the earth; to pound and smelt it, to melt and mould it; to forge it; to roll it, and to fashion it into every form that the most wayward caprice can desire. Do we traverse the deep? the powers of steam lend wings to the ship, and bid defiance to the natural opponents, the winds and the tides. Does the windbound ship desire to get out of port? they throw their arms around her, and place her on the open sea. Do we traverse the land? they are harnessed to our chariot, and we outstrip the flight of the swiftest bird, and equal in speed the fury of the tempest. The substance by which these powers are rendered active is one which nature has provided in boundless quantity all over the earth, and though it has no price, its value is inestimable. This substance is water. FORCE DEVELOPED BY EVAPORATION. To render intelligible the manner in which a mechanical power is developed in the conversion of water into steam, and the circumstances which attend that remarkable physical change, we will suppose A Fig. 1. a quantity of pure water deposited in the bottom, A, of a tube, B A, figure 1. To render the explanation more simple we will suppose that the area of the section of the tube is equal to a square inch, and that the quantity of water deposited in it is a cubic inch. We will further imagine the tube to be glass, so that the phenomena developed in it may be visible. Let a piston, P, be imagined to be fitted in the tube, air tight and steam tight, and to be placed in contact with the surface of the water, so as to exclude all communication between the water and the air above the piston. In this case the piston would be pressed upon the water by the pressure of the atmosphere upon a square inch of surface added to the weight of the piston itself. But the former pressure is equal to fifteen pounds, and therefore the pressure on the surface of the water will exceed the weight of the piston by fifteen pounds. Now to simplify our explanation by excluding all reference to the atmospheric pressure, and the particular weight of the piston, B, we shall suppose both of these exactly counterpoised by the weight, w, so that the piston shall be placed in contact with the surface of the water, without, however, exerting any pressure upon it. These conditions being understood, let a weight, say of fifteen pounds, be placed upon the piston P, and let a fire, a lamp, or any other regular source of heat, be applied to the bottom of the tube. If a thermometer were immersed in the water under the piston, the following effects would then be observed: The thermometer would rise, the piston maintaining its position, and this would continue until the thermometer would rise to the temperature of 2120. Upon rising to that temperature the thermometer would remain stationary, and at the same time the piston, P, would begin to rise, leaving a space apparently empty between it and the surface of the water. The lamp, or fire, still continuing to impart the same heat to the water, the thermometer nevertheless will remain stationary at 2120, but the piston will continue to rise higher and higher in the tube, and if the depth of the water in the bottom of the tube be measured, it will be found that it is constantly diminished. If a sufficiently exact measurement of the decrease of the depth of water, and the height to which the piston is raised, could be made, it would be found that the one would bear a fixed and invariable proportion to the other, the height of the piston being always one thousand six hundred and sixty-nine times the decrease of the depth of water. In fine, if this process were continued for a sufficient time, and if the tube had sufficient length, the water would altogether disappear from the bottom of the tube, and the piston would be raised one thousand six hundred and sixty-nine inches, or one hundred and thirty-nine feet very nearly. For the convenience of round numbers, in a case where the most extreme arithmetical accuracy is not needed, we shall then assume that the piston, loaded with fifteen pounds, has been raised one hundred and forty feet. After this has taken place the tube below the piston will appear to be quite empty, the water having disappeared, and no visible matter having taken its place. If, however, the tube and its contents were weighed, they would be found to have the same weight precisely as they had when the water was deposited under the piston. The phenomenon is easily explained. The heat applied to the tube has converted the visible liquid water into invisible steam. It is a great but very common error to suppose that the whitish, cloudy vapor which is seen to issue from the safety valve of an engine, or the funnel of a locomotive, or the spout of a boiling kettle, is steam. The semi-transparent matter which floats in the air, and continues to be visible for some time after it escapes from the boiler, is, in fact, not steam, but water existing in very minute particles produced by the condensation of the steam by the contact of the colder air. When those particles coalesce and form small drops of water, they either fall to the ground or are evaporated at a lower temperature, and in either case disappear. If the vapor issuing from the safety valve of an engine, or the spout of a boiling kettle, be closely examined, it will not be found to have that cloudy, semi-transparent appearance until it has passed to some distance from the point from which it issues. Pure steam is, in fact, a transparent and invisible elastic fluid like air, and this explains how it is, that in the tube, ▲ B, the space below the piston, after the evaporation of the water, appears to be empty. It is, however, no more empty than if it were filled with air. It is filled with the invisible elastic vapor into which the water has been converted by the heat which has been applied to it. It remains now to show what is the quantity of mechanical force evolved in this conversion of water into steam, and what quantity of heat has been absorbed in producing it. From what has been stated above, it appears that the water, in passing into vapor, has swelled into one thousand six hundred and sixty-nine times its original bulk, being subject to a compressing force of fifteen pounds upon the square inch. In thus expanding, the weight of fifteen pounds has been raised one hundred and forty feet, an effect which is mechanically equivalent to one hundred and forty times fifteen pounds, that is, two thousand one hundred pounds raised one foot. To estimate the quantity of heat absorbed in producing this effect, let us suppose that in the commencement of this process, the water under the piston has the temperature of 32°, and that the lamp, or other source of heat, which is applied to it, acts with such uniformity as to impart exactly the same quantity of heat per minute. Let the time which elapses between the first application of the lamp and the moment at which the water attains the temperature of 2120 and begins to be evaporated, he observed, and also the interval between the commencement of evaporation and the total disappearance of the water. It will be found that the latter interval is five and a half times the former. It follows, consequently, that to convert water at 2120 into steam requires five and a half times as much heat as is necessary to raise the same water from 320 to 2120, or, what is the same, the quantity of heat which would convert water at 2120 into steam would increase the temperature of the same water by five and a half times 180°, that is, by 990°, if it had remained in the liquid state. It follows, also, that to convert water at 320 into steam will take six and a half times as much fuel as would be sufficient pressure. For all practical purposes, then, to boil the same water. It may be asked, what becomes of the enormous quantity of heat thus imparted to the water during the process of its evaporation, seeing that the water itself receives no increase of temperature, being maintained steadily at 2120, and that the steam into which it is converted has the same temperature? This is answered by showing that the entire quantity of heat which thus disappears to the thermometer is absorbed by the steam, and must, in fact, be regarded as the immediate cause of its maintaining the elastic or vaporous form. That it is actually contained in the steam, though its presence is not indicated by the thermometer, is incontestably established by the result of the following processes: Let the steam, at 212°, which has been evolved from a cubic inch of water at 320, be mixed with five and a half cubic inches of water at the temperature of 32°. The steam will be at once reconverted into water, and the mixture will be six and a half cubic inches of water, the temperature of which will be 2120. Thus it appears that the steam at 2120, when reconverted into a cubic inch of water at 2120, parts with as much heat as suffices to raise five and a half cubic inches of water from 320 to 2120, which is exactly the quantity of heat which disappeared while the water was converted into steam. The heat which is thus contained in steam, without affecting the thermometer, is said to be the LATENT, and the latent heat of steam is therefore stated to be about 1,000°, the meaning of which is, that to convert boiling water into steam as much heat must be imparted to it as would raise it 1,000 higher in temperature if it did not undergo that change of state. we shall be sufficiently accurate in stating, that when the weight on the piston P is doubled, it will be raised by the evaporation of a given quantity of water to half the height. In general, in whatever proportion the weight on the piston is increased, the height to which it is raised by the evaporation of a given quantity of water will be decreased, and in whatever proportion the weight is diminished, the height will be increased. It follows, therefore, that in all cases, whatever be the pressure under which the evaporation takes place, the same mechanical force is developed by the evaporation of the same quantity of water. Strictly speaking, there is a little more force with greater pressures, but the difference is so small, and so nearly balanced by certain practical disadvantages attending high pressures, that it may be disregarded. Since the amount of force developed by each cubic inch of water evaporated is equivalent to two thousand one hundred pounds raised one foot, we shall be sufficiently near the truth in stating, in round numbers, that such a force is equivalent to raising a ton weight a foot high. It appears, also, that under a pressure of fifteen pounds per square inch, water swells into one thousand six hundred and sixty-nine times its bulk when it is converted into steam. Since a cubic foot is one thousand seven hundred and twentyeight cubic inches, and since the mean atmospheric pressure is a little under fifteen pounds, it may be stated with sufficient precision for all practical purposes, that a cubic inch of water, evaporated under the mean atmospheric pressure, will produce a cubic foot of steam. II.-FORCE DEVELOPED BY EXPANSION. Steam, in common with all vapors and gases, exerts a certain mechanical force by its property of expansibility. To render this source of mechanical power intelligible, let us suppose the piston P loaded at first with sixty pounds for example, and under this pressure let the In the preceding explanation we have supposed the piston P to carry a weight of fifteen pounds. Let us now consider in what manner the phenomena would be modified if it were loaded with a greater or less weight. If it were loaded with thirty pounds, the conversion of the water under it into steam would not commence until the temperature is raised to 25140, and when the whole of the water is evap-water be evaporated, and the piston raised orated, the piston would be raised to the height of only eight hundred and thirtythree inches, being a very little more than half the height to which it was raised when the evaporation took place under half the VOL. XI.-20 to the height of thirty-five feet. The power thus developed will be that due to evaporation alone. But after the evaporation has ceased, and when the piston, with its load of sixty pounds, is suspended the liquid to the vaporous state, is swelled into a vastly increased volume, so, on the other hand, in passing from the vaporous to the liquid state, it suffers a proportionate diminution in volume. Thus, if the evaporation take place under a pressure of fifteen pounds, a cubic inch of water is dilated into a cubic foot of steam. Now if, by the application of cold, this steam is converted into water, it will resume its original dimensions, and will become a cubic inch of water. This change of vapor into water has been called CONDENSATION, as the matter of which it consists, con at the height of thirty-five feet, let fifteen pounds be taken from it, so as to leave a load of only forty-five pounds. The pressure below the piston being then greater than its load, it will be elevated, and as it is elevated, the steam below it, increasing in volume, will be diminished in pressure in the same proportion, until the piston is raised in height equal to one third part of one hundred and forty feet, when the pressure below will be equal to the load upon it, and it will remain suspended. During this expansive action of the steam, therefore, forty-five pounds have been raised through a height equal to a difference be-tracting into a much smaller volume, is tween one third and one fourth, that is, through one twelfth of one hundred and forty feet. At this point let fifteen pounds more be supposed to be removed from the piston, so that its load shall be reduced to thirty pounds. The pressure below it being, as before, greater than its load, the piston will be raised, and will continue to rise, until it rise to a height equal to half of one hundred and forty feet, when the pressure, reduced by expansion, will become equal to the load, and the piston will again become suspended. In this interval thirty pounds have therefore been raised by the expansive action of the steam, through the difference between one half and one third, that is, through one sixth of one hundred and forty feet. Finally, suppose fifteen pounds more to be removed, and the piston will rise with the remaining fifteen pounds to the height of one hundred and forty feet, so that, in this last expansive action, fifteen pounds are raised through a height equal to the half of one hundred and forty feet. It is evident that the result of the expansive action may be indefinitely varied by varying the extent of its play. Meanwhile, whatever may be its amount, it is clearly quite independent of the process of evaporation, and, indeed, of every property by which vapors are distinguished from air or gases, inasmuch as these latter, being similarly compressed, would similarly expand, and would develop in their expansion precisely the same force. III. FORCE DEVELOPED BY CONDENSA- As heat converts water into steam, so, on the other hand, will cold convert steam into water; and as water, in passing from rendered proportionally more dense. This property has supplied another means of rendering steam a mechanical agent. Let us suppose that, after the piston P, fig. 1, has been raised one hundred and forty feet high by the evaporation of a cubic inch of water, the counterpoise, w, having descended through the same height, an additional weight of fifteen pounds is placed upon w, and, at the same time, the lamp withdrawn from the tube and cold applied to its external surface. The steam by which the piston was raised will then be converted into water, or condensed, and will, as at first, fill the bottom of the tube to the height of an inch. The space within the tube above the surface of the water, extending to the height of one hundred and forty feet, will then be a vacuum, and the atmospheric pressure acting above the piston, not being resisted by any corresponding pressure below it, will force the piston down with a force of fifteen pounds, and will raise the weight w, loaded with the additional fifteen pounds, through the same height. Thus it appears that when steam is condensed, or reconverted into water, by producing a vacuum it develops a mechanical force equal to that which was developed in the conversion of water into vapor. The mechanical power developed by the evaporation of water has been sometimes called the direct power, and that produced by the conversion of vapor into water the indirect power of steam, because the immediate agent in the former case is the elastic force of the steam itself, while the agent in the latter case is the atmospheric pressure, to which effect is given by the vacuum produced by the condensation of steam. The three sources of mechanical power which have been explained, have been used, water may be obtained, and this, in fact, is what is accomplished in steam engines as they are practically worked. The direct and indirect powers of steam may also be easily combined as well in the ascent as in the descent of the piston. If we suppose the upper part of the tube, instead of being open to the atmosphere, to communicate with a reservoir of water, to which, like the bottom of the tube, a lamp or other source of heat is applied, steam may be admitted above the piston P as well as below it. Now, if such be the case, it is easy to imagine how the piston can be at the same time affected by the direct and indirect power of the steam. Thus, if we suppose that a vacuum has been formed above it, by the condensation of steam, admitted from the upper reservoir, while steam produced from the lower reservoir acts below it, the piston will be forced upward by the combined effect of the direct action of the steam below and the indirect action of the condensed steam above, and when the piston has been thus raised, we can imagine that while steam is admitted above it from the upper reservoir, that which is below it may be condensed, in which case it will be forced down by the combined effect of the direct action of the steam above it and the indirect action of the condensed steam below it, and it is evident that such alternate action may be indefinitely continued. sometimes separately and sometimes together, in different forms of steam engine. In the class of engines commonly called high-pressure engines, the direct power alone is used. In a class of engines, now out of use, called atmospheric engines, the indirect power alone was used. In the engines most generally used in the arts and manufactures, known as low-pressure or condensing engines, both powers are used. To obtain the mechanical effect of the vacuum produced by the condensation of steam, it is not necessary that the atmospheric pressure should be used. If we suppose that while the vacuum is produced below the piston P, steam having a pressure equal to that of the atmosphere be admitted to the upper side of it, the piston will be urged downward into the vacuum with the same force exactly as if the atmosphere acted upon it. And, in effect, this is the method by which the indirect force of steam is rendered effective in all engines as at present constructed, the piston being in no case exposed to the atmosphere. In the preceding illustration of the power of steam, we have supposed the piston P to have the area of a square inch, and to be raised continuously to the height of one hundred and forty feet. But it is evident that such conditions are neither necessary nor practicable. If the piston had an area of ten square inches, the same amount of evaporation would raise it to the tenth part of the height; but the force with which it would be raised, being at the same time increased in a ten-condensing, or low-pressure engines, are fold proportion, the mechanical effect would be the same, for it is evident that whether fifteen pounds be raised one hundred and forty feet, or ten times fifteen pounds be raised the tenth part of one hundred and forty feet, the same mechanical effect would be produced. The piston acted upon by the steam, instead of being continuously driven in one direction, may be alternately elevated and depressed, and still the same amount of power will be developed. Thus the evaporation may be continued until the piston has been raised ten feet. The steam which raised it may then be condensed, and the piston having descended to the bottom of the tube, it may again be raised ten feet by evaporation as before, and this may be continued indefinitely. In this way, by means of a short tube or cylinder, the mechanical effect attending the evaporation of any quantity of Such is the effect of the broad principle upon which all engines of the class called constructed. In their details there are numerous points of great practical importance and of much interest in a mechanical point of view. These arrangements, however, need not here be further noticed. The apparatus by which the combustion of the fuel is effected, and by which the heat evolved is transmitted to the water to be evaporated, are furnaces and boilers of very various forms and construction, according to the circumstances in which they are applied, the one being adapted to the other, so that as much of the heat shall arrive at the water as the circumstances of their application permit. The quantity of water which would be evaporated, if all the heat evolved in the combustion of a given weight of fuel could be transmitted to the water, is the theoretical evaporating power of the fuel; and the quantity of water actually evaporated by |