times as much fuel as would be sufficient It may be asked, what becomes of the 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. 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 2510, and when the whole of the water is evaporated, 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 pressure. For all practical purposes, then, that when the weight on the piston P is we shall be sufficiently accurate in stating, doubled, it will be raised by the evaporation of a given quantity of water to half portion the weight on the piston is inthe height. In general, in whatever procreased, 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. whatever be the pressure under which the It follows, therefore, that in all cases, evaporation takes place, the same mechanical force is developed by the evaporation of the same quantity of water. speaking, there is a little more force with Strictly small, and so nearly balanced by certain greater pressures, but the difference is so practical disadvantages attending high pressures, that it may be disregarded. each cubic inch of water evaporated is Since the amount of force developed by 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. of fifteen pounds per square inch, water It appears, also, that under a pressure swells into one thousand six hundred and sixty-nine times its bulk when it is converted into steam. Since a cubic foot is eight cubic inches, and since the mean atone thousand seven hundred and twentymospheric 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. gases, exerts a certain mechanical force power intelligible, let us suppose the piston 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 between 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. IIL-FORCE DEVELOPED BY CONDENSATION. As heat converts water into steam, so, on the other hand, will cold convert steam into water; and as water, in passing from Now 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. 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, contracting into a much smaller volume, is 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 The enormous consumption of coals produced by the application of the steam engine in the arts and manufactures, as well as to railways and navigation, has of late years excited the fears of many as to the possibility of the exhaustion of our coal-mines. Such apprehensions are, however, altogether groundless. If the present consumption of coal be estimated at sixteen millions of tons annually, it is demonstrable that the coal-fields of this country would not be exhausted for many centuries. it is the practical evaporating power. The girt with an iron railway, a train carrying theoretical evaporating power varies with two hundred and forty passengers would be the quality of the fuel. A given weight drawn round it by the combustion of about of certain species of coal will evolve in three hundred tons of coke, and the circuit combustion a greater or less quantity of ❘ would be accomplished in five weeks. heat than other species. In general, it may be stated that the strongest coals, meaning by that term those which have the greatest evaporating power, are those which are richest in carbon. The practical evaporating power of a given species of coal varies with the form, construction, and magnitude of the furnace and boiler. That portion of the heat which does not reach the water is dissipated in various ways. A part of it is lost by radiation from the grate; a part by radiation from the boiler; a part is carried by the heated gases of combustion into the chimney. The first two sources of waste of heat are reduced to a very small amount by a variety of ingenious contrivances. But the last is indispensable to the maintenance of the combustion, and ought to be considered as the power by which the furnace is worked, rather than a waste of heat. The grate upon which the fuel is placed is surrounded on every side by parts of the boiler, within which water is contained. In some boilers, even the ash-pit is a part of the surface of the boiler, under which there is water. In this case, all the heat radiated from the grate, and the fuel upon it, is transmitted to the boiler; and in all cases the furnace is surrounded on every side, except the bottom of the grate or ash-pit, with surfaces having water within them. The waste of heat by radiation from the surfaces of the boiler, steampipes, cylinder, and other parts of the machinery in which steam is contained, or through which it passes, is diminished by various expedients, which in general consist in surrounding such surfaces with packing, casing, or coating, composed of materials which are non-conductors, or at least very imperfect conductors of heat. In some cases the boiler is built round in brick work. In marine boilers it has been the practice recently to clothe the boiler and steam-pipes with a coating of felt, which is attended with a similar effect. When these remedies are properly applied, the loss of heat proceeding from the radiation of the boiler is reduced to an extremely small amount. The circumference of the earth measures twenty-five thousand miles; if it were be But in speculations like these, the probable, if not certain progress of improvement and discovery ought not to be overlooked; and we may safely pronounce that, long before such a period of time shall have rolled away, other and more powerful mechanical agents will supersede the use of coal. Philosophy already directs her finger at sources of inexhaustible power in the phenomena of electricity and magnetism. The alternate decomposition and recomposition of water, by electric action, has too close an analogy to the alternate processes of vaporization and condensation, not to occur at once to every mind: the development of the gases from solid matter by the operation of the chemical affinities, and their subsequent condensation into the liquid form, has already been essayed as a source of power. In a word, the general state of physical science at the present moment, the vigor, activity, and sagacity with which researches in it are presented in every civilized country, the increasing consideration in which scientific men are held, and the personal honors and rewards which begin to be conferred upon them, all justify the ex pectation that we are on the eve of mechanical discoveries still greater than any which have yet appeared; that the steam engine itself, with its gigantic powers, will dwindle into insignificance in comparison with the energies of nature which are still to be revealed; and that the day will come when that machine, which is now extending the blessings of civilization to the most remote skirts of the globe, will cease to have existence except in the page of history. HAPP OLD BOGIE. APPY the child who knows nothing of Old Bogie! Thrice happy the little ones to whom love ministers, and whose fears are not excited by fancied horrors or imaginary monsters. And yet childhood is naturally fearless. It is education-sadly misapplied word! but it is education that makes cowards. Note, for illustration, the boldness with which children will fondle animals; the confidence with which a little boy will throw his arms about a Newfoundland dog. These are characteristics with which tutors of the young idea should not deal too lightly. Yet how soon is fear created in the child. How soon is its wild imagination led to objects of horror that fasten upon it, and become to it irresistibly fascinating! The story of Old Bogie is, alas! a household story-the religion of every nursery. The grim monster presents himself behind a sheet elevated upon a broom-stick, or crouching in a corner, when the gloom of evening leaves broad black shadows in the room. Then are children gathered about the fire, huddled around the nurse's knee, while she relates his wondrous adventures; how, once upon a time, he carried off a little boy who would not go to bed when his mamma told him; how he ate another child who would not learn his letters; how he came with a big bag to a third, and, seeing that the little fellow declined to take the Saturday bath-that weekly misfortune which children dreadput him into the bag, and took him, screaming, away upon his broad shoulders. And then the children, while listening to these awful histories, glance at the dark corners of the room, and clutching still tighter at the nurse's gown, vow that they will be good; that they will go to sleep directly they are put to bed; that they will never cry again; that they will be model scholars. There is a fascination in their fright, and they eagerly ask for other stories equally terrible. Then comes Old Bogie's great son, Blue-Beard. His is a charming story for children! His blue beard; his keys dangling at his waist; his cimeter; his rolling eyes; do we not all remember them as making up perhaps the earliest image that held fast our young imaginations? We read on still, while the book trembled in our hands, to the chamber of blood, where the ogre's dead wives were dangling by the hair. We saw plainly the spot of blood upon the key. And then how happy we were when the brothers appeared! But the blood lingered in our memories. That clotted key has cost us many a wakeful moment at the dead of night, even when the clothes have been heaped over our head. A worthy son of Old Bogie was Blue-Beard to us; nor have we much fault to find, on the score of blood, in Jack the Giant-Killer. To be sure, the little fellow is a brave fellow; and there* is some kind of rough justice in his career; but it is all blood. Nor are the experiences of his good, timid little cousin, Red Riding Hood, more acceptable to our judgment now. That hungry wolf that ate up the little girl's grandam, and subsequently invited the child to lie beside him, made no mean figure in our gallery of monsters. We had our gallery full; in short, a demon at our elbow always. And what cowards they were! Did they not come to us always in that terrible dark? As soon as the light faded from the heavens and objects lost their outline, how wildly did our imaginations set forth on their fearful journey. On no account could we suffer one moment's solitude. Did the nurse leave the room simply to fetch fuel, did we not cling to her apron, even while she scolded us bitterly for our cowardice? And then, when she opened that black place, the coal-cellar, and she playfully asked us whether it did not resemble Blue-Beard's shambles; did not the plaything we held drop among the coals, and were we not too terrified to pick it up? And did not the nurse, seeing our fear, call out that Old Bogie was there, and did we not scamper off in horror? It was a powerful influence that nurse had over us ever afterward. Not she, but Old Bogie, governed us. Did we soil our blouse in the garden, Old Bogie would certainly visit Did we play with the fire, he would come down the chimney. Through our fear, our craven fear, we were led captives. us. O, what a powerful tutor in Old Bogie! powerful beyond the cane of any Dr. Thwackum in the world. Yet he has no statue, no kind of recognition in the country. It would be difficult, perhaps, to design his mysterious figure, so rich is it as presented to the glowing fancies of nursery-maids and ignorant mothers. A |