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other part of the circuit-the part played by the liquid in fig. 34? In the case of the telegraph, the earth is substituted for the liquid, for if the wire which goes along the lines be attached to the copper plate of the battery, and a wire, attached to the zinc, be carried down into the earth, the circuit is complete. In fig. 38, then, L is the line-wire, and E the earth-wire, both of which are made continuous with the coils of wire on the electromagnet, MM'; the armature, A, is attached to a lever, ll', which turns on the axis k. Whenever the current is made to pass through the wire, the armature is drawn down, bringing the end of the lever with it; this raises the other end, to which is fixed a sharp point, p; opposite this point is a

[graphic][subsumed][subsumed][merged small]

groove in the roller, r, and over this groove is made to pass a slip of paper, PP', which is made to move towards P' by the rollers, rr'. These rollers are worked by clock-work, independently of the rest of the machine. When the point p is raised into the groove on the roller r, a raised mark is made on the upper surface of the paper, which will be a dot or a line according to the time the point is raised, that is, according to the time the circuit is kept complete; as soon as the current ceases, the armature is left by the magnet free to rise, and the end of the lever, with the point, is pulled down by the spring s. By means of the dots and lines thus made on the paper, an alphabet is constructed, and words can thus be written down at the distance of thousands of miles.

E

Fig. 39.

m

H

But now, how is this system of stopping and setting agoing of the current managed at the station from which the message is sent? It is done by what is called a key, fig. 39. This is a lever, ll, which moves on an axis, A, and is worked by

a handle, H. To the key are attached three wires: the line-wire, L; a wire, E, attached to a Morse's Recorder, at its own station; and one, C, attached to the copper plate of a galvanic battery. The ordinary position of the key is as seen in the figure, the nipple n, in contact with the little anvil b, that end of the lever being held down by the spring, 8; so that, when a message is to be received, the current passes from L, by A, l, n, b, and E, to the recorder. But when a message is to be sent to another station, the handle, H, is pressed down, the contact between n and b is broken, and the nipple m, is now brought down on the anvil a. This connects the copper plate of the battery with the line-wire to the distant station; and the current passes from the copper plate through the linewire, through the key at the distant station (which is in the position seen in fig. 39), and through the coils of the electro-magnet of the recorder. From the recorder it passes down the earth-wire, and then back, through the earth, to the zinc plate of the battery. The time during which the sharp point in the recorder is to press upon the paper, as before described, is thus regulated by means of the handle, H.

HEAT.

1. Development of Heat.-HEAT may be produced by mechanical means in three ways, by friction or the resistance of the surfaces of two bodies when rubbed, by percussion or the striking of one body against another, and by compression.

(1) By Friction.-If a smooth metal button be stuck on a cork, and rubbed on a piece of soft deal-wood, as a form, it will become heated by the friction; and if rubbed long enough, will become so hot as to scorch wood and paper, and set fire to a match. Considerable exertion of the arm, however, is required to produce the latter result. This experiment affords an illustration of a general principle in nature, that all energy expended results either in a certain amount of work done or of heat produced. Accordingly, energy must be so directed as to produce the exact result desired. If we wish to produce heat, as in the case of the button, or in warming one's hands, the more energy that is applied to overcome the friction, the greater is the amount of heat produced. If sufficient energy be expended, the heat becomes so great that the rubbed bodies take fire. Savages, for example, light their fires by rubbing two sticks together; forests have been set on fire by the friction of two branches waving in the wind; and destructive fires have been occasioned by friction in a piece of machinery. More generally, however, energy is directed to the performance of work, and in this case all that goes to produce heat is lost. If, when a man is

sawing wood, the blade of the saw be held by the wood, the force required to overcome this friction, although it has the effect of heating the saw, is lost, because the object is not to heat the saw, but to cut the wood. To prevent this friction, the teeth of the saw are set outward to each side alternately, so as to make an opening wide enough to allow the blade to work freely; and sometimes a piece of wood is inserted in the cut, to keep the sides apart. When the friction cannot be altogether prevented, it is eased by rubbing the saw with grease. For the same reason, the axles of wheels of carts, railway carriages, and other machines, are kept carefully greased. (2) By Percussion.—On picking up a lead bullet, or rather the flattened fragment of one, just after it has struck a metal target, it is felt to be hot. The heat of the flattened ball is the exact equivalent of the force with which the bullet was moving when it struck the target, together with that communicated to the spot struck. Again, when a piece of cold iron is hammered, it becomes hot-that is, the energy expended in the blows is converted into heat in the iron, just as happens when a button is rubbed. (3) By Compression.-When the density of a body is increased by compression, heat is developed according as the volume of the body becomes diminished. When books are squeezed between the plates of a hydraulic press (see fig. 19, page 17), they are found to be heated; in other words, the force applied to the press has been converted into heat. Similarly, heat is evolved when a weight is laid on a metal pillar.

From a consideration of the foregoing and many similar facts, the conclusion has been arrived at, that heat is a form of motion. Thus, the heat produced by a bullet striking a target is simply the motion, which the bullet had before it struck the target, transferred to the atoms of the lead as well as to those of the metal struck; and the heat of the hammered iron is simply the motion of the hammer transferred to the atoms of the iron; and similarly in any case of friction or compression.

2. Change of Condition.—It must be distinctly understood that all bodies have a greater or less amount of heat. We are obliged to conceive of a point at which there is an entire absence of heat, but of that point we have no experience, and beyond it the heat of bodies differs merely in degree. The temperature of the human body is about 90°, and we are accustomed to speak of bodies with a lower temperature than this as cold, and of all having a higher temperature as warm or hot. Taking for granted, then, that heat is motion among the atoms of the body, let us consider how different bodies are affected by it. In all bodies, the atoms vibrate backward and forward, and these vibrations have greater or less velocity and extent, according to the amount of heat in the body. The result of these vibrations is that the atoms repel each other, so as to make the body composed of them, when heated to more than its ordinary temperature, occupy a larger space. Iron, for example, expands when heated, as was

noticed under Density (page 5). There is thus a contest going on between the binding power of cohesion and the repelling power of heat. At first, with a small amount of heat, the cohesion holds its own; but as the heat increases, the vibrations become more violent, and the atoms are strongly pushed apart. Cohesion, then, has less power, because it has to act at a greater distance; therefore, as the repulsion of the heat increases, the attraction of cohesion diminishes, till the atoms gain sufficient freedom to be able to slide or roll upon one another. The body is then said to be in a liquid state.

In the liquid state, the power of cohesion has not been altogether conquered; the atoms, although they are movable on one another, still resist being torn asunder. But if the heat be still further increased, the last feeble efforts of cohesion are overcome, and the atoms fly apart in the form of vapour. When a liquid has assumed the gaseous form, it is clear that the space it occupies is very much increased; thus, water converted into steam occupies a space about 1700 times greater than it did before-that is, a cubic inch of water becomes a cubic foot of steam.

3. Vaporisation.-When sufficient heat has been applied to a liquid to make it assume the form of visible vapour, the first particles fly off from the surface, as is seen in the vapour that rises from all water when it becomes heated at all. Let us see what is going on meantime within the liquid mass. At the bottom of the liquid, where the heat is generally applied, the particles are being more and more repelled from one another, the liquid becomes lighter than that above it, and rises, while the liquid above it sinks down, as seen in the figure, which represents a vessel of water with a lamp under it. While this is going on, small bubbles of vapour rise from the bottom; but as they rise near the surface, where the temperature is lower, they are condensed again to water. The formation and condensation of these first bubbles give rise to the singing sound heard coming from water just before it boils. When, however, the whole of the water has been raised to a certain temperature, the bubbles of vapour that are formed at the bottom rise to the surface, and the water is then said to boil. Sometimes the bubbles are seen to rest on the surface; that is, there is a small quantity of vapour enclosed in a thin coating of the liquid. The repelling power of the heat in the steam of course tends to make it burst the

[graphic]

Fig. 40.

bubble; but it is prevented for a short time from doing so by the pressure

of the air, which amounts to 15 pounds on every square inch. But take a bubble when it is first formed at the bottom of the water; there the pressure on it from without is the weight of all the water above it, as well as that of the air; so that the heat necessary to raise a quantity of water to the boiling-point is exactly the quantity of heat that will introduce among its particles a force of repulsion sufficient to overcome the pressure arising from the weight of the liquid above it and the weight of the atmosphere. Hence, to boil a large quantity of water, its temperature must be higher than in a smaller quantity, because the pressure to be overcome by the steam in the bubbles is greater; hence too, water will boil at a much lower temperature if the pressure of the atmosphere be diminished, as is the case on high mountains.

4. Latent Heat.-When cold water is placed in a vessel over the fire, heat from the fire is communicated to the water, which gradually becomes hotter till it reaches the boiling-point; but, after the water boils, the temperature of the water does not rise. What, then, becomes of all the heat that continues to be communicated to it? We noticed in Section 1 the manner in which different modes of energy could be converted into heat this is a case of the reverse process; here heat is converted into motion. For a certain amount of heat a certain amount of work is done in pulling the particles of the liquid asunder. The heat which is consumed in this way after a liquid has been raised to the boiling-pointthat is, heat which goes to form vapour without raising the temperature of the liquid or of the steam-has been called latent, from the notion, at one time entertained, that heat was a fluid, and, consequently, that the heat which seemed to be lost in this way concealed itself in the vapour. The same thing takes place when a solid is being reduced to a liquid. When heat is applied to a piece of ice, its temperature does not rise above the point at which it began to melt till every Ibit of it is melted. The heat thus absorbed in the melting of ice, is called the latent heat of water; and that absorbed in converting water into steam, the latent heat of steam. When this latent heat is lost in any way, the repulsion existing among the particles diminishes, cohesion regains the mastery, and the steam returns to the form of water, or water to that of ice. It is on this principle that distillation is accomplished. The still usually consists of a copper boiler, in which the fermented liquor is converted into vapour; of a pipe, which conveys the vapour from the top of the boiler; and of the worm, a coiled metal tube packed in a vessel through which there is a constant flow of cold water. The vapour arising from the boiling liquor in the copper is deprived of its heat in passing through the tube in the cold water; in consequence of this, it assumes again the liquid form, and drops or runs in a small stream from the end of the worm into a vessel placed to receive it.

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