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expansion of bodies by heat.* This clay, when much heated, contracts. The contraction is first observed when the clay acquires a red heat, and continues to increase, until it vitrifies; the reduction of bulk being permanent, and amounting, in the whole, to about one fourth. In order to take advantage of this property of clay, Mr. Wedgewood constructed a gauge of brass, consisting of two straight pieces, two feet long, fixed upon a plate a little nearer to each other at one end than at the other, the space between them, at the widest end, being five tenths of an inch, and at the narrowest, three tenths. The converging pieces were divided into inches and tenths of inches. The pieces of clay, the contractions of which were to be measured, were of a cylindrical form, flattened on one side, and of such a size as to be exactly adapted to the wider end of the gauge, so that it might slide further in, in proportion to the degree of heat applied to it. A scale was adapted to this clay, each degree of which is equal to one hundred and thirty degrees of the ordinary scale in thermometers. The temperature of red heat, which corresponds to one thousand seventy-seven and a half degrees of Fahrenheit's scale, was assumed as the commencement of Wedgewood's; and it was found, that the instrument could be used to measure temperatures as high as thirty-two thousand two hundred and seventy-seven degrees of Fahrenheit.

Steam. It is evident, that the forces of expansion and contraction, by heat and cold, which we have now considered, act through spaces so limited, that they can

* Another remarkable exception to this law is found in freezing water. When near the freezing point, water does not contract, as we should expect, from the increase of cold, but, on the contrary, expands; so that a given quantity fills more space, when frozen, than it did previously. It of course becomes specifically lighter, which may suggest to us the reason of this apparent anomaly. If ice were heavier than water, it would subside, as soon as formed, in successive flakes to the bottom : this process would continue, until the whole of the water, however deep, would become solid. The effects would evidently be most disastrous.

be used as mechanical agents very rarely, and only under peculiar circumstances.*

A much more important agency, which heat exerts in the mechanic arts, results from its power of changing the form of bodies. With the operations of this power we are all familiar, in the case of water. Below the temperature of thirty-two degrees of the common thermometer, that substance exists in the solid form, and is called ice. Above that temperature, it passes into the liquid state, and is called water; and when raised to the temperature of two hundred and twelve degrees, under ordinary circumstances, it passes into the aeriform state, and is called vapor. It is to this last change that we wish, at present, principally to call the attention of the reader.

*It may not be irrelevant to notice, here, the injurious effect which is sometimes exerted in the arts, by the expansive power of heat. In warm weather, for example, it lengthens the pendulum rod of a clock, and causes it to go too slow; a derangement which we shall better appreciate, when we are told, that a difference of the one hundredth part of an inch, in the length of the rod, will occasion a loss of ten seconds in twenty-four hours. This irregularity in the going of a clock is corrected, most commonly, by means of what is termed the gridiron pendulum,—the rod being composed of several parallel bars, like those of a gridiron. These bars, being of different metals, expand unequally, and serve, therefore, to compensate the irregularities of each other. Again, when we suddenly heat one side of a glass vessel, the great expansion causes it to break. If the heat is applied to both sides, at the same time, so that they heat and expand, equally, there is little danger of breaking. In Nature, the expansive power of heat produces the most salutary effects, by creating currents of air, which carry off superfluous heat from one part of the earth, while they serve to mitigate the severity of cold at other parts.

The effect of heat, in expanding bodies, is strikingly exemplified, also, in the immense system of steam-pipes, which are frequently employed to heat manufactories, extending, in some cases, to three hundred feet, in a straight line. "When fire-proof factories, of iron and brick, were first built, in England, the columns, which supported the successive floors, being hollow,, were intended to admit steam, and to be the channels of communicating heat to the apartments. It was soon found, however, that the lengthening and shortening of a columnar range of eighty or ninety feet high, by a changing temperature ranging as high as one hundred and seventy degrees of Fahrenheit, was so considerable, as to impair the stability of the most solid edifices. Hence horizontal steam-pipes were substituted, being suspended near the ceiling, by swinging rods of iron, and so adjusted, as to give free play to the expansion and contraction."-Dr. Ure.

In the transition of water from the liquid state to the state of vapor, or steam, an immense change of bulk takes place. In this change, a solid inch of water enlarges its size about one thousand seven hundred and twenty-eight times, and forms one thousand seven hundred and twenty-eight solid inches of steam. This expansion takes place, accompanied with a certain force or pressure, by which the vapor has a tendency to burst the bounds of any vessel which contains it. The steam which fills one thousand seven hundred and twenty-eight solid inches, at the temperature of two hundred and twelve degrees, will, if cooled below that temperature, return back to the liquid form, and occupy only one solid inch, leaving one thousand seven hundred and twenty-seven solid inches vacant; and if it be included in a close vessel, it will leave the surfaces of that vessel free from the internal pressure, to which they were subject before the return of the water to the liquid form.

If it be possible, therefore, alternately to convert water into vapor, by heat, and to reconvert the vapor into water, by cold, we shall be enabled, alternately, to submit any surface to a pressure equal to the elastic force of the steam, and to relieve it from that pressure, so as to permit it to move in obedience to any other force which may act upon it. Or, again; suppose that we are enabled to expose one side of a movable body to the action of water converted into steam, at the moment that we relieve the other side from the like pressure, by reconverting the steam, which acts upon it, into water; the movable body will be impelled by the unresisted pressure of the steam on one side. When it has moved a certain distance, in obedience to this force, let us suppose that the effects are reversed. Let the steam, which pressed it forwards, be now reconverted into water, so as to have its action suspended: and, at the same moment, let steam, raised from water by heat, be caused to act on the other side of the movable body; the consequence will obviously be, that it

will now change the direction of its motion, and return, in obedience to the pressure excited on the opposite side. "Such is, in fact, the operation of an ordinary lowpressure steam-engine. The piston, or plug which plays in the cylinder, is the movable body to which we have referred. The vapor of water is introduced upon one side of that piston, at the moment that a similar vapor is converted into water on the other side, and the piston moves by the unresisted action of the steam. When it has arrived at the extremity of the cylinder, the steam, which just urged it forwards, is reconverted into water, and the piston is relieved from its action. At the same moment, a fresh supply of steam is introduced upon the other side of the piston, and its pressure causes the piston to be moved in a direction contrary to its former motion. Thus the piston is movea in the cylinder, alternately in the one direction and in the other, with a force equivalent to the pressure of the steam which acts upon it. A strong metal rod proceeds from this piston, and communicates with proper machinery, by which the alternate motion of the piston, backwards and forwards or upwards and downwards, in the cylinder, may be communicated to whatever body is intended to be moved."

"The power of such a machine will obviously depend partly on the magnitude of the piston, or the movable surface which is exposed to the action of the steam; and partly on the intensity of the pressure of the steam itself. The object of converting the steam into water by cold, upon that side of the piston towards which the motion takes place, is to relieve the piston from all resistance to the moving power. This renders it unnecessary to use steam of a very high pressure, inasmuch as it will have no resistance to overcome, except the friction of the piston with the cylinder, and the ordinary resistance of the load which it may have to move. Engines constructed upon this principle, not requiring, therefore, steam of a great pressure, have been generally called, low-pressure engines.' The

reconversion of the steam into water requires a constant and abundant supply of cold water, and a fit apparatus for carrying away that which becomes heated by cooling the steam, and for supplying its place by a fresh quantity of cold water. It is obvious, that such an apparatus is incompatible with great simplicity and lightness, nor can it be applied to cases where the engine is worked under circumstances in which a fresh supply of water cannot be had."*

*The following view, Fig. 16, of Watt's double-acting condensing steam-engine, will render this description more intelligible to the

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young reader. A, boiler; B, steam-pipe, conveying the steam to the cylinder, C; D, eduction-pipe, which conducts the steam from

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