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weight of the air is sometimes not sufficient to raise a column of mercury more than twenty-eight inches, or a column of water much more than thirty-two feet; but when once it has passed that opening, it is no longer the pressure of air on the reservoir which makes it ascend-it is raised by lifting it up, as you would raise it in a bucket, of which the piston formed the bottom. This common pump is, therefore, called the sucking and lifting pump, as it is constructed on both these principles.
The forcing pump consists of a forcing power added to the sucking part of the pump. This additional power is exactly on the principle of the syringe: by raising the piston, the water is drawn up into the pump; and by making it descend, it is forced out. The large pipe, A B, represents the sucking part of the pump, which differs from the lifting pump only in its piston, P, being unfurnished with a valve, in consequence of which the water cannot rise above it. When therefore, the piston descends, it shuts the valve y, and forces the water (which has no other vent) into the pipe, D; this is likewise furnished with a valve, v, which, opening outwards, admits the water, but prevents its return. The water is thus first raised in the pump, and then forced into the pipe, by the alternate ascending and descending motion of the piston, after a few strokes, of the handle to fill the pipe, from whence the water issues at the spout.
Optics is one of the most interesting branches of Natural Philosophy; it is the science of vision, and teaches us how we see objects. In this science, bodies
are divided into luminous, opaque, and transparent. A luminous body is one that shines by its own light-as the sun, the fire, a candle, &c. But all bodies that shine are not luminous; polished metal, for instance, when it shines with so much brilliancy, is not a luminous body, for it would be dark if it did not receive light from a luminous body; it belongs, therefore, to the class of opaque, or dark bodies, which comprehend all such as are neither luminous nor will admit the light to pass through them; and transparent bodies are those which admit the light to pass through them, such as glass and water. Transparent or pellucid bodies are frequently called mediums; and the rays of light which pass through them are said to be transmitted by them, Light, when emitted from the sun, or any other luminous body, is projected forwards, in straight lines, in every possible direction; so that the luminous body is, not only the general centre whence all the rays proceed, but every point of it may be considered as a centre which radiates light in every direction. A ray of light is a single line of light projected from a luminous body; and a pencil of rays is a collection of rays proceeding from any one point of a luminous body.
Philosophers are not agreed as to the nature of light. Some maintain the opinion that it is a body consisting of detached particles, which are emitted by luminous bodies, in which case the particles of light must be inconceivably minute; since, though they must cross each other in every direction, they are never known to interfere with each other. Others suppose it to be produced like sound, by the undulations of a subtle fluid diffused throughout all known space. In some respects, light is obedient to the laws which govern bodies; in others, it appears to be independent of them. Thus, though its course corresponds with the laws of notion, it does not seem to be influenced by those of gravity; for it has never been discovered to have weight, though a variety of experiments have been made with a view of ascer
taining that point. We are, however, so ignorant of the intimate nature of light, that we shall confine our attention to such of its properties as are well ascertained.
To return then to the examination of the effects of the radiation of light from a luminous body;-since the rays are projected in straight lines, when they meet with an opaque body through which they are unable to pass, they are stopped short in their course; for they cannot move in a curve line round the body. The interruption of the rays of light by the opaque body produces therefore darkness on the opposite side of it; and if this darkness fall upon a wall, a sheet of paper, or any object whatever, it forms a shadow; for shadow is nothing more than darkness produced by the intervention of an opaque body, which prevents the rays of light from reaching an object behind it.
If the luminous body, A, be larger than the opaque body, B, the shadow will gradually diminish in size till it terminates in a point; if smaller, the shadow will continually increase in size, as it is more distant from the object which projects it. The shadow of a figure, A, varies in size, according to the distance of the several surfaces, B, C, D, E, on an Badi dogod which it is described. Now what becomes of the rays of light which opaque bodies arrest in their course, and the interruption of which is the occasion of shadows? This leads to a very important property of light, Reflection.
When rays of light encounter an opaque body, which they cannot traverse, part of them are absorbed by it, and part are reflected, and rebound as an elastic ball which is struck against a wall. Light, in its reflection, is governed by the same laws as solid perfectly elastic bodies. If a ray of light fall perpendicularly on an
opaque body, it is reflected back in the same line towards the point whence it proceeded; if it fall obliquely, it is reflected obliquely, but in the opposite direction, the angle of incidence being equal to the angle of reflection. If the shutters be closed, and a ray of the sun's light admitted through a very small aperture, and reflected by a mirror, on which the ray falls perpendicularly, but one ray is seen, for the ray of incidence and that of reflection are both in the same line, though in opposite directions, and thus are confounded together. The ray, therefore, which appears single, is in fact double, being composed of the incident ray proceeding to the mirror, and the reflected ray returning from the mirror. These may be separated by holding the mirror, M, in such a manner that the incident ray, A B, shall fall obliquely upon it; then the reflected ray, B c, will go off in another direction. If a line be drawn from the point of incidence, B, perpendicularly to the mirror, it will divide the angle of incidence from the angle of reflection, and these angles will be equal.
It is by reflected rays only that we see opaque objects. Luminous bodies send rays of light immediately to our eyes; but the rays which they send to other bodies are invisible to us, and are seen only when reflected or transmitted by those bodies to our eyes.
Let us now examine by what means the rays of light produce vision. They enter at the pupil of the eye, and proceeding to the retina or optic nerve, which is situated at the back of the eye-ball, there describe the figure, colour, and [(with the exception of size) form a complete representation of the object from which they proceed. If the shutters be closed, and a ray of light admitted through a small aperture, a picture may be seen on the opposite wall similar to that which is delineated on the retina of the eye; it exhibits a picture in miniature of the garden, and the landscape would be perfect were it not reversed. This picture is produced by the rays of light reflected from the various objects in the garden, and which are admitted through the hole
in the window shutter. It is called a camera obscura, (dark chamber,) from the necessity of darkening the room in order to exhibit it.
The rays from the glittering weathercock at the top
of the alcove A, represent it at u; for the weathercock being much higher than the aperture in the shutter, only a few of the rays, which are reflected by it in an obliquely descending direction can find entrance there. The rays of light moving always in straight lines, those which enter the room in a descending direction will continue their course, in the same direction, and will consequently, fall upon the lower part of the wall opposite the aperture, and represent the weathercock reversed in that spot, instead of erect in the uppermost part of the landscape; and the rays of light from the steps, B, of the alcove, in entering the aperture, ascend, and describe them in the highest instead of the lowest part of the landscape; whilst the rays proceeding from the alcove, which is to the left, describe it on the wall to the right. Those which are reflected by the walnut-tree, C D, to the right, delineate its figure in the picture to the left, e d. Thus the rays, coming in different directions, and proceeding always in straight lines, cross each other at their entrance through the apertures; those from above proceed below, those from the right go to the left, those from the left towards the right; thus every object is represented in the picture as occupying a situation the very reverse of that which it does in nature, excepting the flower pot, E F, which, though its position is reversed, does not change its