A D

B

inclined planes, meeting at the point C, and united at the base A B. The point C is inserted into the body to be cleft, and by means of violent blows of a hammer upon the base A B the whole wedge forces its way. It is generally understood that the wedge acts upon the principle of a double inclined plane. The less the breadth of the base A B is, in proportion to the length of the two sides A C, B C, the greater is the acquired power. It is calculated accordingly in theory, that, if B C, taken together, be four times the length of A B or what is the same thing, if A C be four times the length of A D, the half of A B, the power will

be equal to four times the resistance; and if the wood cleave at a distance before the wedge (which is the case with most kinds of timber) the advantage acquired is computed to be still greater. But, in truth, where there is so much friction it is difficult to attain a precise calculation upon the subject. Hatchets and chisels, and other sharp instruments having both edges sloped, act upon the principle of the wedge. So also does the knife in so far as it is used to split; but it acts also upon the principle of a fine saw, and therefore it is that it is drawn backward and forward across the body to be cut. It has justly been remarked, as one of the many proofs of wisdom displayed in the creation, that the beaks of birds are formed in the shape of wedges, for the purposes of enabling them to dig into the ground, or into the bark of trees, and to break the shells of fruit.

VI. The last mechanical power we have mentioned is the SCREW. It consists of two parts, the screw more properly so called, and the nut. The screw is a cylinder with a spiral protuberance, which is called its thread, apparently coiled round it in the same manner as the ivy twines round the oak, or a serpent twists itself round a pole. From this last circumstance the thread The nut,

sometimes receives the name of a worm. which is the weight to be moved, is generally a heavy piece of iron, with a hole perforated in the centre, which is so grooved as to accommodate itself to the spiral twistings of the screw upon which the nut moves. To the nut there is affixed a handle, which handle and nut taken together are called a winch. The screw acts upon the principle of an inclined plane, by which the body, in place of rising in a straight line, gradually ascends by a spiral curve to the top.

Cut a piece of paper in the shape of the triangle A B C, of which the side A C obviously represents an inclined plane; apply the side A B to a stick or other cylinder, and wrap the paper round it; and you will see at once that the line A C, representing the inclined plane, has become the spiral of a screw.

The closer the parts of the thread are to each other, the more will be the advantage gained by the screw. The operation of this power will be well understood by the following familiar illustration:-If, in place of attempting to ascend a high hill in a straight and perpendicular direction, we make use of a path which winds spirally round the hill till we reach the summit, our ascent, as every one knows, will be rendered much easier, and this facility will be more and more increased as the different parts of the winding path approach more closely to each other. Hitherto we have only considered the operation of that part of this mechanical power which is more properly called the screw; you will, however, at once perceive that the handle also acts as a lever, and that in proportion as its length is increased greater power is acquired. The power of a screw, therefore, may be augmented either by diminishing the distance between the parts of the thread, or by lengthening the handle. This mechanical force is employed by bookbinders and printers, and in the manufacture of wine, cider, and sometimes cheese, &c.

HEAT.

The conditions which we term hot, warm, and cold, appear to be the results produced by certain vibrations of matter. These conditions are not really opposed to each other, but may be regarded as different degrees of one general phenomenon which we call heat, and which, besides rendering itself sensible to our feelings through the above conditions, always exerts an influence on the expansion of bodies.

On inquiring into the proximate causes of heat, they will be found to be various. Heat renders itself sensible when two bodies are rubbed or knocked together. It is well known that savages obtain fire by the friction of two pieces of wood, and that the smith can make a nail red-hot by the proper management of his hammer. A great quantity of heat is likewise disengaged in the turning or boring of metals. When bodies are reduced to a higher degree of density, a considerable evolution of heat takes place; as, for instance, by the rapid and powerful compression of air, and by the slacking of lime.

Various and important phenomena of heat are the results of chemical combinations which are unceasingly proceeding in nature. The best known of these is the process of combustion, which is commonly applied by us to the production of heat for our own purposes. Even the chemical decomposition of food continually proceeding in the human body is an abundant source of heat. Electricity likewise produces considerable heat, as is proved by the effects of lightning.

The earth, moreover, possesses in itself a certain amount of heat, which is but slightly perceptible on its surface, but becomes more sensible to us at some depth, so that we have reason to assume the existence of a considerable degree of heat in the interior of the earth.

Finally, we regard the sun as the principal source of the heat felt on the surface of the earth, as rays of heat, besides those of light, are daily imparted by it. If the earth were not under the influence of solar heat it would differ widely in its nature from its present state.

Whatever may be the source whence heat is derived, it always exhibits the same phenomena in its relation to other objects.

EXPANSION BY HEAT.-THERMOMETER.

One of the most common phenomena produced by heat, which is sensible to the eye, is the expansion of bodies.

Examples of this expansion may be easily found. A metallic ball, which is a little too large to pass through a ring of metal, will, on the latter being heated, fall through it with ease, the ring being expanded by the heat.

If a vessel be filled completely with a liquid, and the latter heated gradually, it will soon flow over the edge of the vessel, in consequence of its expansion.

A bladder, pressed together, with the opening firmly tied up, but containing still a little air, will, on being warmed, assume the same form as if it were inflated with the mouth, in consequence of the expansion of the enclosed air. The expansion of bodies furnishes a very valuable means of comparing the effects of heat, and likewise of measuring its increase. Heat, as far as it exerts its influence on the comparative expansion of bodies, is termed temperature, and the instrument employed for measuring the latter is called a thermometer.

The thermometer, like other important philosophical instru

ments, as the pendulum and barometer, possesses the advantage of great simplicity.

A glass tube is chosen for the construction of the thermometer, the bore of which is perfectly uniform throughout, having about the width of a moderate-sized needle. A small bulb is blown at one end, and then filled with pure mercury. The mercury is now heated, upon which it expands, and fills the whole tube, which is from 6 to 10 inches in length. As soon as the mercury is at the point of protruding from the tube, the latter is sealed, so that it now contains no air whatever, but only the mercury, which on cooling again contracts, so as to stand to about one-third or onefourth of the height of the tube.

When a tube thus prepared is immersed in melting ice the column of mercury will stand at a certain height, which is accurately noted by a mark made on the glass tube. The thermometer is then placed for some time in boiling water, and the height to which the mercury rises likewise marked.

Whenever the thermometer is introduced into melting ice or boiling water, the mercury will stand at exactly the heights already noted, which shows that a body always occupies the same space at an equal temperature, and that this space decreases proportionately as the body becomes colder.

The point to which the mercury sinks, when the thermometer is immersed in melting ice, is indicated by a nought, and is called the freezing-point. That point to which the mercury rises, when the thermometer is plunged into boiling water is called the boilingpoint.

When, therefore, the thermometer is placed in any other position, we can judge of the surrounding temperature from the point at which the mercury stands in the tube. We call the temperature high if the mercury is near to the boiling-point, and low if it approaches the freezing-point.

In order to measure the temperature with greater accuracy, the space between the two points above mentioned is divided into a number of equal parts, which are called degrees. This division of the tube is also extended beyond the freezing and boiling points; those degrees that are situated above the former are termed heat degrees, and are denoted by the sign +, while those below the freezing-point are called degrees of cold, and are indicated by the mark

In France and in scientific works a thermometer with a scale of 100 divisions, or the Centigrade thermometer, is adopted, in which the boiling-point stands at 100°. But in this country a thermo

meter with a perfectly different scale, constructed by Fahrenheit, is most generally employed. The following comparative table will most clearly show the relation existing between the two scales :

:

MODE OF OBSERVING THE THERMOMETER.

I. The thermometer must be suspended in a place which is not exposed to the influence of the sun's rays either direct or reflected.

II. The best time in the day for observing the thermometer is at 9 o'clock morning and evening, as it has been found that the readings at these hours give the mean temperature of the day.

III. In reading off, the eye of the observer must be on a level with the top of the column of mercury.

THE BAROMETER.

THE barometer-derived from two Greek words meaning weight and measure-is a weather-glass, or instrument to measure the variations in the weight of the air.

In a barometer the column of mercury is left open or exposed to the air at its lower extremity, in order that the air may press upon the exposed surface of the mercury. As this pressure varies, the mercury rises or falls in the tube.

There are two practical purposes for which a barometer is used: one is to indicate changes of the weather, and the other is to determine the height of mountains.

Changes in the weather are indicated in this manner:—when