Count Rumford's rules for the construction of fire-places.

And when it blows in the opposite direction, it will often pour over the hill or the loftier house, like water over a dam, and rush down the chimneys of the house thus unfortunately situated.

Count Rumford lays down the following rules for the construction of fire-places :-"1. The throat of the chimney should be perpendicularly over the fire, as the smoke, vapor, &c. which rise from the fire naturally tend upwards. By the throat is meant the lower extremity of the chimney, where it unites with the upper part of the fire-place. 2. The nearer the throat is to the fire, the stronger will be the draught. 3. Four inches is the proper width to be given to the throat of a chimney, reckoning from the inside of the mantel to the back of the chimney." It is proper to observe that, in regard to this, much depends on the kind of fuel which is used. Green wood, which, besides smoke, sends up a large amount of watery vapor, requires a larger throat than other kinds of fuel. Anthracite coal, which sends up scarcely anything but the carbonic acid produced by its combustion, requires the smallest. "4. The width given. to the back of the chimney should be about one third of the width of the opening of the fire-place in front. In a room of middling size, fourteen inches is a good size for the width of the back, and thirty-nine inches for that of the opening of the fire-place. 5. The angle made by the back of the fire-place and the sides or covings of it should be 135 degrees, which is the best position they can have for throwing heat into the room." For thus the rays of heat which are thrown from the fire towards the side, in a direction parallel to the back, will be reflected directly forward; and the other rays more or less obliquely in various directions, diffusing heat over all parts of the

Currents of air entering above the fire.

Their injurious effects.

room. "6. The back of the chimney should always be built perfectly upright. 7. Where the throat of the chimney has an end, that is to say, where it enters into the lower part of the open canal of the chimney, there the three walls which form the sides and the back of the fire-place should all end abruptly, without any slope. This will render it more difficult for any wind from above to force its way through the narrow passage of the throat of the chimney. The back and covings should rise five or six inches higher than the breast of the chimney. 8. The current of air which, passing under the mantel, gets into the chimney, should be made gradually to bend its course upwards, by which means it will unite quietly with the ascending current of smoke. This is effected with the greatest ease and certainty, by merely rounding off the breast of the chimney." When this is not done, the current of air which passes under the mantel strikes the ascending current of smoke nearly at right angles, and therefore tends to drive part of it down. And not only so, but the mantel itself, when the breast of the chimney is not rounded off, tends to cut the column of smoke in two, and throw part of it out into the room. Many a smoky chimney exhibits the operation of this principle.

When we consider that man is furnished with means, and endowed with power, by which he can, at pleasure, bring into operation heat that was before concealed and inactive-that he is able to regulate the heat thus brought into action, increasing or diminishing its amount as he sees necessary-and when we think of the influence thus exerted on human happiness, the evidence of benevolent design in the Creator is too manifest not to be seen by

Blessings which the control of the element of heat confers.

all who are willing to see it. Were it not for this power of producing heat by artificial means, and of controlling it when produced, a large part of the earth would be wholly uninhabitable; and that part, too, which is now inhabited by the most enlightened nations, and which seems to be best calculated for the full development of the human faculties. In addition to this, the aid which the heat produced by combustion affords in the avocations of domestic life, in the softening of refractory metals, in the progress of the arts, and in many other cases too numerous to mention, clearly exhibits the benevolence of the Creator, and strongly urges the just claims which he has to the gratitude and adoration of his creatures.

GRANITE ROCK.

BY SAMUEL FISH.

Rocks.

Species used in building.

IN passing through the country, at a distance from any populous city, nothing appears more useless than the rocks scattered promiscuously over the ground. With people in general, there is no beauty perceptible in them, and no interest taken in the examination of their respective varieties. They are looked upon as an incumbrance, rather than as an article of usefulness.

By the scientific and the artist they are viewed in a different light. The geologist takes an interest in them. for the sake of the information they furnish in regard to the formation of the globe; the sculptor and architect for the use which is made of them in the arts which they respectively practice.

Though granite, particularly for architectural purposes, is used more extensively than any other species of rock, yet other kinds are abundantly used, not only for building, but for a great variety of purposes. Primitive limestone, sometimes called statuary marble, is a valuable article for statuary purposes; porphyry is an excellent, though expensive article for the columns of splendid edifices; clayslate, from its being easily separated into sheets, is much used for the outside covering for the roofs of houses; gypsum, or plaster of paris, is used in agriculture, and transition limestone, for the purpose of making quick

Geology.

False ideas respecting mineral formations.

lime, for a building stone, and for slabs to fire-places, tables, &c. The uses to which the different kinds of rocks are applied, are too numerous to mention.

Granite is but a single species of the great variety of rocks belonging to the globe we inhabit. To give a proper idea of this substance, it will be necessary to enter into some account of rocks in general. A person taking a careless view of the rocks scattered over the earth, would consider them too numerous to admit of a syste matic arrangement. A more careful observation shows this to be a mistake. Nothing is more easy than to arrange them into distinct classes, and exhibit them under their appropriate names. Though they seem to exhibit an endless variety, they are all composed of about nine different minerals. The various combinations of these minerals, or simple substances, is what principally occasions this variety.

Until within a few years, nothing has been so much neglected as this branch of natural philosophy. Mankind, to a very great extent, have been contented to live in ignorance in regard to the component principles of the globe. Very inaccurate and inconsistent ideas have been entertained in respect to rocks. Some persons suppose that they have ever existed just as we behold them at the present day. Others suppose that they grow-vegetate like plants. Even the great Mr. Locke, the most profound metaphysical philosopher that ever lived, entertained such an idea.

Though they cannot be said to grow, they may be said to be formed-just as many other things are formed-out of materials which previously existed. All that now composes the rocks, existed at some ancient period in the form of something else. The outer shell or crust of the earth, as it is commonly called, was consolidated out of a