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that we witness over the whole creation, an admirable accordance to the modes, for the support of life and health which God has thought fit to adopt.

The peculiarities, in the functions of respiration in birds, are exceedingly curious. In this class of animals, the lungs are small, Aattened, and lie close to the breast, but there is no diaphragm, and there is no alternate expansion of the lungs, as in mamntalia. In them, the lungs have several openings, by means of which they communicate with various air-bags, or cells, which fill the whole of the cavity of the body from the neck downwards. These cells are filled by air which passes into, and out of them, through the lungs, and which, in its passage, produces those changes on the blood circulating through the lungs, which are necessary for the health of the animal. By admitting different quantities of air into these cells, and also into the bones, which are hollow, for the purpose of admitting air into them from the lungs, birds have the power of increasing or diminishing their specific gravity, so that they can, not only walk on the earth, but soar in the heavens, in all the varieties of density of atmosphere, which a greater or smaller proximity to the earth necessarily occasions. No strength of wing could poise a terrestrial animal in air, unless there were the power of admitting air into the inmost recesses of the body, as happens in birds ; and this has been so carefully attended to in them, that the cells extend even among the muscles of the body, where they are particularly large in the soaring animals, as the eagle, hawk, stork, and lark. Those birds that pounce and those that dive, are enabled to do so with great velocity, by suddenly compressing their body, which drives out, and increases their specific gravity. The barrels of the quills in birds, too, are hollow, and contain air ; and it is said, that it is in some measure owing to the power of diminishing or increasing the contained quantity, that the turkey, bulfinch, &c. are able to produce the quick and voluntary erection of their plumage. We may mention here, that the power which birds have of admitting a large quantity of air into their bodies, enables them to keep up a much

stronger, and more continued current of air through the larynx, than any other animal can do ; and gives them, therefore, a volume of voice, which is very great, compared with their small dimensions,

In fishes, as already stated, the air is applied to the gills through the medium of the water. The gills are covered with a large flap, or operculum, which is edged with a fringe which can be accurately applied to the part beneath, so as entirely to shut up the slit, or opening into the gills. When the animal breathes, that is, when it wishes water to be applied to the gills, it acts with the muscles of this flap so as to render it convex; this cannot be done, it is clear, without producing a vacuum under the flap; and as the animal is in water, and there is an opening in the mouth which communicates with the gills, the water rushes in among the gills, filling up the space made by the changed form of the flap, and thus applying itself to the minute ramifications of blood-vessels diffused over the gills. When the air contained in this water is no longer equal to its purpose, the water passes away through the airopening at the edge of the operculum, which the animal has the power of making ;" and by a repetition of the process, a fresh supply of water is obtained, and the function of respiration kept up. Fish could not live in water from which the air has been expelled by boiling; and when a small pond is frozen over, the fish die unless an opening is made to admit the air.

There is, in fish, a part of structure somewhat analogous to the air-cells in birds, namely, the air-bladders, or swimming-bladders, which are given to them, as the cells are to birds, for the purpose of increasing, or diminishing their buoyancy. These bladders are placed close to the back-bone; they vary in size, shape, and number ; and are wanting, or very small, in such fish as are generally confined to the lowest depths. They form what is called the sound of fish; a part which gourmands prize highly. When the air-bladder is ruptured, the animal loses the power of raising itself, and lies on its back, from the additional weight given to that part of the body, by the removal of the air. The air, in

the air-bladders of fish, cannot be admitted and thrown out at pleasure, as in the case of birds. The air is procured from the vessels circulating in the membrane which composes the air-bladders; these vessels having the power of secreting air. The air-bladder is ordinarily full, and is then capable of being acted upon, and compressed, either by the abdominal muscles, or by, a muscular structure peculiar to, this organ; and thus the air is condensed pursuant to the will of the animal, and an alteration made in the specific gravity accordingly.

It is a curious fact that the nature of the air varies very much, according to the depth which fish generally inhabit. Those which live in shallow water, have azote, with a very small proportion of oxygen. As the depth increases so does the oxygen ; and after the depth of 150 feet, the average proportion is as much as 70 per cent, while the mean result afforded by fish caught at less depth, is only 29 per cent. Pike, carp, roaches, and perch, which are fresh-water, and therefore shallow-water fish, have only from 3 to 5 of oxygen.

There is a curious mode of respiration employed by frogs, toads, cameleons, and some others of the amphibious tribe, which is, that the animal, instead of breathing through its mouth, keeps its mouth shut, receives air through its nose, and by means of the muscles of the jaws forces it into the lungs, from which it is returned, through the nostrils, by the action of the muscles of the abdomen-there being no diaphragm. With this conformation, those animals would be suffocated if their mouths were kept open.

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SECTION IV.

INTRODUCTION TO NATURAL PHILOSOPHY.

Natural Philosophy, in its most extensive sense, has for its province the investigation of the laws of matter, that is, the properties of matter; and it may be divided into two great branches. The first and most important (which is sometimes called Natural Philosophy, by.way of distinction, but more properly Mechanical Philosophy) investigates the sensible motions of bodies. The second investigates the constitution and qualities of all bodies, and has various names, according to its different objects. It is called chemistry, if it teaches the properties of bodies with respect to heat, mixture with one another, weight, taste, appearance, and so-forth ; Anatomy and Animal Physiology, if it teaches the structure and functions of living bodies, especially the human ;-for, when it treats of the functions of other animals, we term it Comparative Anatomy. It is called Medicine, if it teaches the nature of diseases, and the means of preventing them, and of restoring health: Zoology, if it teaches the arrangement or classification, and the habits of the different lower animals : Botany, including Vegetable Physiology, if it teaches the arrangement or classification, the structure and habits of plants: Mineralogy, including Geology, if it teaches the arrangement of minerals, the structure of masses in which they are found, and of the earth composed of these masses. The term natural history is given to the three last branches taken together ; but chiefly, as far, as they teach the classification of different things, or the observation of the resemblances and differences of

the various animals, plants, and ungrowing substances in nature,

Here we may make two observations. The first is, that every such distribution of the sciences is necessarily imperfect; for one runs unavoidably into another. Thus, Chemistry shows the qualities of plants with relation to other substances, and to each other : and Botany does not overlook those same qualities, though its chief object be arrangement. So Mineralogy, though principally conversant with classifying metals and earth, yet regards also their qualities in respect of heat and mixture. So Zoology too, beside arranging animals describes their structures like comparative anatomy. In truth, all arrangement and classification depend upon noting the things in which the objects agree and differ and among those things in which animals, plants, and minerals agree or differ, must be considered the anatomical structure of the one, and the chemical qualities of the other. Hence, in a great measure, follows the second observation, namely, that the sciences mutually assist each other. Thus, arithmetic and algebra aid geometry, and the purely mathematical sciences aid mechanical philosophy; mechanical philosophy, in like manner, assists chemistry and anatomy, especially the latter : and chemistry very greatly assists physiology, medicine, and all the branches of natural history.

The first great head, then, of natural science, is mechanical philosophy; and it consists of various subdivisions, each forming a science of great importance. The most essential of these, which is indeed funda, mental, and applicable to all the rest, is called dynamics, from the Greek word signifying power or force. It teaches the laws of motion in all its varieties. The application of dynamics to the calculation, production, and direction of motion, forms the science of mechanics, sometimes called practical mechanics, to distinguish it from the more general use of the word, which comprehends every thing that relates to motion and force.

The application of dynamics to the pressure and motion of fluids, constitutes a science, which receives different appellations according as the fluids are heavy

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