longing to it: for, in consequence of its great degree of hardness, it is capable of cutting and polishing not only the hardest glass, but even the hardest gems: and if we consider how useful a substance glass is, how universally employed as a means of at the same time admitting light and excluding the air from the interior of our houses; but that in consequence of its hardness and brittleness it would with great difficulty be divided by any common mechanical instrument, so as accurately to fit the frames in which it is fixed for the above purposes, we at once see the value of a substance which easily and readily accomplishes that end. A small diamond no larger than a mustard seed, brought to a point and fixed in a convenient handle, enables the glazier to cut a plate of glass into pieces of any shape that he pleases: and the same instrument will serve his daily use for many successive years. Nor is it among the least of the glories of this gem, that it gave occasion to that remarkable conjecture of sir Isaac Newton respecting its chemical nature. That philosopher having observed, that the refractive power of transparent substances is in general proportional to their density; but that, of substances of equal density, those which are combustible possess the refractive power in a higher degree than those which are not, concluded from a comparison of the density and refractive power of the diamond, that it contained an inflammable principle; which opinion was subsequently confirmed by direct experiment. It will be remembered by the chemical reader that on the same ground he made the same conjecture with respect to water, and with the same success. And never, perhaps, did the eye of philosophy penetrate more unexpectedly the thick veil which is so often found to hide the real character of various forms of matter: for, imperishable as from its name the adamant was supposed to be, who would have antecedently expected that it might be dissipated into air by the process of combustion? and, with respect to the M other subject of his conjecture, if any principle was opposed to combustibility in the opinion of mankind it was water-" Aquæ contrarius ignis." SECTION IV. The Distribution and relative Proportions of Sea and Land; and the geological Arrangement and physical Character of some of the superficial Strata of the Earth. As it is clearly a just object of the present treatise to select the most familiar and most obvious instances of the principle intended to be illustrated, I shall in entering upon the abstruse department of geology, consider only those phenomena which offer themselves to the eye in every part of the world; and which are either at once intelligible, or easily demonstrable, to the commonest observer. Of such phenomena the most prominent are the general distribution of the sea and the land; and the relative proportions of their superficial extent. With reference to the sea, although we may never know all the ends which are answered by its saltness, and why its depth should be greater in some parts than others; and although we can perhaps form no more than a conjecture as to the advantages derivable from the tides; (the prevention, for instance, of a stagnant state of the water;) or from the accumulation of ice near the poles; (the cooling, probably, of the general mass of the atmosphere, and the consequent production of currents of air;) yet of its mode of distribution, and of the relative extent of its surface, we readily apprehend the reason; simply in considering that all those forms of water which contribute to the fertilization of the earth, or the support of animal life, are derived from the ocean. Were the superficial extent of this therefore much less than it is, the quantity evaporated would not be sufficient for the intended purposes; or, were the dis tribution different from what it is, were the sea, for instance, to occupy one hemisphere and the land the other, the water evaporated would not be so equally diffused through the atmosphere as it is at present. And, with respect to the land, how beautifully does the particular arrangement and character of its surface conspire with its general distribution to equalize the diffusion of the water that is discharged upon it from the atmosphere! The truth of the proposition contained in those lines, "Rusticus expectat dum defluat amnis, at ille depends on the nature of the particular arrangement and character, to which allusion has just been made. On the one hand, the general surface of the land ascending from the sea on all sides towards some central ridge or district, called the watershed of the country, all the rain that does not sink beneath the surface is accumulated into rivers; which naturally descend towards, and ultimately reach, the sea: and, on the other hand, the superficial strata being in general incapable of immediately absorbing the rain which falls upon them, the descent of the water is the necessary result of the inclination of the surface. But if, from partial causes, such an inclination of the land is either wanting, or the course of rivers is impeded by the unrepressed growth of reeds and sedge, the adjoining district is overflowed, and at length converted into a stagnant marsh. It is from such a physical cause, that, at this moment, the ancient site of Babylon attests the truth of prophecy; being still, as it has been for ages," a possession for the bittern, and pools of water." But that which is called the watershed of any large tract of land is not simply the most elevated portion of the whole surface: it consists also, in a greater or less degree, of ranges of mountains; down the highly inclined sides of which the rain immediately descends in numerous torrents, which by their gradual accu mulation produce rivers. And, as best calculated to secure the permanent effect, the substance of these mountains is in general so hard, and impermeable to water, that, with reference to the present system of the earth, they may justly be characterized by the epithet "everlasting." But if, instead of being thus durable, they were of a soft or friable substance, they would soon cease to exist as mountains; and if they were porous, instead of compact, they would absorb much of that rain which now contributes to the formation of rivers. From that portion of the rain which, in comparatively flat districts, sinks beneath the surface of the earth, reservoirs of water are formed: from which, either spontaneous springs arise, or into which, artificial excavations called wells are sunk: and of the utility of such reservoirs, those beds of gravel which occur in every part of the world afford upon the whole the best illustration. SECTION V. Beds of Gravel. FEW subjects would at the first view appear more barren of interest than a bed of gravel; consisting, as it usually does, of nothing but fragments of broken pebbles and sand, heaped together in apparently inextricable confusion. Yet such beds, dispersed as they are very generally over the surface of the regular strata, administer materially to the wants of man; in affording him the means of supplying himself readily with that important necessary of life, water. From the irregularity in the form and size of the component parts of gravel, and from the slight degree of cohesion by which they are united, the whole mass is necessarily porous: and hence, readily transmitting the rain which falls on its surface, becomes charged with water to an extent proportional to the quantity of rain which has penetrated it; being enabled to retain the water thus accumulated, in consequence of its resting on some substratum, as clay, which is impermeable to water: so that, if an excavation sufficiently deep be made into any part of the gravel, the water immediately drains into this excavation, and rises at length to the level of the general mass of water contained in the whole bed; by which easy process, in such instances at least, those reservoirs, called wells, are formed: and these reservoirs are never exhausted, so long as the whole bed of gravel retains any considerable proportion of water. A very ready illustration of this fact is afforded by the familiar instance of those excavations which children are accustomed to make in the sand of the sea beach, while yet charged with moisture during the ebbing of the tide. The inhabitants of a town which, like Oxford, is built partly on a comparatively shallow bed of gravel, and partly on a deep stratum of clay, can well appreciate the value of the former substratum of their habitations, with reference to the facility of procuring water: for while they, whose dwellings are built on the gravel, can readily obtain water by sinking a well immediately on the spot; they whose dwellings are on the clay, must either procure water from a distance, or incur a very serious, and, finally perhaps, useless expense, in attempting to penetrate the clay. With respect to its general uses, gravel seems only to be employed in the repairing of roads and walks; in the composition of some kinds of mortar; and as a convenient occasional ballast for sailing vessels: so that, if we confine our view to the means afforded by gravel beds of supplying the ordinary wants of man, their history may be comprised in a From the observation of an analogous arrangement in the general strata of the earth, namely, that those which are pervious to water alternate with those which are impervious to water, Mr. William Smith, "the father of English geology," became acquainted with the origin of springs, and the true principles of draining. |