"When a beam gives support laterally, its strength is propor. tioned to its breadth, and to the square of its depth conjointly. If, then, such a beam were enlarged ten times in each of its linear dimensions, its ability to sustain a weight placed at its extremity would, on account of the increased distance from the point of insertion, be only one hundred times augmented, but the load to be put upon it would be one thousand times greater; and thus, although the parts of the model be quite strong enough, we cannot thence conclude that those of the enlarged machine will be so.

"It may thus be stated as a general principle, that, in similar machines, the strengths of the parts vary as the square, while the weights laid on them vary as the cube of the corresponding linear dimension.

"This fact cannot be too firmly fixed in the minds of machine makers; it ought to be taken into consideration even on the smallest change of scale, as it will always conduce either to the sufficiency or to the economy of a structure. To enlarge or diminish the parts of a machine all in the same proportion, is to commit a deliberate blunder. Let us compare the wing of an insect with that of a bird: enlarge a midge till its whole weight be equal to that of the sea-eagle, and, great as that enlargement must be, its wing will scarcely have attained the thickness of writing paper; the falcon would feel rather awkward with wings of such tenuity. The wings of a bird, even when idle, form a conspicuous part of the whole animal; but there are insects which unfold, from beneath two scarcely perceived covers, wings many times more extensive than the whole surface of their bodies.

"The larger animals are never supported laterally; their limbs are always in a position nearly vertical: as we descend in the scale of size the lateral support becomes more frequent, till we find whole tribes of insects resting on limbs laid almost horizontally. The slightest consideration will convince any one that lateral or horizontal limbs would be quite inadequate to support the weight of the larger animals. Conceive a spider to increase till his body weighed as much as that of a man, and then fancy one of us exhibiting feats of dexterity with such locomotive instruments as the spider would then possess !

"The objects which I have hitherto compared have been remote, that the comparisons might be the more striking; but the same principles may be exhibited by the contrast of species the most nearly allied, or of individuals even of the same species. The larger species of spiders, for instance, rarely have their legs so much extended as the smaller ones; or, to take an example

from the larger animals, the form of the Shetland pony is very different from that of the London dray-horse.

"How interesting it is to compare the different animals, and to trace the gradual change of form which accompanies each increase of size! In the smaller animals, the strength is, as it were, redundant, and there is room for the display of the most elaborate ornament. How complex or how beautiful are the myriads of insects which float in the air, or which cluster on the foliage! Gradually the larger of these become more simple in their structure, their ornaments less profuse. The structure of the birds is simpler and more uniform, that of the quadrupeds still more so. As we approach the larger quadrupeds, ornament, and then ele. gance, disappear. This is the law in the works of nature, and this ought to be the law among the works of art.

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Among one class of animals, indeed, it may be said that this law is reversed. We have by no means a general classification of the fishes; but, among those with which we are acquainted, we do not perceive such a prodigious change of form. Here, how. ever, the animal has not to support its own weight; and whatever increase may take place in the size of the animal, a like increase takes place in the buoyancy of the fluid in which it swims. Many of the smaller aquatic animals exhibit the utmost simplicity of structure; but we know too little of the nature of their functions to draw any useful conclusions from this fact."

Shoes and Buckles.

The business of a shoemaker is of great antiquity. The instru ment for cleaning hides, the shoemaker's bristle added to the yarn, and his knife, were in use as early as the twelfth century. He was accustomed to hawk his goods, and it is conjectured that there was a separate trade for annexing the soles. The Romans in classical times wore cork soles in their shoes, to secure the feet from water, especially in winter; and as high heels were not then introduced, the Roman ladies who wished to appear taller than they had been formed by nature, put plenty of cork under them. The streets of Rome in the time of Domitian were blocked up by cob. bler's stalls, which he therefore caused to be removed. In the middle ages shoes were cleaned by washing with a sponge; and oil, soap, and grease were the substitutes for blacking. Buckles were worn on shoes in the fourteenth century. In an Irish abbey a human skeleton was found with marks of buckles on the shoes, In England they became fashionable many years before the reign

of Queen Mary; the laboring people wore them of copper; other persons had them of silver, or copper gilt; not long after, shoe roses came in. Buckles revived before the revolution of 1689, remained fashionable till after the French revolution in 1789, and finally became extinct before the close of the eighteenth century.

The Croton Aqueduct.

Some of our readers may have had their curiosity excited with respect to the great aqueduct now in construction for the supply of New York with water. The following description of the manner in which the work is performed, with the illustrative cut connected with it, will prove acceptable. We have been indebted for them to Mr. Miner, editor of the Railroad Journal, from whom we have before received several similar favors.

The ground on which New York stands consists chiefly of loose sand, intermixed in many places with coarse gravel and boulders, or roundish stones of different sizes, apparently brought by a flood of water from some primitive region. Hornblend rock predomi nates. Granite and gneiss rocks are found in original masses in some parts. Long Island consists of sand and loose stones, without a trace of any fixed rock, except at Hurlgate, and perhaps at one or two other places.

Primitive rocks and soils generally furnish good water; and the springs of this city, though few and public, are abundant, and many of them were originally good. The increase of population, however, has caused the deterioration of the water: for where the rain once fell on fields of grass or groves of wood, it now meets with crowded streets or narrow lots occupied by crowded habitations, and contracts impurities which it carries with it far down into the sands where the springs flow. Some of the wells in the middle and upper parts of the city, which yielded excellent water within the memory of living inhabitants, have become so much affected in later years, that many of the people purchase drinking water at a penny a pailful, of men who bring it in carts from springs yet untainted by the encroaching city. As the water of the wells is unfit for washing as well as for drinking, every family requires a cistern; and thus it has been thought desirable, for many years, that an abundant supply of good water should be obtained for the city.

The Water Works in Chambers street, under the direction of the Manhattan Company, have furnished, for some years, water of an inferior quality to the inhabitants of many streets in the lower

parts of the city, at certain prices; and water for the use of fireengines has since been provided, in a large reservoir on the height of ground, from which it is distributed in hydrants to different districts. It was proposed, some years ago, to obtain a supply from several ponds in the town of Rye: but, after an examination, the Croton river was preferred, although the distance was great, the route obstructed by serious impediments, and the work tedious and very expensive. The friends of the enterprise, however, rightly judged, that nothing could be so expensive to the health and convenience of the inhabitants, and therefore in fact so great a pecuniary loss to the city treasury, as the longer neglect of the great work. It was therefore commenced; and about five thou sand men have sometimes been employed on it at one time.Family Visiter.

Description of the mode of constructing the Croton Aqueduct, from the American Railroad Journal.-The materials used are good building-stone, of the proper degree of hardness and dura. bility, free from all metals, particularly iron. Gneiss is preferred to any other, both because it is more plentiful and more easily worked. Some limestone is also used, but not until it has the

express permit of the Resident Engineer. Brick is the next material; it is required to be from the centre of the kiln, such as is thoroughly burnt, free from lime or any other impurity, and to possess a clear ringing sound when struck. The worst accepted are such as cost from five to seven dollars a thousand. Next is the cement, from which the concrete and masonry generally are formed. The commissioners' specifications are very explicit relative to the manufacture of this article, requiring that the name of the manufacturer should be known; that the cement shall not have been made more than six months before being used; that it shall be transported from the factory in water-tight casks; and, in addition to all this, that each parcel or cargo received shall be thoroughly tested, either by officers appointed for the purpose, or by the Resident Engineer himself. These are the principal materials, stone, brick, and cement. The stone is required to be always clean, and in hot weather, kept wet, and when laid in the wall requiring mortar, it must "swim" in the cement-that is, when the stone is lifted up from its bed, no point or surface of the stone must touch the stone below it, each stone must be surrounded by cement. When the weather is hot, the top of the wall must be kept moist, and in cold weather all the masonry must be covered so effectually, as to protect it perfectly. The brick must be laid true and even, allowing three-eighths of an inch joint, or thereabouts. In hot weather, they are to be soaked in water, and to be kept wet while being laid. The cement is mixed in different proportions, according to the work required. For stone work, the proportions are one part of cement to three of sand, (the sand to be medium size, sharp grained and clean-river sand is accepted.) For brick work, the proportions are one of cement to two of sand; for concrete, one part of cement, three of sand, and three of clean building-stone, broken about as fine as that used for Macadamizing. Concrete is used for forming artificial foundations, is mixed with as little water as possible, and when laid in any part of the work, is left undisturbed forty-eight hours; at the expiration of this time it has become so hard, that a blow with a pickaxe will not break it: it becomes quite a rock./

The aqueduct, maintaining a uniform descent, requires that in places the earth should be cut away, and in crossing valleys, that they should be filled up. In the former case, the sides of the cut are left standing at a slope of one half to one; that is, if the perpendicular height of the side of the cut be six feet, it will fall from directly above its base three feet. It is one-half horizontal to one vertical. The base of the cut is always thirteen feet wide. Pegs, showing the bottom of the side walls, and of the reversed arch in