property of hollow tubes is also accompanied with greater stiffness; and the superiority in strength and stiffness is so much the greater as the surrounding shell is thinner in proportion to its diameter. Hence we find that the bones of men and other animals are formed hollow, which renders them incomparably stronger and stiffer, gives more room for the insertion of muscles, and makes them lighter and more agile, than if they were constructed of solid matter. In like manner the bones of birds, which are thinner than those of other animals, and the quills in their wings, acquire by their thinness the strength which is necessary, while they are so light as to give sufficient buoyancy to the animal in its flight through the aerial regions. Our engineers and carpenters have, of late, begun to imitate nature in this respect, and now make their axles and other parts of machinery hollow, which both saves a portion of materials and renders them stronger than if they were solid.*

The departments of Hydrostatics and Hydraulics, which treat of the pressure and motion of fluids, and the method of estimating their velocity and force, require to be thoroughly understood by all those who are employed in the construction of common and forcing-pumps, water-mills, fountains, fire-engines, hydrostatical-presses; and in the formation of canals, wet-docks, and directing the course of rivers; otherwise they will constantly be liable to commit egregious blunders, and can never rise to eminence in their respective professions. Such principles as the

* The mechanical reader who wishes particular information on this subject is referred to the article Strength of materials in Ency. Brit. 3d edit. which was written by the late Professor Robison.

following:-that fluids press equally in all directions, -that they press as much upwards as downwards, -that water, in several tubes that communicate with each other, will stand at the same height, in all of them, whether they be small or great, perpendicular or oblique, that the pressure of fluids is directly as their perpendicular height, without any regard to their quantity, and that the quantities of water discharged at the same time, by different apertures, under the same height of surface in the reservoir, are to each other nearly as the areas of their apertures,will be found capable of extensive application to plumbers, engineers, pump-makers, and all who are employed in conducting water over hills or vallies, or in using it as a mechanical power, by a recognition of which they will be enabled to foresee, with certainty, the results to be expected from their plans and operations; for want of which knowledge many plausible schemes have been frustrated, and sums of money expended to no purpose.

The following figures and explanations will tend to illustrate some of the principles now stated:-1. Fluids press in proportion to their perpendicular heights, and the base of the vessel containing them, without regard to the quantity. Thus, if the vessel

ABC, fig. 2, has its base BC equal to the base FG of the cylindrical vessel DEFG, fig. 1, but is much smaller at the top A than at the bottom, and of the same height; the pressure upon the bottom BC is as great as the pressure upon the bottom of the vessel DEFG, when they are filled with water, or any other liquid, notwithstanding that there will be a much greater quantity of water in the cylindrical than in the conical vessel; or, in other words, the bottom BC will sustain a pressure equal to what it would be if the vessel were as wide at the top as at the bottom. In like manner, the bottom of the vessel HIKL, fig. 3, sustains a pressure only equal to the column whose base is KL, and height KM, and not as the whole quantity of fluid contained in the vessel; all the rest of the fluid being supported by the sides. The demonstration of these positions would occupy too much room, and to many readers would appear too abstract and uninteresting; but they will be found satisfactorily demonstrated in most books which treat of the doctrines of hydrostatics.

2. The positions now stated form the foundation of the hydrostatical paradox, namely, that a quantity of fluid, however small, may be made to coun

terpoise a quantity, however great." Thus, if to a wide vessel AB, we attach a tube CD, communicat

ing with the vessel, and pour water into it, the water will run into the larger vessel AB, the same height C and G in both.

and will stand at If we affix an in

clined tube EF, likewise communicating with the large vessel, the water will also stand at E, at the same height as in the other two; the perpendicular altitude being the same in all the three tubes, however small the one may be in proportion to the other. This experiment clearly proves that the small column. of water balances and supports the large column, which it could not do if the lateral pressures at bottom were not equal to each other. Whatever be the inclination of the tube EF, still the perpendicular

I

altitude will be the same as that of the other tubes, although the column of water must be much longer than those in the upright tubes. Hence it is evident, that a small quantity of a fluid may, under certain circumstances, counterbalance any quantity of the same fluid. Hence also the truth of the principle in hydrostatics, that "in tubes which have a communication, whether they be equal or unequal, short or oblique, the fluid always rises to the same height." From these facts it follows, that water cannot be conveyed by means of a pipe that is laid in a reservoir to any place that is higher than the reservoir.

These principles point out the mode of conveying water across valleys without those expensive aqueducts which were erected by the ancients for this purpose. A pipe, conforming to the shape of the valley, will

answer every purpose of an aqueduct. Suppose the spring at A, fig. 5, and water is wanted on the other side of the valley to supply the house H, a pipe of lead or iron laid from the spring-head across the valley will convey the water up to the level of the