175

Chap. 1.] Without a knowledge of Atmospheric Pressure .

begin to think, that when a liquid is raised in the pipe of a pump, it mustbe by some force acting below, or befand it, a force a tergo, as it is named,and of which all the preceding machines are examples. Thu? when abücket of water is raised from a well, the force is applied oehind it,i.e. to the bottorn of the bücket, through the cord, bale, andsides, to whichit is attached. It is the same in the screw, the force continually elevatinga portion of it immediately behind the water ; and in the tympanum, no-rm, chain of pots, chain pump, &c. it is the same; the vessels or palletsgo below> i. e. behind the liquid and urge it up before them. It is the sameir. all ordinary motions. I wish to examine a small object laying at thefoot of my garden : now I cannot by moving this ruler in its direction, inthe manner of a pump rod, induce it to move to me, nor can it ever be somoved, until the force of some other body in motion behind it impel it to-wards me, It is the same in the case of a stubborn boy, who not only re-fuses to move as directed, but opposes the natural inertia of his body tothe change, and therefore can only be impelled forward by some force ap-plied directly or indirectly behind him, by dragging or pushing him along.In this way, all the motions in the universe, according to some philoso-phers, are imparted or transferred; those which appear exceptions beingconsidered modifications of it. Still however, the difficulty of establishinga connection between the movements of the sucker in the interior of thepump at one end, and this force, whatever it might be, acting on the wa-ter, outside of it, at the opposite end, would remain; and we should prob-ably at last impute this ascent of water (with the ancients) to some inde-finable energy of nature, both fallacious and absurd ; nor would this besurprising, for in the absence of a knowledge of the atmosphere and ofits properties, there really is as great a mystery in the movements of apump rod being followed by the ascent of the liquid, as in any thing everattributed to the divining rod, or to the wand of Abaris.

In Order to understand the Operation of machines belonging to this partof the subject, and also the principle upon which their action depends, wemust leave, for a few moments, the consideration of pumps and pipes, and allthe contrivances of man, and turn our attention to some of the Creator’s worksas they are exhibited in nature. This may perhaps be deemed a depar-lure from the subject; it is however so far from being a digression, that itis essentially necessary to ascertain the cause of water ascending in thisdass of machines, as well as to understand the philosophy of numerousnatural as well as artificial operations, that are performed by apparatusanalogous to them; as the acts of inspiration and respiration, quadrupedsdrinking, the young of animals sucking their dams, children drawing nour-lshment from their mothers’ breasts : bleeding by cupping, by leeches,or by the more delicate apparatus of a musketoe’s proboscis; and ifthings ignoble may be named, the taking of snuff, Smoking of cigars andpipes of tobacco, and also the experiments of those peripatetic philosophers,who perambulate our wharves, and imbibe nectar through straws fromhogsheads of rum and molasses.

Every person is aware, that the earth on which we live is of a globularor spheroidal figure, and that it is enveloped in an invisible ocean of air orgas, which extends for a great number of miles from every part of its sur-face. This hollow sphere of air is named the atmosphere, and is one ofthe most essential parts in the economy of nature. It is the source aswell as the theatre of those sublime meteorological pbenomena which weconstantly behold and admire. It is necessary to animal and to vegetablehfe. Its material is the ‘ breath of life’ to all things living. Tt is more-over the peculiar element of land animals, the scene of their actions, the