IV

T

THE WORK OF AIR AND WATER

HE store of energy contained in the atmosphere and in the waters of the globe is inexhaustible. Its amount is beyond all calculation; or if it were vaguely calculated the figures would be quite incomprehensible from their very magnitude. It is not, however, an altogether simple matter to make this energy available for the purposes of useful work. We find that throughout antiquity comparatively little use was made of either wind or water in their application to machinery.

Doubtless the earliest use of air as a motive power was through the application of sails to boats. We know that the Phoenicians used a simple form of sail, and no doubt their example was followed by all the maritime peoples of subsequent periods. But the use of the sail even by the Phoenicians was as a comparatively unimportant accessory to the galaxies of oars, which formed the chief motive power. The elaboration of sails of various types, adequate in extent to propel large ships, and capable of being adjusted so as to take advantage of winds blowing from almost any quarter, was a development of the Middle Ages.

The possibilities of work with the aid of running

water were also but little understood by the ancients. In the days of slave labor it was scarcely worth while to tax man's ingenuity to invent machines, since so efficient a one was provided by nature. Yet the properties of both air and water were studied by various mechanical philosophers, at the head of whom were Archimedes, whose work has already been referred to, and the famous Alexandrian, Ctesibius, whose investigations became familiar through the publications of his pupil, Hero.

Perhaps the most remarkable device invented by Ctesibius was a fire-engine, consisting of an arrangement of valves constituting a pump, and operating on the principle which is still in vogue. It is known, however, that the Egyptians of a much earlier period used buckets having valves in their bottoms, and these perhaps furnished the foundation for the idea of Ctesibius. It is unnecessary to give details of this fire-engine. It may be noted, however, that the principle of the lever is the one employed in its operation to gain power. A valve consists essentially of any simple hinged substance, arranged so that it may rise or fall, alternately opening and closing an aperture. A mere flap of leather, nailed on one edge, serves as a tolerably effective valve. At least one of the valves used by Ctesibius was a hinged piece of smooth metal. A piston fitted in a cylinder supplies suction when the lever is raised, and pressure when it is compressed, alternately opening the valve and closing the valve through which the water enters the tube. Meantime a second valve alternating with the first permits the water to enter the chamber

containing air, which through its elasticity and pressure equalizes the force of the stream that is ejected from the chamber through the hose.

SUCTION AND PRESSURE

In the construction of this and various other apparatus, Ctesibius and Hero were led to make careful studies of the phenomena of suction. But in this they were not alone, since numerous of their predecessors had studied the subject, and such an apparatus as the surgeon's cupping glass was familiarly known several centuries before the Christian era. The cupping glass, as perhaps should be explained to the reader of the present day-since the apparatus went out of vogue in ordinary medical practise two or three generations ago consists of a glass cup in which the air is exhausted, so as to suck blood from any part of the surface of a body to which it is applied. Hero describes a method of exhausting air by which such suction may be facilitated. But neither he nor any other philosopher of his period at all understood the real nature of this suction, notwithstanding their perfect familiarity with numerous of its phenomena. It was known, for example, that when a tube closed at one end is filled with water and inverted with the open end beneath the surface of the water, the water remains in the tube, although one might naturally expect that it would obey the impulses of gravitation and run out, leaving the tube empty. A familiar explanation of this and allied phenomena throughout antiquity was found in the

saying that "Nature abhors a vacuum." This explanation, which of course amounts to no explanation at all, is fairly illustrative of the method of metaphysical word-juggling that served so largely among the earlier philosophers in explanation of the mysteries of physical science.

The real explanation of the phenomena of suction was not arrived at until the revival of learning in the seventeenth century. Then Torricelli, the pupil of Galileo, demonstrated that the word suction, as commonly applied, had no proper application; and that the phenomena hitherto ascribed to it were really due to the pressure of the atmosphere. A vacuum is merely an enclosed space deprived of air, and the "abhorrence" that Nature shows to such a space is due to the fact that air has weight and presses in every direction, and hence tends to invade every space to which it can gain access. It was presently discovered that if the inverted tube in which the water stands was made high enough, the water will no longer fill it, but will sink to a certain level. The height at which it will stand is about thirty feet; above that height a vacuum will be formed, which, for some reason, Nature seems not to abhor. The reason is that the weight of any given column of water about thirty feet in height is just balanced by the weight of a corresponding column of atmosphere. The experiments that gave the proof of this were made by the famous Englishman, Boyle. He showed that if the heavy liquid, mercury, is used in place of water, then the suspended column will be only about thirty inches in height. The weight or [65]

VOL. VI. -5

pressure of the atmosphere at sea level, as measured by these experiments, is about fifteen pounds to the square inch.

Boyle's further experiments with the air and with other gases developed the fact that the pressure exerted by any given quantity of gas is proportional to the external pressure to which it is subjected, which, after all, is only a special application of the law that action and reaction are equal. The further fact was developed that under pressure a gas decreases at a fixed rate in bulk. A general law, expressing these facts in the phrase that density and elasticity vary inversely with the pressure in a precise ratio, was developed by Boyle and the Frenchman, Mariotte, independently, and bears the name of both of its discoverers. No immediate application of the law to the practical purposes of the worker was made, however, and it is only in recent years that compressed air has been extensively employed as a motive power. Even now it has not proved a great commercial success, because other more economical methods of power production are available. In particular cases, however, it has a certain utility, as a relatively large available source of energy may be condensed into a very small receptacle.

A very striking experiment illustrating the pressure of the air was made by a famous contemporary of Boyle and Mariotte, by the name of Otto von Guericke. He connected an air pump with a large brass sphere, composed of two hemispheres, the edges of which fitted smoothly, but were not connected by any mech