in the water by the use of the sulphate of alumina, and then allowing the water to flow through a body of coarse sand, by which the coagulated aluminated matter is caught and held in the interstices of the sand, and the bacteria arrested. All objectionable matter is thus arrested by the surface portion of the sand body, which portion is from time to time scraped off, and the whole sand mass occasionally washed out by upward currents of water forced through the same.

By this system great rapidity of filtration is obtained, the rate being 120,000,000 gallons a day per

acre.

The English system consists more in the use of extended and successive reservoirs or beds of sand alone, or aided by the use of the sulphate. This also is extensively used in many large cities.

CHAPTER XII.

PNEUMATICS AND PNEUMATIC MACHINES.

"THE march of the human mind is slow," exclaimed Burke in his great speech on "Conciliation with the Colonies." It was at the beginning of the last quarter of the 18th century that he was speaking, and he was referring to the slow discovery of the eternal laws of Providence as applied in the field of political administration to distant colonies. The same could then have been said of the march of the human mind in the realms of Nature. How slow had been the apprehension of the forces of that kind but silent Mother whose strong arms are ever ready to lift and carry the burdens of men whenever her aid is diligently sought! The voice of Burke was, however, hardly silent when the human mind suddenly awoke, and its march in the realms of government and of natural science since then cannot be regarded as slow.

More than fifteen centuries before Burke spoke, not only had Greece discovered the principles of political freedom for its citizens and its colonies, but the power of steam had been discovered, and experimental work been done with it.

Yet when the famous orator made his speech the Grecian experiment was a toy of Kings, and the steam engine had just developed from this toy into a mighty engine in the hands of Watt. The age of mechanical inventions had just commenced with

the production of machines for spinning and weaving. And yet, in view of the rise of learning, and the appearance from time to time of mighty intellects in the highest walks of science, the growth of the mind in the line of useful machinery had indeed been strangely slow. "Learning" had revived in Italy in the 12th and 13th centuries and spread westward in the 14th. In the 15th, gunpowder and printing had been discovered, and Scaliger, the famous scholar of Italy, and Erasmus, the celebrated Dutch philosopher, were the leading restorers of ancient literature. Science then also revived, and Copernicus, the Pole, gave us the true theory of the solar system. The 16th century produced the great mathematicians and astronomers Tycho Brahe, the Dane, Cardan and Galileo, the illustrious Italians, and Kepler, the German astronomer, whose discovery of the laws of planetary motion supplemented the works of Copernicus and Galileo and illuminated the early years of the 17th century.

In the 17th century appeared Torricelli, the inventor of the barometer; Guericke, the German, inventor of the air pump; Fahrenheit, the inventor of the mercurial thermometer bearing his name; Leibnitz, eminent in every department of science and philosophy; Huygens, the great Dutch astronomer and philosopher; Pascal of France and Sir Isaac Newton of England, the worthy successors of Kepler, Galileo and Copernicus; and yet, with the exception of philosophical discoveries and a few experiments, the field of invention in the way of motor engines still remained practically closed. But slight as had been the discoveries and experiments referred to, they were the mine from which the inventions of subsequent times were quarried.

One of the earliest, if not the first of pneumatic machines, was the bellows. Its invention followed the discovery of fire and of metals. The bladders of animals suggested it, and their skins were substituted for the bladders.

The Egyptians have left a record of its use, thirtyfour centuries ago, and its use has been continuous ever since.

Mention has been made of the cannon. It was probably the earliest attempt to obtain motive power from heat. The ball was driven out of an iron cylinder by the inflammatory power of powder. Let a piston be substituted for the cannon ball, as was suggested by Huygens in 1680 and by Papin in 1690, and the charge of powder so reduced that when it is exploded the piston will not be thrown entirely out of the cylinder, another small explosive charge introduced on the other side of the piston to force it back, or let the cylinder be vertical and the piston be driven back by gravity, means provided to permit the escape of the gas after it has done its work, and means to keep the cylinder cool, and we have the prototype of the modern heat engines. The gunpowder experiments of Huygens and Papin were not successful, but they were the progenitors of similar inventions made two centuries thereafter.

Jan Baptista van Helmont, a Flemish physician (1577-1644), was the first to apply the term gas to the elastic fluids which resemble air in physical properties. Robert Boyle, the celebrated Irish scholar and scientist, and improver of the air pump, and Edwin Mariotte, the French physicist who was first to show that a feather and a coin will drop the same distance at the same time in a reservoir exhausted of air, were the independent discoverers of Boyle's and Mariotte's law of gases (1650-1676).

This was that at any given temperature of a gas which is at rest its volume varies inversely with the pressure put upon it. It follows from this law that the density and tension, and therefore the expansive force of a gas, are proportional to the compressing force to which it is subjected. It is said that Abbé Hauteville, the son of a baker of Orleans, about 1678 proposed to raise water by a powder motor; and that in 1682 he described a machine based on the principle of the circulation of the blood, produced by the alternate expansion and contraction of the heart.

The production of heat by concentrating the rays of the sun, and for burning objects had been known from the time of Archimedes, and been repeated from time to time.

Thus stood this art at the close of the 17th century, and thus it remained until near the close of the 18th,

In England Murdock, the Cornish Steam Engineer, was the first to make and use coal gas for illuminating purposes, which he did in 1792 and 1798. Its utilisation for other practical purposes was then suggested.

Gas engines as motive powers were first described in the English patent to John Barber, in 1791, and then in one issued to Robert Street in 1794. Barber proposed to introduce a stream of carbonated hydrogen gas through one port, and a quantity of air at another, and explode them against the piston. Street proposed to drive up the piston by the expansive force of a heated gas, and anticipated many modern ideas. Phillipe Lebon, a French engineer, in 1799 and in 1801 anticipated in a theoretical way many ideas since successfully reduced to practice. He proposed to use coal gas to drive a