Page:Encyclopædia Britannica, Ninth Edition, v. 2.djvu/241

Rh AQUEDUCT 223 dimensions, and united together into a continued series so closely as to prevent the escape of the water even under a violent pressure, arising from the altitude of the fountain- head. They enable us, therefore, to take advantage of and give effect to the fundamental principle of hydrostatics, that the fluid tends continually to a level, even though it be confined in the smallest or most complicated system of pipes ; so that however low it be carried in any valley, or to whatever distance, still it will rise on the opposite side to the original altitude of the fountainhead a principle which is most important indeed in such works, seeing that by it we are not restricted, as the Romans Avere, almost to a perfect level in the line of the conduit. We have seen that, for the purpose of attaining this level or very gentle declivity all along the conduit, they were under the necessity of raising it by arcades continued in one unbroken series, frequently 30 or 40 miles in extent ; and, in addition to this, they had often to prolong the length of the track by a circuitous route, turning and winding for miles out of its course, for the very purpose of increasing its length. But the use of pipes enables us to dispense with these long arcades all raised nearly to the same level with the fountainhead ; because the conduit may be varied in its level to any extent, and still will rise at last to its original altitude. The pipes, therefore, are merely laid all along the surface of the ground, with a cover of 2 or 3 feet of soil to place them beyond the reach of frost. To prevent, however, the frequent or abrupt alternations of rise and fall, any sudden inequalities in the ground are equalised by cuttings and embankments, but not to anything like the extent that would be required to raise the whole to a level. This, therefore, forms a very great improvement in the method of conducting water, the greatest indeed which has ever been made in this important branch of practical mechanics. That it was not introduced by the Romans, is not to be ascribed, as many have supposed, to their ignor ance of the hydrostatic principle that the fluid would rise to a level in the opposite branches of the same train of pipes. They were well acquainted with this principle, and a portion of a leaden pipe, supposed to have been used in the baths of Caracalla, has been found, which sets this matter at rest ; but, from the low state of the arts at that period, they were unable to give effect to the principle. They had not the means of fabricating pipes of such a magnitude as would have been required for the enormous quantities of water consumed in Rome, and at the same time, of strength sufficient to withstand the pressure from the fountainhead. Lead was the only material that could be used by them for the purpose ; and besides the enormous thickness that so weak a material would have reqmred, and the imprac ticability of their forming pipes, and uniting them together endwise, they were too well acquainted with the tendency of lead to render the water unwholesome by its poisonous impregnation. The use of cast iron was quite unknown. There remained, therefore, no resource but in the aque ducts, which, though attended no doubt with vast expense, and requiring great enterprise, skill, and patience, were yet attainable by these means, and formed when completed a simple and very perfect mode of effecting the object. Now, however, when the manufacture of cast iron has been brought to such perfection, and methods contrived for uniting perfectly together all the pipes into one con nected train, this improved system has been universally adopted. In former editions of this work (under this heading) the works of the Edinburgh Water Company, undertaken in 1819, and designed by Mr Jardine, civil engineer, were described ; but as there is little which can properly come under the popular definition of Aqueduct, they will be more consistently referred to in the article WATERWORKS. Croton Aqiieduct, New York. The Croton Aqueduct, by which the city of New York is supplied with water, was justly regarded at the time of its execution, from 1837 to 1842, as one of the most magnificent works of the kind in modern times (figs. 1 to 5) Fid. 1. Part Section of Croton Waterworks. Its length from the Croton Lake to the receiving reservoir is 38^ miles. The original reservoir, called Croton Lake, is formed by an embankment across the Croton Creek, a small stream of wholesome water falling into the Hudson. It covered 400 acres, and contained about 500,000,000 gallons of water. To the valley of the Har lem River, a distance of 33 miles, the aqueduct (fig. 2,) is built of stone, brick, and cement, arched over and under, 6 feet 3 inches wide at the bottom, 7 feet 8 inches at the top, and 8 feet 5 inches high, and capable of discharging 60,000,000 gallons per day. It is earned over the Harlem Valley in iron pipes, laid upon a magnificent bridge (figs. 3 and 4) 1460 feet long, constructed of arches 114 feet above FIG. 2. Tunnel in soft ground. BRIDGE OVER HARLEM RIVER Scale for Blerdlion* of JBridgf Fig. 3. high-water mark at Yorkville. These pipes passed into a receiving reservoir capable of holding 150,000,000 gallons, and from thence the water was conducted for 2| miles to a distributing reservoir, containing 20,000,000 gallons, by a double line of iron pipes, 3 feet in diameter. From this reservoir the water was distributed to the city. The Croton water was originally introduced into New York in 1842, whrp the population was about 450000 persons, and from that time to 1848, 18,000,000 gallons per day gave an abundant supply. In 1872, with a popu lation of 1,000,000 persons, the quantity required was 88,000,000 gallons per day. Thus, between 1848 and 1872, while the population little more than doubled, the consumption of water increased nearly five times. This fact, coupled with severe droughts in 1870 and 1871, when the natural volume of the Croton River fell to 27,000,000