ultimately prove beneficial, especially in places where they shall originate in the public exigences, rather than in the spirit of avaricious speculation.

Among the benefits which large towns may derive from establishing water-works, must be enumerated, their furnishing. ample means for cleansing all subterraneous drains, and therefore removing the annoyances to health and comfort which would otherwise result from them. The stupendous sewers of Rome have often been applauded for their utility; and those of London are more extensive, though less in magnitude. As early as the time of Henry VIII., the care and management of these structures was particularly provided for, by Act of Parliament. Probably such was the case long before; but subsequent Acts have confided their superintendance to a considerable number of commissioners, who are invested with powers to preserve them in good condition, by constructing and repairing any that may be necessary, and likewise to compel the inhabitants to pay the expense that may be incurred. Some of the sewers have a perpendicular diameter of five or six feet, so that the currents produced by rain, and the abundance of water which constantly flows into them, from the great supply of the different water-works, effectually cleanses them; yet as these operations are not commonly exposed to public view, their advantages are not duly appreciated.

This compendious and imperfect survey will, perhaps, exhibit the superiority of modern science and art, in a comparison with those of the ancients. The Roman aqueducts, the machinery of Egypt,-the Souterazi of Constantinople, and some other contrivances of former times, strongly excite our curiosity, and claim admiration; but how obviously inferior are they, in many respects to the ingenious inventions of a later period for similar pur

poses. Chemistry having disclosed by what means the potent agency of steam may be employed and regulated, for almost indefinitely augmenting mechanical force, the skilful application of this great elastic power, has facilitated the execution of plans for affording an exuberant supply of water to any city, whatever may be its extent, -the loftiness of its buildings, or the number of its population. By the aid of steam-engines every difficulty of situation is readily overcome; and thus the genius and skill, of a recent era, have in a great degree superseded the labour of ages,-tended to obviate the expenditure of millions, besides rendering the devices of our progenitors useless, for the purposes which originally occasioned their construction. Hence, the most important, useful, and admirable objects may be accomplished, without having recourse to the painful and degrading drudgery of slaves, as was heretofore the case in some countries. The toil of domestics is also diminished, by the waters of distant rivers, and fertile springs, being now readily and constantly conveyed into the interior parts of our dwellings,-even to our sleeping-rooms and fire-sides. Thus have science and art largely contributed to the exigences, comforts, conveniences and enjoyments of social life, though we may not have structures resembling either in form, or magnitude, the superb Aqueducts, Therma or Naumachia, of the Romans.

CHAPTER XVII.

Agency of Nature in accomplishing apparently simple but great purposes. Remarkable effects of Evaporation. Experiments of Saussure; Calculations of Bishop Watson and Dr. Halley. Estimate of Rain in different latitudes by M. Humbolt and Dr. John Dalton. Abundance of Water suspended in the atmosphere, when the sky appears very clear. Velocity and effects of Winds. Constantly varying modifications of the Earth's surface, but its equilibrium preserved. Difference in the properties of Water; causes of its being hard or soft. Rainwater. Rivers. Stagnant Water. Men prefer it transparent, but some Animals occasionally render it turbid before they drink. Practice in Italy of mixing Salt with it. Qualities of Water in its natural state. Statements of Dr. Henry and Dr. Turner. Transparent Water desirable for beverage, &c; mode of obtaining it in Turkey. Springs, natural filters. Importance of Filtration of Water procured from rivers.

SEVERAL very striking facts relating to the subject of water, exhibit the incessant agency of nature for accomplishing momentous purposes; whilst others favourably display the varied energies of human intellect, in forming contrivances to remove obstacles, and overcome difficulties, which, in peculiar cases, apparently interfere with the wants and comforts of mankind. The large quantities of water collected to supply Constantinople and Greenock corroborate this assertion; and may authorize a few remarks concerning the origin of those numerous rills, which generally fill their capacious reservoirs. Although it may escape the notice of an unreflecting observer, the source whence they may be traced is the evaporation constantly operating over the surface of our terraqueous globe. The atmospheric air seems to be

a principal agent in producing this effect, inasmuch as it contains such a portion of heat as not only converts the water into vapour, but also occasions the union of air with water, when both come into contact. A familiar instance illustrative of this fact frequently occurs after casual showers in windy weather; for the public streets and roads rapidly become dry, even in cloudy days, when the direct rays of the sun seems to have no particular influence.

Different philosophers have made a number of experiments to ascertain the capacity of atmospheric air for holding water in a state of solution. The results of those by M. Saussure led him to conclude that eleven grains of water would unite with a cubic foot of air; but in general the quantity varied from five to ten grains. Bishop Watson estimated the produce of evaporation from an acre of ground at more than 1600 gallons, during the space of twelve hours in a hot summer's day; besides his experiments for the purpose were made whilst the earth was apparently very dry, and when no rain had fallen for a considerable time.

Though the circumstances already mentioned show that abundance of moisture is constantly present in the atmosphere, yet there are others that demonstrate the wonderful extent of natural exhalation. Three-fourths of the surface of our habitable globe being occupied by water, and covering an area of about 145 millions of English square miles, the quantity evaporated from the Mediterranean Sea alone will afford a criterion for a reasonable conclusion on the subject. Some experiments made by Dr. Halley countenanced his inference, that one-tenth of an inch of the surface of that sea was daily raised in the form of vapour. Hence, he observes, that Every ten square inches of the surface of the water

yields in vapour per diem, a cube inch of water; and each square foot, half a wine pint; every space of four feet square, a gallon; a mile square, 6914 tons; a square degree,—suppose of sixty-nine English miles,— will evaporate thirty-three millions of tons: and if the Mediterranean be estimated at forty degrees long and four broad, allowances being made for the places where it is broader by those where it is narrower, there will be 160 degrees of sea; and consequently the whole Mediterranean must lose in vapour, in a summer's day, at least 5280 millions of tons. And this quantity of vapour, though very great, is as little as can be concluded from the experiment produced: and yet there remains another cause, which cannot be reduced to the rule,—namely, the winds, whereby the surface of the water is licked up, somewhat faster than it exhales by the heat of the sun, as it is well known to those that have considered those drying winds which sometimes blow.

"The Mediterranean receives these considerable rivers: the Iberus, the Rhone, the Tiber, the Po, the Danube, the Niester, the Borysthenes, the Tauris, and the Nile, all the rest being of no great note, and their quantity of water inconsiderable. We will suppose each of these nine rivers to bring down ten times as much water as the river Thames, not that any of them is so great in reality,--but to comprehend with them all the small rivulets that fall into the sea, which otherwise I know not how to allow for.

"To calculate the water of the Thames, I assume that at Kingston Bridge, where the flood never reaches, and the water always runs down, the breadth of the channel is a hundred yards, and its depth three, it being reduced to an equality, in both which suppositions I am sure I take to the utmost. Hence, the profile of the water in this place