Illustrations of the Croton Aqueduct

by F. B. Tower

Composition. The composition of water is determined both by analysis and synthesis. If this liquid be submitted to the influence of a volcanic battery, it is decomposed into two gases, namely one volume of oxygen and two volumes of hydrogen. These gases, in the proportions just mentioned, may be made to recombine, and form water by heat, electricity, or spongy platinum, as water consists of one equivalent of hydrogen, 1 and one of oxygen, 8 = 9; and in volume, of one volume of hydrogen, and half a volume of oxygen, condensed into aqueous vapor or steam we can easily calculate the specific gravity of steam, for its density will be, .0689 (Sp. gr. of hydrogen) + .5512 (half the Sp. gr. of oxygen) = .6201.

Water as affected by the laws of Heat.

As the extensive and important functions which water discharges in the economy of nature, depend mainly on the manner in which it is affected by the laws of heat, a few remarks on this subject may not be inappropriate to this place.

Heat is communicated through water in a different manner, from that observed in relation to solids, for it is not conducted as in them, from one particle to another, but carried with the parts of the fluid by means of an intestine motion. Water expands and becomes lighter by heat, and therefore it is, that if the upper portion of water be cooled below the lower, the former descends, and the latter rises to take its place. Thus a constant counter-current is kept up, and the whole body of water has to cool down to near the freezing point, before congelation can take place. This equalization of temperature, moreover, takes place much more rapidly, than it would do in a solid body; hence alternations of heat and cold, as day and night, summer and winter, produce in water, inequalities of temperature much smaller than those which occur in a solid body.

Hence it is, that the ocean, which covers so large a portion of the earth’s surface, produces the effect of making the alternations of heat and cold much less violent than they would be if it were absent. The different temperatures of its upper and lower parts produce a current which draws the seas, and by means of the seas, the air, towards the mean temperature. This circulation is also carried on between distant tracts of the ocean; as we see in the case of the Gulf Stream, which rushing from the Gulf of Mexico across the Atlantic to the western shores of Europe, carries with it a portion of the heat of equatorial climes to the colder northern regions, and bringing back in return a portion of the cold from the same higher latitudes. Thus, large portions of the earth are rendered habitable to man, which, without the existence of such a law, would be doomed to perpetual frost and solitude. This influence of the ocean on temperature, explains satisfactorily some peculiarities in the climates of certain tracts and islands, for example, why London is cooler in summer, and hotter in winter than Paris. But though water expands by heat and contracts by cold, there is even a limit to this law, for had there not been, the lower parts of water would have frozen first, and thus entire lakes, rivers and oceans, perhaps, become solid, and had they become thus frozen, they would have remained so; for, as the heat at the surface would not have descended far through the colder parts, the main body of the ice must forever have remained solid, as in the arctic circle. To obviate this great disadvantage, water contracts by the increase of cold till we come near the freezing temperature, (40° F.) when it begins to expand and continues so to do till it freezes; at 32° F. Hence, water at 40° is at its greatest density and will lie at the bottom, with cooler water or ice floating above it. However much the surface be cooled, water colder than 40° cannot descend to displace water warmer than itself. Hence we never can have ice formed at the bottom of deep water, though it is not uncommon to find it thus situated, in shallow streams or rivers of rapid flow. Here the temperature of the whole body of water is brought down to the freezing point, and in freezing the ice adheres to the sides and bottom of the stream. What a beautiful provision is this, that the coldest water should rise to the surface, and there freeze and remain, exposed to the warmth of the sun-beams and the air, to be speedily dissolved upon the return of spring! This is owing to the well known fact, that in the act of freezing a still further expansion takes place, so that the specific gravity of ice is less than water of any temperature, and consequently floats upon the surface. We thus see that by the contraction of water by cold, the temperature of various times and places is equalized, though were that contraction without limit, a great portion of the earth would be bound in fetters of ice. Such a disastrous result, is prevented by the substitution of expansion for contraction, when the temperature is reduced to 40°, and the benevolent purposes of an all-wise Designer, are made still more manifest by the further expansion of water in the act of freezing. As water becomes ice by cold, it becomes steam by heat. We generally understand by steam the vapor of hot water, but steam or vapor rises from water at all temperatures, however low, and even from ice. The expansive force of this vapor increases rapidly as the heat increases, but yet in all cases the surface of water is covered with an atmosphere of aqueous vapor, the pressure, or tension of which is limited by the temperature of the water. If, therefore, the vapor is not confined, causing the surface of water to be pressed upon, evaporation will take place, and thus there must, according to this law, always exist an atmosphere of aqueous vapor, the tension of which may be compared with that of our common atmosphere. Now the pressure of the latter is measured by the barometrical column, about 30 inches of mercury, while that of watery vapor is equal to one inch of mercury at the constituent temperature of 80 degrees, and to one fifth of an inch at the temperature of 32 degrees.

If the atmosphere of air by which we are supported were annihilated, there would still remain, an atmosphere of aqueous vapor, arising from the waters and moist parts of the earth, but in the existing state of things this vapor rises in the atmosphere of dry air, and thus its distribution and effects are materially influenced by the vehicle in which it is thus carried.

The moisture thus floating at all times in the air, serves for the support of vegetable life, even in countries where rain seldom if ever falls. It is absorbed by the leaves of living plants, which thus increase in weight even when suspended in the atmosphere and disconnected with the soil. During intense heats, and when the soil is parched and dry, we see the life of plants thus preserved until the earth is again refreshed with showers, and the roots supplied with their wonted moisture.

Clouds, are produced when aqueous vapor returns to the state of water; and this process is called condensation. Whenever the temperature becomes lower than the constituent temperature, requisite for the maintenance of the vapory state, some of the vapor, or invisible steam, will be condensed, and become water. This may be seen illustrated in the condensation of the steam, as it issues from the spout of a tea-kettle. Clouds not only moderate the fervor of the sun, but they also check radiation from the earth, for we find that the coldest nights are those which occur under a cloudless winter sky. The use of clouds in the formation of rain, is too obvious to need pointing out more particularly. Snow is frozen vapour aggregated by a confused action of crystalline laws, and ice is water, solidified while in its fluid state, by the same crystalline forces. These are bad conductors of cold, and when the ground is covered with snow, or the surface of the soil, or if the water is frozen, the roots or bulbs of plants beneath are protected by the congealed water from the influence of the atmosphere, the temperature of which in northern winters, is usually very much below the freezing point; and this water becomes the first nourishment of the plant, in early spring. The expansion of water during its congelation, at which time its volume increases one twelfth, and its contraction in bulk during a thaw, tend to pulverize the soil, to separate its parts from each other, and to make it more permeable to the influence of the air.

When ice changes to water, or water to steam, although at an invariable degree of temperature, yet the change is not sudden, but gradual. When the heat reaches the point, at which thawing or boiling takes place, the temperature makes a stand; a portion of it disappears, or becomes latent, as it is called; thus the temperature of ice cannot be raised, till the whole is thawed, nor that of boiling water, till it has all been converted into steam; all the heat that is applied being absorbed in producing these changes. Were it not for this law of latent heat, thaw and evaporation would be instantaneous, we should be overwhelmed with floods, at the first glow of warmth in the spring, and in heating water the whole would flash instantaneously into steam upon reaching the boiling point.

It is through the same relations of water to heat, that springs are supplied—for these undoubtedly draw their principal supplies from rain. Mr. Dalton has calculated that the quantity of rain which falls in England is 36 inches a year. Of this he reckoned that 13 inches flow off to the sea by the rivers, and that the remaining 23 inches are raised again from the ground by evaporation. The 13 inches of water are of course supplied by evaporation from the sea, and are carried back to the land through the atmosphere. Vapor is perpetually rising from the ocean, and is condensed by cold in the hills and high lands, as is easily recognized by the mists and rains, which are frequent in such regions; whence it descends through their pores and crevices, till it is deflected, collected and conducted out to the sea, by some stratum or channel which is water-tight, thus keeping up a perpetual and compound circulation. In every country these two portions of the aqueous circulation have their regular and nearly constant proportion; and their due distribution appears to be necessary to its organic health, to the habits of vegetables and of man. This circulation goes on from year to year as regularly as that of the blood, in the veins and arteries of the human system, and though maintained by a very different machinery, is no less clearly adapted to its purposes. In short the properties of water which regard heat make one vast watering engine of the atmosphere, (Whewell.)