Liquids seek their own Level.

175

and a lower one must rise, until just balanced by those around; that is, until all become alike. Besides, just as a ball rolls down a slope or inclined plane, so all the particles of a fluid glide down among each other till each occupies the lowest possible situation.

This explains why the elevation and depression of a liquid surface, called a wave, continues to rise and fall, or to oscillate, for some time with gradually diminishing force. A column raised above the general level, as it cannot be supported, must sink; but in sinking, like a falling pendulum, it acquires momentum which carries it as far below the general level. Pressed up again, and acquiring new momentum in its rise, it has once more to fall, and so this alternation continues, until the lateral sliding of the particles, and the friction among them, gradually destroy the oscillation.

307. The surface of a liquid is always level when at rest-but only when at rest. If we pour some oil over water in a tumbler, the separating surface is perfectly flat and we may pour some spirits over the oil, yet each will kccp separate and sharply defined when the whole is once tranquil. On the other hand, the slightest breath disturbs the perfect smoothness of a lake, so that it no longer mirrors the distant landscape in harmony, but sends to the eye a confusion of images. So the hollow shape given, by stirring, to the surface of tea in a cup, subsides into the perfect level as the motion

ceases.

The law of fluid level may be also stated as follows :

308. "If various tubes and vessels communicate with one another, water admitted into any one of them will rise to the same level in all."

Fig. 79 represents a variety of tubes and vessels, opening into the box, G. Water poured

into any one would fill the box, and would then rise to the same level in all, so that if it stand at a in the first, and at ƒ in the last vessel, all the surfaces between will form with a fa horizontal line. The reason of this has been

already given in Art. 301,

where it was stated that the pressure depends not on the shape of

the vessel, but only on the vertical height of liquid.

176

Illustrations of Liquid Level.

If a tube twenty miles long, and rising and descending among the inequalities of a country, were filled with water, and could have its ends brought together for comparison, it would exhibit two liquid surfaces having precisely the same level; and on either end being raised, the fluid would sink in it, and cause an overflow from the other.

309. Many important phenomena find an explanation in this apparently simple statement.

An easy mode of determining the horizontal or level line at any spot is to have an open tube, bent up at

its ends, a and b (fig. 80), and nearly filled with liquid. By then looking along the two liquid surfaces, or through floating sights resting on them, an observer can

Fig. 80.

tell whether one or more objects are in the same horizontal line with a b, or not.

If there were two lakes at different levels on adjoining hills, a pipe connecting them through the valley would soon bring them to the same level; and if the bottom of one were above the bank of the other, it would empty the upper lake into the lower.

It was at one time supposed that the Dead Sea in Palestine had a subterranean communication with the Mediterranean, from which it is about fifty miles distant; but apart from the fact that the water of the Dead Sea contains a much greater proportion of salt than the water of the inland sea, the difference of level is altogether inconsistent with this supposition. According to the measurement made by Lieutenant Symonds in 1841, the surface of the Dead Sea is 1312 feet below the surface of the Mediterranean. Mr. Moore found no bottom in it at the great depth of 2220 feet. Hence it follows that the basin of the Dead Sea represents an enormous chasm in the earth upwards of 3500 feet in depth, below the level of the Mediterranean on the Syrian coast.

A projector once thought to solve the problem of perpetual motion by using a vessel shaped as in fig. 81. He reasoned thus: A pound of water in the goblet, a, must more than counterbalance an ounce which the tube, b, will contain, and must therefore be constantly pushing the ounce forward into a again, and keeping up a circulation, which will

Fig. 81.

cease only when the water dries up-a result easily preventable. He forgot that a common teapot is nearly such a vessel, and yet does not overflow.

A glass tube, running down the outside of a cask or cistern, and connected with it at the bottom, shows at once the level of the mass of liquid within. By a similar contrivance the engineer sees the level of water in the boiler of his engine, and thus knows how to regulate its supply.

So a chemist is thus able to determine the precise quantity of gas contained in a gas-holder when the gas has been collected over

water.

In like manner a pipe brought from a river into a neighbouring cellar or pit, will indicate the height of the water in the river.

A gigantic illustration of the principle that "water always seeks its level," is seen in the ramifying system of pipes by which water is now distributed through all large towns. Brought or pumped up to an elevated site near the town, it rises by the mere effect of its perfect mobility and of gravity to every cistern not above the level of the reservoir, however tortuous its course may be.

We are not to suppose that it was ignorance of this law of liquids, that led the ancients to construct those enormous aqueducts, some of which are scarcely inferior in magnitude to the great wall of China or the Egyptian pyramids. The want of a suitable material such as iron imposed on them the necessity of all this enormous labour; just as the invention of printing was delayed not by the want of the idea so much as of the material means of making impressions readily.

On the possession and knowledge of the qualities of this tough and workable substance which we call iron depends the health and wealth of imperial London. Like the arterial and venous circulation in the animal body, is the supply of pure water and the drainage of impurities through all the districts of our huge metropolis; although we are now so habituated to the fact, that we do not think on how little turns our superiority to our less favoured ancestors.

310. "Levelling."

In the cutting of canals, in the making of railways, and in many other engineering operations, it is of essential importance to have a delicate means of determining the level or horizontal direction at any place. For this purpose engineers and builders use the spirit

178

Uses of the Spirit-level.

level (fig. 82); it is a tube of glass like a c, containing spirit of wine and a small bubble of air, b. The tube is slightly convex or

a

b

Fig. 82.

curved on the upper side, so that, when it is laid on a horizontal surface, the bubble, rising to the highest part, stands at the centre of the tube, which the maker of the instrument has marked with a slight scratch. If the surface on which it rests deviates ever so little from the horizontal, the centre of the tube is no longer its highest part, and the bubble instantly moves towards the higher end. Spirit of wine is used in preference to pure water, because it is more limpid, and has less adhesion to the glass than water, and thus furnishes greater delicacy of movement. Such a tube properly fixed in a frame, with a telescope attached to it, becomes the engineer's guide in his most important operations.

A perfectly level surface on the earth really means one in which every point is equidistant from the centre of the earth. It is therefore truly a spherical surface like that of the earth; but so large is the sphere, that if a slice of it two miles in diameter were cut off and laid down on a true plane, the centre of the slice would be only four inches higher than the edges. Any small portion of it may therefore be practically regarded as a perfect plane.

Thus a hoop surrounding the earth would bend away from a perfectly straight horizontal line four inches in the first mile. In cutting a level canal, therefore, which may be considered as part of such a hoop, there must be everywhere a falling from the straight line called a tangent, in the proportion now described. All rivers also must have this curvature, and a little more, to produce the running motion.

A very slight declivity from the level suffices to give the running motion to water. Three inches per mile, in a smooth straight channel, gives a velocity of about three miles per hour. The Ganges, which gathers the waters of the lofty Himalayas, is, at eighteen hundred miles from its mouth, only eight hundred feet above the level of the sea—that is, about twice the height of St. Paul's; and to fall gradually these eight hundred feet, in its long course, the water takes nearly a month. The gigantic Rio de la Plata has so gentle a descent to the ocean, that in Paraguay, fifteen hundred miles from its mouth, large ships arrive which have sailed against the current all the way, by the force of the wind alone. On the gently inclined plane of the stream, they have been gradually lifted by the soft wind,

even against the current, to an elevation greater than that of our loftiest spires.

311. "Canals."

When the difference of level between two places is very considerable, and the ease and convenience of water-intercourse are desired, recourse is had to the construction of canals, divided into portions at different levels like the steps of a stair. Boats are raised or lowered from one level to another by the contrivance called a lock, which is merely a portion of the canal, of sufficient length for the boat to lie in, provided with high walls, and with flood-gates at both ends. When the gates below are shut, and water is admitted from above, the lock becomes part of the high level, ready as such to receive a boat, or to deliver one: and when the upper flood-gates are shut, and the water is gradually allowed to escape from below, the lock becomes part of the low level, and a boat may enter it or leave it by its lower gates. The rising at these gates varies from six to twelve feet.

The cutting of canals is one of the great items in the mass of modern improvement, which both mark and hasten the progress of civilization. To show their importance as facilitating intercourse, we need only say here, that a horse which can draw but one ton on our best roads, can draw thirty with the same speed in a canalboat.

One of the grandest works of this description is the Isthmus of Suez maritime canal, which was commenced in April, 1864, and completed in November, 1869, chiefly under the direction of a French engineer, M. de Lesseps. It is not quite a hundred miles in length, extending from the new harbour of Port Said on the Pelusian coast of the Mediterranean to the Port of Suez at the head of the Red Sea. Its construction was attended with enormous engineering difficulties, and is said to have cost about sixteen millions sterling. There are no locks, but in some parts it traverses sandy deposits and in others high rocky ground. Its depth throughout is 26 feet, and its width 246 feet at the base, and 328 feet at the top of the banks. By means of this canal large vessels can now pass directly from sea to sea. The project of another great ship canal across the Isthmus of Panama has been lately revived; but the difficulties n joining the Atlantic with the Pacific Ocean are likely to be greater than those which were encountered in connecting the Mediterranean with the Red Sea.