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Natural Siphons—Pressure Engines — Vapor.

[Book V.

ceivable strength. These receive and transmit liquid columns whosehydrostatic pressure would shiver the strongest conduits made by man,while the volumes of water that play within and pass through them renderutterly insignificant all the products of artificial engines. We know thatrivers sometimes discharge themselves into subterraneous tubes, which,transporting the fluid to a distance, again vomits it up. In this mannerwater is often conveyed to places where its appearance is difficult toaccount for, because of the level of all the neighboring regions beingfar below the aperture of discharge—this being sometimes on the summitof mountains, and often at their sides.

But the transmission of water from one level to another through pipes,is one of the simplest operations in natural as it is in artificial hydraulics.The flexure of the tubes fabricated by nature convert some of them intosiphons, and these often decant the contents of caverns in which waterslowly accumulates. The liquid rises tili it flows over the highest bendin the tube, and the siphon being thus charged continues in Operation, likeone of ours, until the reservoir that supplies it be emptied, or the contentsreduced to a level with the external orifice of the discharging leg. Theaction then ceases until the cavern be again filled and the Operationrenewed. Hence intermitting springs, and some of those that ebb andflow.

Natural machines analogous to water-rams, pressure engines, and foun-tains of compression are doubtless also in Operation in the bowels of theearth. In the intricate and infinitely variegated chasms and fissuresthrough which water is falling and gases collecting, the principles of thesemachines must necessarily be often excited, and on scales of magnitudecalculated to strike us with awe. It is not improbable that some of thosehorrible eruptions mentioned in history and others that have occurred atsea without human witnesses were effected by machinery of this descrip-tion. The subaqueous eruption which occurred on the south-west coastof Italy , in 1831, was probably an example. A column of water, 800yards in circumference, was forced to an elevation of sixty feet, and anisland formed of the solid materials displaced.

But natural devices are not confined to such as raise liquids by the mo-mentum they acquire in flowing through tubes, or oscillating in waves,nor by the hydrostatic pressure of one volume transmitted by means ofairs to another. There are some in which water is raised by solar heat.The liquid is converted into steam or vapor, in which state it is renderedlighter than air, and consequently ascends. This may be considered asnature’s favorite plan. It is in Operation everywhere, and always. By itwater is drawn from every pari of the earth’s surface—both sea and land,and by it oceans of the liquid are kept suspended above us in the formof clouds, until it again returns in showers of rain and drifts of hail andsnow. Of the quantity thus elevated, we may form some rüde idea fromthe calculations of Halley respecting that drawn daily from the surface ofthe Mediterranean, viz. between five and six millions of tons ! a resultwhich he deduced from experiments. Every person knows that canalsrequire an extra supply of water to meet the expenses of evaporation.By experiments on the canal of Languedoc in France , the annual quan-tity thus borne off was found to be'nearly three feet in depth over itswhole area. Clouds of vapor or steam are often observed hanging overmarshy ground, until the wind rises and bears them away. In hot sea-sons copious Stearns may be seen ascending just after a shower; but ingeneral aqueous vapor thus generated, is invisible as it is impalpable.In clear weather, we are not sensible of its presence or of its movements.