476
The Trombe.
[Book Y.
may here be explained : in the first case the hollow hands closely encom-passed the mouth and received the warm air from his ehest; in the latter,his food was at a distance from his lips, and consequently the heat of hisbreath was absorbed by the surrounding air and that which was carriedalong with it to his soup.
A blast of wind directed over the surface of a placid pond or lake notonly ereates a current on the latter, but sometimes bears away part of thewater with it. A vessel sailing before the wind is aided in her course,though it may be but slightly, by the liquid current produced on theocean’s surface. Storms of wind long continued heap up the sea againstthe mouths of rivers, and cause them to overflqw their banks, while lowtides often result from the same agent driving the ocean away in oppositedirections. These effects of wind were observed in remote ages. “ Heraiseth the stormy wind which lifteth up the waves.” The river Jordanwas “ driven back ” by wind, so that “ all the Israelites passed over on dryground.” By its agency, a passage for the same people was openedthrough the Red Sea . “ And Moses stretched out his hand over the sea,and the Lord caused the sea to go back, by a strong east wind all thatnight, and made the sea dry land, and the waters were divided.” Exodusxiv. 21.
On the other hand, rivers and water-falls bear down immense quantitiesof air with them. Strata of this fluid on the surfaces of rapid streams ae-quire a velocity equal to that of the latter, and in some places aerial cur-rents thus produced are very sensible. At Niagara they are sufficient todrive mills or supply blasts for a long line of forges. In 1829, while as-cending the path on the Canadian side, in Order to pass under the grandchute, we entefed suddenly into one of those invisible currents under theTable Rock, and were nearly prostrated by it. It is the ascent of this airloaded with minute particles of water, (which are borne up by it in thesame manner that it is itself carried down) that contributes to the forma-tion of the solar and lunar rainbows seen at the great North American andother cataracts. Heavy rains bring down oceans of air, and in the showerbellows, or trombe, blasts of wind are produced on the same principle.Could we see the air brought down by heavy showers, we should beholdit rebounding from the earth, something like smoke when driven againsta wall or any other plane surface.
As the trombe illustrates this part of our subject,a figure of one may as well be given. The pipe Adischarges water from a reservoir into a funnel placedon the vertical tube C. The end of A terminatesin the funnel, and opposite to it is made a numberof openings in C, two of which are shown in the cut.The lower end of C enters the close vessel D, anddischarges its Contents on a stone placed directlyunder it. As the water from A passes down C, itdraws air along with it through the top of the funnel,and also through the holes in the upper part of C.As the liquid dashes against the stone, the air sepa-rates and rises to the top of the vessel, whence it isforced by successive volumes through B to the fire,while the water collects at the bottom and is let offby a regulating valve or cock. This machine it willbe perceived is a miniature imitation of some of na-ture’s operations ; for cascades, water-falls, and alsoheavy showers of rain, are all natural trombes.
No. 198. The Trombe.