Page:Popular Science Monthly Volume 89.djvu/532

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��Popular Science Monthly

��steam which can lie used in an cnj^'ine of suitable desip;n. It must not he su|)- posed that he discovered the paradox, nor that he is the first to utilize it in a practical wa>'.

When Tyndall in one of his most bril- liant writings defined heat as "a mode of motion," he meant that the infinitesi- mal molecules of which all matter is composed are in a state of vibration. To understand his definition we must imag- ine the mole- cules of all bodies, even of so cold a mass as a block of ice, moving about at a high velocity. As soon as the temperature of the body is raised, its molecules vi- brate faster, collide with one another, and are made to move in longer paths. Thus the phe- nomenon of expansion under the inllueiice of heat is produced. When the tcm])era- ture is raised still liigher, so that the solid melts and becomes a licpiid, the molecules move in paths so very much greater that there is less common interference. Last- ly, when the licpiid is made to boil, many of the molecules are actually thrown off, and strike against the walls of the en- closing vessel, so violent is their mo\T- ment. The pressure of steam or ol aii\' confined gas, then, must be regarded as a plicnonienon due entirely to millions and niillidiis of blows struck b\' millions and millions of in\isil)le intiniiesimal mole- cules. If a thimbleful of boiling water were magnified to the size of a cathedral the steam within it might seem to a gigantic eye like myriads of bullets shot in all directions. Hecause countless Inillcts strike the walls of this huge thimble not singh', but at once in ver\' rapid succession the effect of stead\'
 * )ressure is produced. A single finger taj)

���Water was easily pumped for irrigating purposes in Egypt by means of Mr. Shuman's Sun Power Plant

��may not e\en move an open door. A billion simultaneous finger taps will shut it — shut it, moreover, as if it had been pressed by a hand.

At what temperature the molecules will fiy off from a boiling liquid depends entirely on the pressure to which the licjuid is subjected. The atmosphere weighs down on all earthly things with a pressure that amounts to about fifteen

pounds to the scpiare inch at thele\elofthc sea. If water is heated in the open air at sea level the flying mole- cules must be able to over- c o m e that pressure; otherwise the water does not boil. The tem- jierature at w h i c h the y can fly o(T at sea level, at which water, in other words can boil, is two hundred and twehe degrees Fahrenheit. On the top of a high mountain where theatmos- l^here jiresses down with less force bei-ause there is less ot it, the molecules will lly off much more readily than at the \c\q\ of the sea, with the result that water will boil much below two hundred and twehe degrees. If it were jjossible to remove the pressure of the atmosphere at sea le\el altogether, water could be made to boil ,u the temperature of an (irdinar\- room without heating it. That feat has actualK' been accomjilished in the laboratory' b>' puni|iing out the air in the water \essel.

What Mr. Shuman hastlone, therefore, is to remo\e part of the atmosphere's pressure from the hot water so that steam may be gcnera1i'<l. That steam he sup- plies to an enginr w liirh he has designed for the express purjjose of utilizing steam at low pressure, .'\fter doing its work the steam is condensed into water and is ])asse(l back to thegreenhouse-like healer.

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