Popular Science Monthly
��517
��would Ikuo been reduced to a mere cinder. If tlic amount of lieat received is to be measured, tills radiation must be checketl. A iieat Irap must be desij^ned. One of the earliest instruments made for that purpose was devised by the late Professor Samuel Pierpont Langley, of the Smithsonian Institution, somewhat on the lines of a gardener's greenhouse. His heat trap was simply a box provided with a double glass pane and [lacked with cotton to reduce loss of heat by radiation.
��successful if constructed on the principle of the gardener's greenhouse and Lang- ley's box. Mr. Frank Shuman has given us a type of solar power plant in which a thin film of water is heated in a cast-iron trough surrounded by window glass. So intense is the heat impounded by the double glass that the water is quickly raised to the boiling point (two hundred and twelve degrees Fahrenheit) or very near it.
After the water is brought to about
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���The water which is heated by the parabolic reflectors is stored in well-insulated tanks. A low-pressure steam-engine was designed by Mr. Shuman which would take this hot water and use it to drive a piston even though the pressure gained was only four pounds absolute
��The layer of air between the two sheets of glass served as a heat insulator, and the glass itself [prevented the heat which enlered the box from escaping. On Pike's Peak, where the thermometer recorded fifty-nine degrees Fahrenheit, the temperature in the box rose to two hundred and thirty-five degrees. Had he succeeded in trapping all the heat, which is practically impossible, he might have obtaineil enough to melt solder. Since Langkn's time, experiments con- ducted by Mr. C. C. Abbott of the Smithsonian Institution have given much better results.
These facts having long been known, it has occurred to more than one inventor that a solar power plant might prove
��the boiling point in the trough, it is con- veyed to a steel storage-tank in the in- ventions of Mr. Shuman. That tank is not simply an enlarged covered pot, but a vessel so constructed that as little heat as possible can escape from the water within. Just as we keep ourselves warm in winter by wearing clothes to prevent a too abundant radiation of our bodily warmth, so Mr. Shuman swathes his storage-tanks in an insulating material which keeps the water hot for many hours.
But how can an engine be driven with nothing but hot water? Mr. Shuman l)erforms the feat by the paradox of making the water boil without flame after he has stored it. Thus he generates
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