. The mastery of water. comes gradually cooler until it freezes over. Thecrust of ice protects the lower layers from the cold airabove, so that the water of a frozen pond is nearlyalways above freezing-point, and fish can live in it untilthe ice melts again. CHAPTER XXXII. Safety Valves and the Steam Engine. The changes in volume which have been described,however, are trivial beside that which occurs when wateris converted into steam. By experiments which cannotbe described here, it has been found that a cubic footof water at the boiling-point produces 1,648 cubic feetof steam. If the vessel i
. The mastery of water. comes gradually cooler until it freezes over. Thecrust of ice protects the lower layers from the cold airabove, so that the water of a frozen pond is nearlyalways above freezing-point, and fish can live in it untilthe ice melts again. CHAPTER XXXII. Safety Valves and the Steam Engine. The changes in volume which have been described,however, are trivial beside that which occurs when wateris converted into steam. By experiments which cannotbe described here, it has been found that a cubic footof water at the boiling-point produces 1,648 cubic feetof steam. If the vessel in. which it is boiled is closed. 192 THE MASTERY OF WATER. and no means of escape for this steam is provided, thepressure rises until it may burst the boiler. For thatreason every boiler for producing steam under pressureis fitted with a safety valve. This is simply a valvewhich is held down by a weight or a spring, so thatwhen the pressure in the boiler reaches a certain amount,it rises and permits the steam to
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