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neither moves, looks up, nor betrays any sign of your presence. The thermometer which he is watching is five feet in length. Only the top or marked portion extends above the cylinder, the tube which carries the mercury reaching down to the hot acids and nitroglycerin. In the cylinder has been placed about a ton and a half of sulphuric acid mixed with a ton of nitric. Into this mixture are now being sprayed 700 pounds of glycerin, the glycerin injector-pipe being joined by another carrying compressed air. As fast as the glycerin spray enters the mixture it seizes the nitrogen of the nitric acid and combines to nitroglycerin, and the sulphuric takes up the water which is thus set free. The process requires fifty-five minutes, during which the 700 pounds of glycerin becomes about 1,500 of nitroglycerin. Great heat is caused by the chemical action, and the absolute necessity is that the heat shall be kept down or it will explode the newly formed nitroglycerin. To this end the cylinder is surrounded by a water-jacket, through which cold water is rushing constantly, and four concentric coils of lead pipe occupy the interior of the cylinder, carrying four steady rushes of cold water.

If the heat, through vagaries in the glycerin, rose above the danger point, the thermometer would instantly reveal this to the man on watch. If the thermometer rose ever so little above twenty-two degrees centigrade, the man would turn on more air and shut off the inflow of glycerin. If it continued to rise slowly and he could not stop it by more air and water, he would give a warning shout, "Stand by," to a man watching below. If it continued, he would shout "Let her go," and the man would open a valve; this would sweep the whole charge down to a "drowning-tank" lower down the hill, which would drown the coming explosion in excess of water. The two men the meanwhile would bolt to a safe position behind banks. If the heat rose rap-