The question of light presented a still greater difficulty. A candle exhausting the air too rapidly, Bushnell was obliged to find some less embarrassing substitute. He finally noted the points of the compass by two pieces of fox-fire wood--that is, wood that emitted a phosphoric light. The same method was used to determine the depth of the water. For this Bushnell constructed a peculiar contrivance, the secret of which is unknown, consisting of a glass tube, filled with water, in which a cork floated up and down. This cork was also covered with fox-fire, and by its rise and fall the operator could determine the depth to which his vessel was submerged. The boat was propelled back and forth by a paddle in front, shaped like the arm of a windmill. The operator turned this with a crank, and could go either forwards or backwards, as he desired. It was precisely the same principle as the modern screw, with the exception that it was placed in the bow instead of the stern of the boat. Another “oar,” identically the same, was arranged at right angles with the first, by means of which the vessel could be guided up or down. The progress of the boat was necessarily slow, but it is said that a strong man, with a favoring tide and current, could propel it three knots an hour. The rudder, also turned by a crank, could be used for sculling when desired. In the bottom of the vessel were two large water tanks, into which the water was let by a spring. It was by this means that the boat made its descent. There were two pumps, which the operator worked with his feet, for the expulsion of the water when he desired to rise. In case these failed to work, the two hundred pounds of lead on the bottom could be released, after which the rise was very sudden. The operator sat upright, with his head in the crown of the vessel, and by a dexterous use of his hands and feet, had little difficulty in completely mastering his boat. It required considerable practice, however, and unusual strength.