The inventor claimed that this new machine would be found useful in relieving mines from water, in throwing bombs, in ship-propulsion, attaching revolving paddles--i. e., paddle-wheels--to the sides of the vessel, which wheels were to be driven by several of his engines, in order to secure continuous motion, the piston-rods being fitted with racks which were to engage ratchet-wheels on the paddle-shafts. "The principal difficulty," he says, answering anticipated objections, "is that of making these large cylinders." In a reprint describing his invention, in 1695, Papin gives a description of a "newly-invented furnace," a kind of fire-box steam-boiler, in which the fire, completely surrounded by water, makes steam so rapidly that his engine could be driven at the rate of four strokes per minute by the steam supplied by it. Papin also proposed the use of a peculiar form of furnace with this engine, which, embodying as it does some suggestions that very probably have since been attributed to later inventors, deserves special notice. In this furnace, Papin proposed to burn his fuel on a grate within a furnace arranged with a _down-draught_, the air entering above the grate, passing _down_ through the fire, and from the ash-pit through a side flue to the chimney. In starting the fire, the coal was laid on the grate, covered with wood, and the latter was ignited, the flame, passing downward through the coal, igniting that in turn, and, as claimed by Papin, the combustion was complete, and the formation of smoke was entirely prevented. He states, in "Acta Eruditorum," that the heat was intense, the saving of fuel very great, and that the only difficulty was to find a refractory material which would withstand the high temperature attained. This is the first fire-box and flue boiler of which we have record. The experiment is supposed to have led Papin to suggest the use of a hot-blast, as practised by Neilson more than a century later, for reducing metals from their ores. Papin made another boiler having a flue winding through the water-space, and presenting a heating surface of nearly 80 square feet. The flue had a length of 24 feet, and was about 10 inches square. It is not stated what were the maximum pressures carried on these boilers; but it is known that Papin had used very high pressures in his digesters--probably between 1,200 and 1,500 pounds per square inch. In the year 1705, Leibnitz, then visiting England, had seen a Savery engine, and, on his return, described it to Papin, sending him a sketch of the machine. Papin read the letter and exhibited the sketch to the Landgrave of Hesse, and Charles at once urged him to endeavor to perfect his own machine, and to continue the researches which he had been intermittently pursuing since the earlier machine had been exhibited in public.