THE PERIOD OF APPLICATION. 51
drop. The fire being removed, the steam condenses, and a vacuum is formed below the piston, and the latch, E, being disengaged, the piston is driven down by the superincumbent atmosphere and raises the weight which has been, meantime, attached to a rope, L, passing from the piston-rod over pul- leys, TT. The machine had a cylinder two and a half inches in diameter, and raised 60 pounds once a minute; and Papin calculated that a machine of a little more than two feet diameter of cylinder and of four feet stroke would raise 8,000 pounds four feet per minute-i. e., that it would yield about one horse-power.
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 antici- pated 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 sur- rounded 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 fur- nace 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
