A gun-barrel mounted on a carriage with wheels, and moving on a perfectly horizontal railway, is placed at right angles to another short railway, at any convenient distance (suppose fifty feet, or yards); on the second railway a light carriage moves freely with any desired velocity, being drawn forward by means of a weight and pulleys: a cord is attached to the front of this carriage, which passes over a pulley at the end of the railroad, and is continued up a high pole or staff over another pulley at the top, at which end the weight is attached. A long rectangular frame covered with paper is fixed perpendicularly on the carriage, so that when it moves forward it passes across the direct line of the barrel, and forms a long target. A percussion lock is attached to the barrel, which is fired by a detent, or hair-trigger, and the wire which pulls it is disengaged at the same instant to admit of recoil. This wire is carried straight on to the target railroad, and fixed to a small lever, against which the front part of the target-carriage strikes as it is carried onwards by the weight. This constitutes the whole apparatus. When required to be used, the barrel is loaded with gunpowder accurately weighed, and a brass ball that fits the bore correctly: the weight is then disengaged, and the target moves quickly along, discharging the barrel as it passes, and the ball goes through it. With the same powder tried at the same time, the ball constantly goes through the same hole, or breaks into it. If the next powder tried be slower of ignition than the preceding, the ball will pass through another part of the target more in the rear; if quicker, more in advance; thus affording a means of ascertaining this important quality of gunpowder with considerable accuracy: the velocity of the target-carriage can be easily regulated by increasing or diminishing the weight which draws it forward. The differences in the distances between which the balls strike the target with different kinds of powder was frequently as much as ten or twelve inches; but it is not an apparatus commonly used, having been merely constructed for experimental purposes. ON THE TIME REQUIRED FOR IGNITION OF GUNPOWDER.