But these proofs are only to test the strength of a barrel; the test of its accuracy is a much more delicate operation. Of course the machinery by which it is bored and rifled works with the most admirable precision; but yet it is necessary to put this machine-work to trial. There are, amongst others, two highly ingenious methods for doing this. In the one case it is placed on a stand which is so constructed that on it the barrel can be made to revolve rapidly. The barrel is pointed towards a window, and in front of it is a fixed sight. The workman looks through it while it is revolving; and if the sight remains steady to his eye, that is a proof that the barrel may be said to be straight. But there is yet another method. The mechanism of this testing apparatus is rather difficult to describe, but is something of this fashion. The barrel is made to revolve as before; but this time there is inserted in it a spindle, on which is fixed a short arm with a point which touches very lightly the interior of the barrel. If there is any inequality, or if the barrel is not perfectly straight, this short arm is of course shaken, and when this is the case, the motion is further communicated to a long arm at the end of which is an indicator, which is looked at by the workman through a magnifying glass. [Illustration: Gatling Gun on Field Carriage.] Barrel, stock, and lock being at last completed and tested, the rifle is put together; but even then it is subjected to one more trial. This is carried out on the proof-ground in the marshes, and takes the form of an actual discharge of the weapon at a target. The rifle is screwed to a fixed and firm support, and then a certain number of rounds are fired at ranges of five hundred and one thousand yards respectively. In this test the hitting of the centre of the target, or 'bull's-eye,' is not the end in view, as it is in ordinary target practice. That sort of shooting depends of course on the steadiness with which the marksman holds the rifle. In this case, however, the fixed _rest_ may be directed on any portion of the target, and the _grip_ will always be the same. The only object of the test is to see whether the rifle throws the bullet at each round on or near the same spot. A marker at the butt examines the position of each shot, and the smaller the space on which they strike, the better the weapon.