The field in which experiments are carried on is very different from that of a battle. Experiments, as detailed, sometimes turn out most fallacious when put to the use for which they are intended; and in no case is this more apparent than in breech-loading arms: thousands of rounds may be fired in a few days with great success; but extend that over twelve months, a certain number being fired every day, and the gun being cleaned after each day’s practice, and long before thousands are fired, the gun displays weak points--points which could not be discovered in the lesser experiment. So it is in practice: a gun left dirty for hours is undergoing rapid destruction; the unctuous deposit from gunpowder has such an affinity for iron that minute galvanic cells are formed on its surface in a very short time: half an hour after a gun has been discharged in a damp atmosphere these operations may be seen to be going on with rapidity, and an old gun on the hexagonal principle (if one should last long enough to grow old) would not be a very desirable weapon, in point of safety. The comparative cost of ammunition for the hexagonal rifle and the Enfield, is a point of no little importance. Calculation gives the former at something equivalent to 4-1/2_d._ or 5_d._ at each discharge, while the latter cannot exceed 1-1/4_d._, or at most 1-1/2_d._--a serious question for the Chancellor of the Exchequer. That this sum may be lessened by the employment of machinery is not unlikely; but this can only be done to a limited extent, it being essential that mathematical nicety, as well as the right degree of hardness, should be strictly observed, otherwise the power of penetration will be sacrificed: and of this property a great deal has been made. There are few who do not know that a pound hammer will soon drive to the head a fine-pointed slender nail; whilst a short, thick, stumpy nail requires three times the force: again, if fine steel polished nails were constructed, a still smaller amount of force would suffice. If such effects are carefully studied, much may be done with very little means.