We have already discussed, under the general head of tactics, the position and use of artillery on the battle-field a few additional remarks must suffice. As a general rule, batteries should be placed in positions from which they can employ their fire to advantage, and also be free to move in any direction that the progress of the battle may require. Advantage should always be taken of natural or artificial obstacles, such as hedges, clumps of trees, logs, mounds of earth, &c., to cover and conceal the guns till the moment they open their fire. Elevated positions are, contrary to the common opinion, generally unfavorable, for artillery cannot fire to advantage at any considerable angle of depression. The slopes in front should be of considerable length, otherwise the balls would do very little execution upon that portion of the column of attack which occupied the valley. The ground should also be smooth, for if rough the balls will either bury themselves in the earth, or ricochet at a high angle of deflection, thus destroying a considerable part of the effect of the fire. The counterforts or spurs of hills are favorable for artillery, as they enable it to see, with an enfilading fire, the slopes of the principal range. Batteries should seldom be placed so as to fire over other troops, for they will not only be intimidated by this fire, but also exposed to the opposing fire of the enemy's artillery. A large number of pieces should never be crowded into the same place, but an interval should be left between the guns of forty or fifty feet, according to the locality. The most favorable position for this arm in ordinary ground, is in the intervals between the regiments or brigades of the line, and far enough in advance of this line not to draw upon the other troops the fire of the enemy's artillery. The flanks of the line are also favorable for the action of this arm. Sometimes artillery has been employed to form a part of the line of battle; but such instances are exceptions, and can never be comprised in general rules. Whenever this disposition has been made, it has resulted from the defective character of the other arms, or from some peculiar circumstance in the battle which enabled a bold and skilful commander to deviate from the ordinary rules of tactics. Such was the case with Napoleon at Wagram. In Saxony, in 1813, he was several times obliged to substitute his artillery to supply the want of other arms. In the defence and attack of field-works, and in the passage of rivers, artillery plays an important and indispensable part; but it here becomes an auxiliary to the dispositions of the engineers, or at least acts in concert with that arm. The troops of artillery, in all well-regulated army organizations, should equal about two-thirds of the cavalry, or one-seventh of the infantry.[36] [Footnote 36: To qualify himself for the duties connected with his arm of service, the artillery officer must make himself thoroughly acquainted with--.