Experience is the best guide in estimating the amount of hay or grain that may be taken from a given field:  the produce of an acre is, of course, very different for different soils and climates.  In distributing the burdens to the several pack-horses and wagons employed in conveying the forage to the army, it is important for the foraging officers to know the relative weight and bulk of each article.

Ordinary pressed hay in this country will average about . 12 lbs. per cubic foot.  Wheat . . . weighs. . 60 lbs. per bushel.  Rye . . . . " . . . . 56 " " Maize or Indian corn . 56 " " Barley . . . " . . . . 50 " " Oats . . . . " . . . . 35 " " Meal, flour, and ground feed of all kinds, are purchased by the pound.

As it would be exceedingly dangerous to send forward the regular train of the army for the conveyance of forage collected by these foraging parties, the country wagons and pack-horses are usually pressed into service for this purpose.

Troops of horse are sometimes sent into the vicinity of meadows and grain-fields for temporary subsistence:  in such cases the horses and cattle may be farmed in the neighborhood, and the grass and grain issued in regular rations, immediately as taken from the field; but in no case should the animals be turned out to pasture.

In a country like ours, where large bodies of new and irregular forces are to be suddenly called into the field in case of war, it is important to establish very rigid rules in relation to forage and subsistence; otherwise the operations of such troops must be attended with great waste of public and private property, the want of means of subsistence, the consequent pillage of the inhabitants, and a general relaxation of discipline.  Regular troops are far less liable to such excesses than inexperienced and undisciplined forces.

Marches.—­Marches are of two kinds:  1st.  Route marches,—­2d.  Marches within reach of the enemy.  The former belong to the domain of strategy; the latter to that of tactics; both, however, are connected with logistics in every thing that concerns the means of their execution.

When an army is moving on a line of operations, it should be in as many columns as the facility of subsistence, celerity of movement, the nature of the roads, &c., may require.  Large columns cannot move with the same rapidity as smaller ones, nor can they be so readily subsisted.  But when an army is within striking distance of the enemy, concentration becomes more important than celerity, and the forces must be kept in mass, or at least within supporting distances of each other.  We find only two instances in the Seven Years’ War, in which Frederick attempted attacks by several columns at considerable distances from each other; and in both these instances (at Torgau and at Namiest, against Laudon, during the siege of Olmutz) he was unsuccessful.  His usual mode was to bring his columns near together as he approached the enemy, and to form his troops into line at the moment of attack.  Such was his order of march at Prague, Kollin, Rosbach, Leuthen, Zornsdorf, and Kunersdorf.  The following is one of Frederick’s orders respecting marches, (October 2d, 1760.)