With two fleets both moving, it could hardly happen that they could engage except when sailing on the wind, that is, with the wind on one side and not behind them. In that position an admiral had a much greater control over the movements of his squadron. Thus the formation which gradually came to be accepted as normal was the close-hauled line of battle. In order that each vessel, while retaining the power of striking at the enemy, was also to have the necessary freedom of movement, a space had to be left between them in which they could turn when occasion arose, and which would give to each the time to avoid the ship immediately ahead of her, if it, by any chance, became disabled. As it was desirable to employ the greatest number of men in fighting, and as few as might be aloft, while it was obviously convenient to diminish to the utmost the surface presented to an enemy's fire, it was a practice imposed by the conditions of sea battles to go into action with a diminished spread of sail. It was further necessary that every vessel should have the power of increasing her rate of speed. If one vessel was crippled, the next behind her had to push up and take her place, which could only be done where there were some immediate means of increasing the rate of speed. This margin was secured by employing a detachment of men to spill the wind out of one of the sails, so that it did not produce its whole effect in dragging the ship on. Spilling the wind meant the keeping one corner of the sail loose, so that it flapped and did not hold the wind. When the speed of the ship had to be increased, the sail was sheeted home, or, in the old phrase, they "let everything draw." It follows, from what has been said, that a fleet was always compelled to regulate its speed by that of the slowest vessel in the line. The great majority of battles fought by ships under sail were conducted very slowly. It was seldom that the line moved at more than two and a half or three miles an hour.