No one knows how the speed and force of a falcon's stoop are gained. All we can say is that it is the fastest movement made by any living thing in the world. It is not flying, and it is not falling, but a combination of the two, with some other impulse which we do not understand. Mere weight must be at least a most important element, for a heavy hawk seems always to come down quicker as well as far more forcibly than one of the same species which is lighter. But weight is only one factor in the agglomeration of influences which make the stoop of the peregrine and the ger so swift. It must be seen to be believed in. There is no conceivable way of measuring its speed, but it is such that the momentum of it alone carries the hawk with half-closed wings right past a grouse at his best pace, making that pace appear absolutely slow by comparison. The descent from above is often made so that the hawk is at the end of it a few feet or yards behind the grouse, and nearly on a level with him. Hence the course of the pursuer bends forwards horizontally, but with such deviation from the straight line as is necessary to correspond with the flight of the pursued. It is so regulated that it may pass through that part of the air where the quarry is expected to be. Of course the expectation may be falsified. The hawk may suppose that the grouse will swerve to the right, whereas he may swerve to the left. But, just as a fine fencer will divine by some subtle skill whether his adversary is going to parry in _carte_ or _tierce_, or to make a single or double disengagement, so the good game-hawk judges from some slight movement or attitude where the grouse intends to be at the moment when she rushes past. This power is not so surprising in a haggard, but some eyesses seem to be instinctively gifted with a share of it. Others acquire it rapidly both in stooping at the lure and in their actual flights. But with eyesses it is rather the exception to be really good footers, whereas with haggards and many red passage hawks it is almost the rule.