Now, having briefly sketched his general manner, let us return and examine the details. In the first place, he usually rises slightly, with outstretched wings, as if about to soar at the moment of commencing hovering. The planes of the wings are then inclined, and meet the air. At the instant of stopping, the tail is depressed. It appears reasonable to conjecture that the slight soaring is to assist the tail in checking his onward course, and to gain a balance. Immediately the wings beat rapidly, somewhat as they do in ordinary flight but with a more forward motion, and somewhat as birds do when about to perch on an awkward ledge, as a swallow at an incomplete nest under an eave. The wings look more, in front, as if attached to his neck. In an exaggerated way ducks beat the air like this, with no intention of rising at all, merely to stretch their wings. The duck raises himself as he stands on the ground, stretches himself to his full height, and flaps his wings horizontally. The kestrel's wings strike downwards and a very little forwards, for his natural tendency is to slip forwards, and the object of slightly reversing his vanes is to prevent this and yet at the same time to support him. His shape is such that if he were rigid with outstretched wings he would glide ahead, just as a ship in a calm slowly forges ahead because of her lines, which are drawn for forward motion. The kestrel's object is to prevent his slip forwards, and the tail alone will not do it. It is necessary for him to "stroke" the air in order to keep up at all; because the moment he pauses gravitation exercises a force much greater than when he glides. While hovering there are several forces balanced: first, the original impetus onwards; secondly, that of the depressed tail dragging and stopping that onward course; thirdly, that of the wing beating downwards; and fourthly, that of the wing a very little reversed beating forwards, like backing water with a scull. When used in the ordinary way the shape of the wing causes it to exert a downward and a backward pressure. His slip is when he loses balance: it is most obviously a loss of balance; he quite oscillates sometimes when it occurs; and now and then I have seen a kestrel unable to catch himself, and obliged to proceed some distance before he could hover again. Occasionally, in the slip he loses a foot or so of elevation, but not always. While actually hovering, his altitude does not vary an inch. All and each of these movements and the considerations to which they give rise show conclusively that the act of hovering is nothing more or less than an act of balancing; and when he has his balance he will rest a moment with outstretched wings kept still. He uses his wings with just sufficient force neither to rise nor fall, and prevents progress by a slightly different stroke.