Mr F. J. Stubbs makes some useful suggestions (50). He points out that Gätke and others imagine that a bird flying with the wind would suffer inconvenience through the wind ruffling up its feathers. It is surely evident that a bird supported in a moving medium could not progress at any slower rate than that medium; the bird is not flying on one stratum of air with another moving with a different speed immediately above it; it is actually in a current, not on it. If the bird flies at 20 miles an hour, and the wind or moving air is progressing at 10 miles an hour, the bird will cover a distance of 30 miles in one hour, though the force exerted by the bird is the force necessary for 20 miles in a calm. Conversely, if a 10-mile air current meets it, it will unconsciously be carried only 10 miles. If the speed of the bird is the same as the opposing force of the wind, it will remain stationary; I have seen ducks in a blizzard rise head to wind and fly rapidly, making no progress but maintaining their position over the water, to which they dropped again when the storm passed. Some black-headed gulls on the same water did not attempt this manoeuvre, and in a few seconds had vanished down wind. The swimmer, in a swiftly-flowing river, may hold himself in position so long as he can swim at the same rate as the stream he is contending with, but he cannot make headway if the speed of the water exceeds his. He may, however swiftly the stream moves, swim in any direction, but his actual progress will be down stream; if he aims to swim directly across, his real course will be diagonal. The fact that birds fly in any direction in a wind, and when at low elevation pay little heed to the direction of the wind, when the breeze is light, simply means that they can fly faster than the medium they are in; if the medium travels faster than they do, they will be carried in it to their advantage or disadvantage. Even in these days of upper-air investigation we really know very little about the speed, direction and steadiness of upper-air currents, but we do know that at a moderate elevation--some two or three thousand feet--the strength is usually greater than nearer the earth. Mr Abel Chapman, in "Wild Norway," makes this pertinent remark--"Except by aid derived from the operation of physical laws, the nature and extent of which are unknown to me, and by taking advantage of 'Trade-wind' circulation in the upper air, I believe that migration is impossible for short-winged forms of sedentary habits--but that aid, and those advantages, may facilitate, and perhaps vastly accelerate, a process which is otherwise impossible."