I have often noticed in high winds that light and fleecy clouds come into view, say, about 2,000 feet above the surface of the earth, and pass rapidly and steadily by preserving their shape completely. This would certainly indicate that there is no rapid local disturbance in the air in their immediate vicinity, but that the whole mass of air in which these clouds are formed is practically travelling in the same direction and at the same velocity. Numerous aeronauts have also testified that, no matter how hard the wind may be blowing, the balloon is always practically in a dead calm, and if a piece of gold-leaf is thrown overboard, even in a gale, the gold-leaf and the balloon never part company in a horizontal direction, though they may in a vertical direction. Birds may be divided into two classes. First, the soaring birds, which practically live upon the wing, and, by some very delicate sense of touch, are able to feel the exact condition of the air. Many fish which live near the top of the water are greatly distressed by sinking too deeply, while others which live at great depths are almost instantly killed by being raised to the surface. The swim-bladder of a fish is in reality a delicate barometer provided with sensitive nerves which enable the fish to feel whether it is sinking or rising in the water. With the surface fish, if the pressure becomes too great, it involuntarily exerts itself to rise nearer the surface and so diminish the pressure, and I have no doubt that the air cells, which are known to be very numerous and to abound throughout the bodies of birds, are so sensitive as to enable soaring birds to know at once whether they are in an ascending or a descending column of air. The other class of birds consists of those which only employ their wings occasionally for the purpose of taking them rapidly from one place to another. Such birds do not expend their power so economically as the soaring birds. They do not pass much of their time in the air, but what time they are on the wing they put forth an immense amount of power and fly very rapidly, generally in a straight line, taking no advantage of air currents. Partridges, pheasants, wild ducks, geese, and some birds of passage may be taken as types of this kind. This class of birds has relatively small wings, and carries about two and a half times as much weight per square foot of surface as soaring birds do.