Suppose that the local influence which causes the up and down motion of the air should be sufficiently great to cause the air to rise at the rate of 2 miles an hour, and that the wind at the same time should be blowing at the rate of 10 miles an hour, the motion of the air would then be the resultant of these two velocities. In other words, it would be blowing up an incline of 1 in 5. Suppose, now, that a bird should be able to so adjust its wings that it advanced 5 miles in falling 1 mile through a perfectly calm atmosphere, it would then be able to sustain itself in an inclined wind, such as I have described, without any movement at all of its wings. If it were possible to adjust its wings in such a manner that it could advance 6 miles by falling through 1 mile of air, it would then be able to rise as relates to the earth while in reality falling as relates to the surrounding air. In conducting a series of experiments with artillery and small guns on a large and level plain just out of Madrid, I often observed the same phenomena, as relates to the wind, that I have already spoken of as having observed at sea, except that the lines marking the centre of an ascending or a descending column of air were not so stationary as they were over the water. It was not an uncommon thing, when adjusting the sights of a gun to fire at a target at a very long range, making due allowances for the wind, to have the wind change and blow in the opposite direction before the word of command was given to fire. While conducting these experiments, I often noticed the flight of eagles. On one occasion a pair of eagles came into sight on one side of the plain, passed directly over our heads, and disappeared on the opposite side. They were apparently always at the same height from the earth, and in soaring completely across the plain they never once moved their wings. These phenomena, I think, can only be accounted for on the hypothesis that these birds were able to feel out with their wings an ascending column of air, that the centre of this column of air was approximately a straight line running completely across the plain, that they found upward movement more than sufficient to sustain their weight in the air, and that whereas, as relates to the earth, they were not falling at all, they were in reality falling some 4 or 5 miles an hour in the air which supported them. Again, at Cadiz in Spain, when the wind was blowing in strongly from the sea, I observed that the sea-gulls always took advantage of an ascending column of air. As the wind rose to pass over the fortifications, the gulls selected a place where they would glide on the ascending current of air, keeping themselves always approximately in the same place without any apparent exertion. When, however, they left this ascending column, it was necessary for them to work their wings with great vigour until they again found the proper place to encounter a favourable current.