I have often noticed that gulls are able to follow a ship without any apparent exertion; they simply balance themselves on an ascending column of air, where they seem to be quite as much at ease as they would have been roosting on a solid support. If, however, they are driven out of this position, they generally commence at once to work their passage. If anything is thrown overboard which is too heavy for them to lift, the ship soon leaves them behind, and in order to catch up with it again they move their wings very much as other birds do; but when once established in the ascending column of air, they manage to keep up with the ship by doing little or no work. In a calm or head wind we find them directly aft of the ship; if the wind is from the port side they may always be found on the starboard quarter, and vice versâ.
One Sunday morning, while living at Kensington, I noticed some very curious atmospheric effects. The weather had been intensely cold for about a week, when suddenly the atmosphere became warm and very humid. The earth being much colder than the atmosphere, water was condensing on everything that it touched. I went to the bridge over the Serpentine in Hyde Park, and was not disappointed in finding a large number of sea-gulls waiting about the bridge to be fed. On all ordinary occasions these birds manage to move about with the expenditure of very little energy, but on this occasion every one of them, without a single exception, no matter in what position he might be, was working his passage like any other bird, just as I had expected. It is only on very rare occasions that the surface of the earth is sufficiently cold as relates to the atmosphere to prevent all upward currents of air.
Everyone who has passed a winter on the northern shores of the Mediterranean must have observed the cold wind which is generally called the mistral. One may be out driving, the sun may be shining brightly, and the air warm and balmy, when suddenly, without any apparent cause, one finds himself in a cold descending wind. This is the much-dreaded mistral, and if at sea it would be marked by a glassy line on the surface of the water. On land, however, there is nothing to render its presence visible. The ascending column of air is, of course, always very much warmer than the descending column, and this is taking place in a greater or lesser degree everywhere and at all times. A decided upward trend of air is often encountered by those who are experimenting with kites, the kite often mounting higher than can be accounted for on the hypothesis that the wind is moving in a horizontal direction. I have heard this discussed at considerable length. When a kite is flown in an upward current, it behaves in many respects like a soaring bird.
From the foregoing, I think, we may safely draw the following conclusions:—
First, that there is a constant interchange of air taking place, the cold air descending, spreading itself out over the surface of the earth, becoming warm, and ascending in other places.
Second, that the centres of the two columns are generally separated from each other by a distance which may be from 500 feet to 20 miles.
Third, that the centres of greatest action are not in spots, but in lines which may be approximately straight, but sometimes abound in many sinuosities.
Fourth, that this action is constantly taking place over both the sea and the land; that the soaring of birds, the phenomenon which has heretofore been so little understood, may be accounted for on the hypothesis that the bird seeks out an ascending column of air, and while sustaining itself at the same height in the air, without any muscular exertion, it is in reality falling at a considerable velocity through the air that surrounds it.
It has been supposed by some scientists that birds may take advantage of some vibratory or rolling action of the air. I find, however, from careful observation and experiment, that the motion of the wind is comparatively steady, and that the short vibratory or rolling action is always very near to the earth and is produced by the air flowing over hills, high buildings, trees, etc.
Tools and instruments used by mechanicians are very often made of the material most used in their profession; for instance, a blacksmith’s tools are generally of iron, a carpenter’s tools largely of wood, and a glass-blower uses many things made of glass, and so on. Mathematicians are no exception to this general rule, and seem to imagine that everything can be accomplished by pure mathematical formulæ.