Page:Popular Science Monthly Volume 43.djvu/294

282 the great eccentricity of the orbit of Mars, and the consequent heavy fall it makes when plunging toward the sun. Situated farther from the sun than we are, Mars must be regarded as an older member of our system; and since it is smaller than the earth, it is only natural that its surface crust should be thicker than that of our planet. Granting this, then the internal pulp would not have such a power to compensate for the rapid fall as the earth does internally, for there would not be so much of it, so that an external compensation, assuming the crust to be too thick to alter its form, would have to take place at the surface. On the surface, of course, the water is the only power; therefore we should expect, to put it in Mr. Kingsmill's own words, "that the water in the ocean would be projected into the Martial hemispheres, and as the planet approached the sun, tides would sweep round the planet; that the canals should sometimes appear and>sometimes be duplicated. . . is only, a priori, what might be anticipated."

Factors of a River's Character.—Where a river shall go, what kind of a channel it will cut, how much work it will do, says A. P. Brigham, in his paper on Rivers and the Evolution of Geographic Forms, are matters determined, in an infinite number of ways, by the underlying strata. A river flowing on horizontally bedded rocks will tend to have, in its youth, a narrow cañon. Alternations of hard and soft strata give, in early stages of river life, alternations of rock benches and talus slopes; and many terrace-like horizons on the sides of the valley mantled commonly by soil, have this origin. Thus a terrace may be built up or carved out, and it may consist of alluvium, glacial rubbish, or bed rock. Tilted rocks give different types of river valleys in infinite variety. These types may be said to be just now beginning to attract a fair share of the interest of geographers and geologists. They will, in years to come, afford some of the most intricate as well as most fascinating problems which are open to inquiry.

The Critical Point in a Thunderstorm.—The belief that danger from lightning ceases as soon as the rain begins to fall heavily—expressed in the words of a mother reassuring her children, "Don't cry any more, God is sprinkling the earth with holy water"—prevails extensively among the Flemish peasants. Usually, according to M. P. J. De Ridder, lightning flashes from storm-clouds at the line between the heavy rain of large drops and the finer rain—or from the edge of the heavy rain. This is always the case in cumulo-nimbus storms, and as the number of storms of that kind exceeds all others, the belief of the peasants is at least worthy of attention. In nimbus storms, on the other hand, the critical point is at the latter end. In those of them which are developed in the veil of strato-cirrus, as when the sky is slowly covered, the rain falls at first without intensity and increases gradually, with distant thunder, while the storm itself does not seem to make much headway. But suddenly the rain falls more rapidly, and the dangerous moment has come. The roar of the thunder becomes terrible, the storm ceases, and the sky is cleared.

The Agaves.—The name of aloes is commonly given to plants of peculiar appearance which have long, fleshy leaves, with spines on their tips and sides; but this does not explain why the name has been given to species to which it does not belong, such as the agave, which do not resemble them. In Central America, their real country, where they have been cultivated on a large scale from the most remote times, they are called pitu, ozal, istle, metl, maguey, etc. Probably, soon after the discovery of America, a species of agave was introduced in the south of Europe, and became quite at home there. Linnæus, not realizing that all the agaves are American, gave it the specific name of Agave americana. Now there are more than a hundred species on horticulturists' catalogues, but many of these are only varieties. The uses to which these plants are found applicable are constantly increasing. In the United States and Europe they are only garden ornaments. In Mexico they hold the first place as wine plants and as textile plants. The filamentous substance obtained from their leaves is known all over the world as aloes fibers. These fibers, of length and thickness depending on the variety and locality, are so elastic and durable as to be in great demand for ropes, brushes, harness, and coarse woven