Page:Popular Science Monthly Volume 37.djvu/438

424 a layer more soluble than the rest, along which the rock splits and peels. Compact rocks like granite are traversed by parallel joints on which the weather acts destructively; and the less compact clays, dried to powder, are blown by the wind and swept more rapidly into rivers, lakes, and the sea. The rain-streams, always running over the same channels, furrow the hill-side and wear it away still more rapidly; and a sheep-walk in a few years may become a deep ravine. The material carried down, when spread at lower levels, helps to form plains like meadows, and these in time may be cut through and partially carried away. Streams descending from peaty regions are charged with carbonic acid, and destructive to limestone. In all mountain regions the rivers are liable to enormous increase in volume from heavy rainfalls, when they carry off proportionately larger quantities of earth. The work of springs is like that of rain and rivers. The sink-holes in limestone regions carry the water down underground to do a similar work there; and this underground flow of water often helps in the production of landslips, especially when the ground is covered with bowlder-clay.

Teaching the "New Botany."—In the "New Botany," as described by Prof. W. J. Beal, in Garden and Forest, pupils are set to studying plants before books. Previous to the first lesson, "each pupil is furnished or told where to procure some specimen for study. If it is winter, and flowers or growing plants can not be had, give each a branch of a tree or shrub, which may be two feet long. The examination of these is made during the usual time for preparing lessons, and not while the class is before the teacher. For the first recitation each is to tell what he has discovered. The specimens are not in sight during the recitation. In learning the lesson, books are not used; for, if they are used, no books will contain a quarter of what the pupil can see for himself. If there is time, each member of the class is allowed a chance to mention anything not named by any of the rest. The pupils are not told what they can see for themselves. An effort is made to keep them working after something which they have not yet discovered. If two members disagree on any point, on the next day, after further study, they are requested to bring in all the proofs they can, to sustain their different conclusions. Give other specimens for the next lesson, keep reviewing, and generalize as details and facts accumulate. I like to give two species for careful comparison. . . . After a few weeks, reviews may be made in connection with chapters in some book. I make it a rule to give pupils specimens for study and comparison regarding every chapter in Gray's Structural Botany before the book-lesson is studied. I place no stress on making these investigations in the order in which the chapters of a text-book are arranged. Free use is made of our botanic garden, the crops in the vegetable-garden, fields, and experiment station, and the thickets along the river. Special topics are often assigned, in which each student has to go many times to observe and record observations on his growing plant." Illustrations by drawing are a prominent feature of the whole course.

Leaning Towers.—The leaning tower of Pisa is not the only building of its kind. There are many towers in northern Italy that deviate from the perpendicular, so that a writer has spoken of the country as "a land of towers staggering in all directions like tipsy men"; and there are in England few spires of any great altitude that are quite upright. The inclination of the Italian towers is a result of the character of the foundation soil, and of mistakes in building. The soil and subsoil of northern Italy down to the water-level are composed of rounded stones brought down by torrents and rivers from the Alps. A broad foundation is a primary condition of the stability of buildings erected upon it. The builders of the towers had classic models in their eyes, and did not contemplate the broadening of bases or the adding of buttresses to insure stability. Consulting appearances, and not venturing to depart from the conventional, they built straight up. The pressure concentrated on the narrow base was too much for the shifting stones beneath. They yielded at the weaker points, and the towers bent over. If the builders had minded the example of their Gothic neighbors and widened their bases, the load would have been more evenly