Page:Popular Science Monthly Volume 23.djvu/66

56 of using the legs should be adopted by man as is displayed in the model swimming of that amphibian.

In analyzing the stroke of the frog, we notice that there is no vertical motion; the whole direction of the force is in a plane exactly horizontal, and is accomplished by virtually opening and closing the space between the knees—offering the sole of the foot as a resistance while kicking, and placing the feet in a position of least resistance while recovering.

In accomplishing the first of these conditions—the opening and closing of the space between the knees—the knees should be thrown out, and the contraction of the legs made slowly, in order to cause as little resistance as possible to the headway already attained.

It will be found that, if we alternate the stroke of the arms and legs by giving propulsion with one while recovering with the other, a more constant buoyancy will be attained, and, for long swims, it will be found far less fatiguing.



HE carboniferous formation represents the most wonderful episode in the history of our globe. It gives us an impression comparable in strangeness to that produced by those wonderful civilizations which blossomed out so suddenly and so splendidly in the infancy of mankind. Only a rare concurrence of circumstances could have brought on the expansion of plant-growth which characterized its epoch. The world of plants was still young and imperfect. Vegetation was characterized by the abundance of green parts susceptible of rapid growth, and of an almost indefinite development. It was, however, destitute of two characters which have been acquired by the later plants: those of the periodical and gradual increase of parts destined to endure, and of an absolute specialization of the reproductive apparatus. The vegetable kingdom was the first factor in the production of coal, but not the only one; and two other factors must be taken into account in studying its genesis. One of these related to the conditions of the environment, the climate, and the temperature; the other to the situations in which the plants that were converted into coal were placed. Had either of these conditions been essentially different or left out, we would have had no coal. The influence of situation is shown by the fact that the coal-beds are always intermittent; that they are limited in extent, and pass laterally into shales 