Page:Popular Science Monthly Volume 35.djvu/741

Rh the original constitution of the matter which was drawn together to form our planet. The character of all inorganic substances, as of all living creatures, is only consistent with the actual constitution and proportion of the various substances of which the earth is composed. Other proportions than those present in the constituents of the atmosphere would have required a different organization in all air-breathing animals, and probably in all plants. Any considerable difference in the quantity of water, either in the sea or distributed as vapor, must have involved corresponding changes in the constitution of living creatures.

The Medium of Electro-magnetic Action.—It was decided by experiment, during 1888, according to Prof. G. F. Fitzgerald, in the British Association, that electro-magnetic action takes place, not at a distance, but through an intervening medium. The experiments were made by Hertz in Germany, who observed the interference of electromagnetic waves quite analogous to those of light, and proved that electro-magnetic actions are propagated in air with the velocity of light. "By a beautiful device Hertz has produced rapidly alternating currents of such frequency that their wave-length is only about two metres. I may pause for a minute to call your attention to what that means. If they vibrated three hundred thousand times a second, the waves would be each a kilometre long. This rate of vibration is much higher than the highest audible note, and yet the waves are much too long to be manageable. We want a vibration about a thousand times as fast again, with waves about a metre long. Hertz produced such vibrations, vibrating more than a hundred million times a second." While this rate is too slow for visibility or light, and the vibrations are also inaudible, the experimenter was able to detect them by resonance. He constructed a circuit whose period of vibration for electric currents was the same as that of his generating vibrator, and "was able to see sparks, due to the induced vibration, leaping across a small air-space in this resonant circuit." By this combination—of a vibrating generating circuit with a resonant receiving circuit—which the author had recommended at the Southport meeting of the Association to be used for this very investigation, Hertz was able to observe the interference between waves incident on a wall and the reflected waves. The phenomenon is the same as what are known as Lloyd's bands, in optics, which are due to the interference between a direct and a reflected wave. "It follows, hence, that just as Young's and Fresnel's researches on the interference of light prove the undulatory theory of optics, so Hertz's experiment proves the ethereal theory of electro-magnetism. It is a splendid result. Henceforth I hope no learner will fail to be impressed with the theory—hypothesis no longer—that electro-magnetic actions are due to a medium pervading all space, and that it is the same medium as the one by which light is conducted."

Washing Men and Children by Machinery.—One of the latest inventions in sanitation is machinery for personal washing. A French colonel, according to Mr. Edwin Chadwick, ascertained that he could wash his men with, tepid water for a centime, or one tenth of a penny a head, soap included. The man undresses, steps into a tray of water, and soaps himself, when a jet of tepid water is played upon him. He then dries and dresses himself in five minutes, against twenty minutes in the bath, and with five gallons of water against seventy in the usual bath. In Germany they have an arrangement under which half a million of soldiers are regularly washed. By an adaptation of apparatus to the use of schools, a child may be completely washed in three minutes.

Modern Deterioration of Eye-sight.—Dr. R. Brudenell Carter, when questioned about the causes of modern deterioration of eyesight, replied that the circumstances of civilization are unfavorable to the cultivation of eye-sight. We are not as dependent on keenness of vision as our ancestors were. Much of the work of dwellers in towns is done upon objects close to them, from which they obtain large retinal images, whence they become comparatively insensible to small ones. They often work by defective light, and are thus driven to approach the object still more closely; and it is by such approximation that the malformation which produces short sight is mainly brought about. The increase