Page:Popular Science Monthly Volume 20.djvu/526

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be accomplished by receiving a spot of light, which is reflected from the mirror attached to the needle, upon a cylinder covered with sensitized paper. This cylinder is made to slowly revolve by clock-work, so that twenty-four-hour observations can be obtained. Nothing is so characteristic of modern methods in physical science as continuous registration of physical phenomena. Early observers were forced to content themselves with scattered observations taken at different intervals, and important variations might occur at the times when no observer was watching the apparatus. Now the movements of the human heart can be recorded by a little apparatus which will combine in one curve a thousand continuous observations, and the slightest anomalous fluctuations can be studied from the record which is obtained.

Provided, therefore, with these modern and more refined means of observation, we are in a condition to study the fluctuations of that subtile manifestation of energy which is ever present in the air, and seems to lurk in all matter. The influence of the electrical state of the air upon atmospheric changes is doubtless far-reaching. We are accustomed to think of thunder-storms as the only manifestations of atmospheric electricity; but there are influences which it exerts, more silent than those which are announced by the crackle of lightning, yet none the less extended. A pretty experiment, described by Lord Rayleigh, illustrates the effect of electricity upon the coalescence of rain-drops. A narrow stream of water is allowed to issue from a reservoir and form a parabola, which strikes the ground in drops about four feet from the orifice. When this is a fine stream, it breaks up into a shower of drops at about two feet from the orifice. On rubbing a bit of sealing-wax with a cloth, and presenting the rod of sealing-wax near the stream, the drops immediately cease to separate, and the stream is continuous from the orifice to the point where it strikes the ground. This is a very striking experiment, and undoubtedly has a bearing upon the formation of large drops of rain, which are often noticed during a thunder-storm after a discharge of lightning. Instead of a rod of sealing-wax, a piece of writing-paper can be readily charged by placing it upon a table and rubbing it vigorously with the palm of the hand.

None of the theories which endeavor to explain the source of the electricity of the air are satisfactory. Professor Tait, in a recent lecture, is of the opinion that evaporation is the source. Under the influence of the sun's heat, this operation of nature is conducted on an immense scale, and seems to him to be competent to furnish the supply of atmospheric electricity.

It is maintained by others that the atmosphere of the earth has a permanent charge, which it received in the beginning, or which was developed by cosmical changes; and this charge manifests itself here and there, and fluctuates under different conditions of heat and moist-