Page:Popular Science Monthly Volume 2.djvu/220

208 by the great power of the wind. The case therefore neither confirms nor disproves the Espian theory, and we may still believe the well-authenticated cases where, under favorable circumstances of very moist air and absence of wind, rain has been produced by large fires." Prof. Lapham also remarks, "The telegraph-wires indicated no unusual disturbance of the electrical condition of the atmosphere." Upon reading this last remark, the question occurs to us, Can there not be a change in the electrical state of the atmosphere which, although too small to manifest itself upon telegraph-wires, may occasion storms?

Some experiments made by the writer in the physical laboratory of Harvard College, on the influence of flames upon the electrical state of the air, may throw some light upon this subject. Two pieces of apparatus were used, one of them "the new quadrant electrometer" of Sir William Thomson, and the other a "water-dropper," also an invention of that distinguished philosopher. The electrometer is a very complicated piece of apparatus. Let me describe it in as clear a manner as possible. I do not know that I shall succeed in conveying to the uninitiated any idea of that instrument, for it has many parts. I shall endeavor merely to explain its principles roughly. Conceive of a light aluminum needle, suspended by two single cocoon threads in the centre of a glass jar, which is filled to nearly one-sixth of its capacity with strong sulphuric acid. A very fine platinum thread drops from the aluminum needle and dips in the acid. Let us see what we have now. An aluminum needle suspended in mid-air by two filaments of silk very near each other, and so fine that they can hardly be perceived by the naked eye. Further, this needle has an extremely fine metallic wire running down from it and terminating in a little weight, also of the metal platinum, which is immersed in the sulphuric acid. Thus we see that the needle is very free to swing in a horizontal plane, and it will be readily perceived that, if there were but one filament of silk supporting it, it might swing round a complete circumference, or indeed make many revolutions under the influence of a strong repellant or attractive force; the two filaments by their torsion allow the needle to swing only to a certain distance, and compel it to return to its original position when the force is removed. One can readily conceive of this by suspending a bar in an horizontal position by two vertical ropes, and then endeavoring to turn it in an horizontal plane.

Let us now charge the aluminum needle with positive electricity. To do this, we shall conduct into the sulphuric acid by means of a metallic wire a slight positive charge, and the acid, being a good conductor, will convey this charge by the extremely fine metallic wire to the aluminum needle suspended above the acid in mid-air. Now, if we present a substance charged with negative electricity to the needle, it will, as is well known, be speedily attracted; and if the substance presented has a positive charge it will be repelled: the unlike charges attracting and the like repelling each other. Thus, we see that we