Page:Popular Science Monthly Volume 18.djvu/588

572 bill authorizing such representation, which will come up at the next session of Congress. One steamboat company and one railroad company have instituted a compulsory examination of all their men.-The State of Connecticut has adopted a law for the examination of railroad-men, with the following requirements for a certificate in the first class (engineers, firemen, and brakemen): 1. Healthy eyes and eyelids without habitual congestion or inflammation; 2. Unobstructed visual field; 3. Normal visual acuteness; 4. Freedom from color-blindness; 5. Entire absence of cataract or other progressive disease of the eye. For second class certificates (conductors, station-agents, switchmen, etc.) the first two conditions are the same, but in the third condition is required only "visual acuteness at least equal to three fifths without glasses and normal with glasses in one eye and at least one half in the other eye, with glasses," and in the fourth "freedom from color-blindness in one eye, and color-perception at least equal to three quarters in the other eye."

Form of the Lightning-Rod.—The subject of the proper form of lightning-conductors, long a disputed one among scientific men, has recently been experimentally investigated by Mr. W. H. Preece, with the result of confirming the position of Faraday, that the section of a rod is the essential element. The advocates of rods of large surface, such as ribbons, tubes, etc., among whom was the late Professor Henry, conclude, from the fact that static electricity resides upon the surface, that electricity of high tension, such as a lightning-discharge, is better conducted away by a large extent of surface. Mr. Preece stated that no direct experiments had, so far as he was aware, ever been made to settle the question, which was an important one, as the acceptance of the surface theory had led to the employment of unsightly and costly conductors, when a simple rod would answer all purposes. The experiments were made in the laboratory of Dr. Warren de la Rue, and had the advantage of his advice and assistance. In the first experiment copper conductors thirty feet in length, in the form of a solid rod, a thin tube, and ribbon, each of precisely the same mass, were used. The electricity was obtained from 3,240 chloride-of-silver cells, and accumulated in a condenser of a capacity of 42·8 micro-farads. The sudden discharge of this quantity of electricity produced results similar in character to lightning. It was capable of completely deflagrating 2 inches of platinum wire of .0125 inch diameter, and of raising to different degrees of incandescence greater lengths. Such wire, affixed to a white card so as to record the effect, was used to measure the discharge after it had passed through the conductor. Each form of conductor gave exactly the same result in the deflagration and heating of the platinum, showing that different extents of surface had no effect. As it might be thought that, in copper conductors of such length as those used, differences in conductivity could not be readily detected, the experiments were repeated with lead conductors, the resistances of which were twelve times that of copper, with the same results. An experiment, to determine how closely variations in the discharge could be estimated, showed that a change of resistance of five per cent, could have been easily detected. Mr. Preece, therefore, concludes that no more effective lightning-conductor than a simple rod or wire rope can be devised.

The Phenomena of Thunderstorms.—In a recent lecture at Glasgow, Professor Tait reviewed the present state of our knowledge of thunderstorms, and pointed out the chief conditions upon which the phenomena seemed to depend. The different degrees of conductivity of the air to which the zigzag form of the flash is due he thought might be produced by local electrification, which would have the same effect as heat in rarefying the air and making it a better conductor. Sheet-lightning is probably the reflection of a flash of forked lightning, itself invisible to the observer. Summer lightning is, in some cases, of a similar character, but in others, when the sky is clear, it seems to be due to discharges taking place in an upper strata of the atmosphere, the thunder being inaudible both on account of the distance and its originating in an atmosphere of but small density. The discharge in the form of a luminous ball is of rare occurrence, and but little is known of