"We do not even yet know what electricity is, but we know a great deal about the way it behaves and the effects it produces. There are two kinds of electricity, which we call _positive_ and _negative_. A body is said to be _charged_ when it has an excess of either kind, and the two kinds have a tendency to unite and neutralize each other's effects. Thunderclouds become heavily charged with electricity. We are not quite sure how this happens, but it is now commonly believed that the strong uprising currents of air that occur in the storm, in the process of breaking up the water-drops in the cloud also separate positive from negative electricity; leaving the former in excess in the part of the cloud next to the earth, and carrying the latter far aloft. "By a process called 'induction' the positive charge in the cloud draws an excess of negative electricity to the surface of the ground underneath. The stronger the contrast between these opposite charges, the harder they try to break through the interposing barrier of the air (which is a poor conductor of electricity) and to neutralize each other. At length they succeed in doing so. A powerful stream of electricity flows for an instant between cloud and earth. Its passage heats the air and makes it luminous--just as the passage of an electric current heats the filament of an electric lamp and makes it luminous. This is lightning. "These discharges occur not only between the clouds and the earth, but also, and probably more often, between clouds charged with opposite kinds of electricity. "The sudden expansion of the heated air along the path of the discharge affects our ears just as does the sudden expansion of the air at the mouth of a gun when it is fired. In each case a wave is sent through the air in all directions from the place of disturbance, and our ear-drums are set in vibration. That is thunder." * * * * * Take courage then, you timid ones, who wince in the lightning's flash and tremble under the thunder's roll. Thunder is simply a vibration of your ear drums--and, when you hear the thunder, be assured, all danger is over. [Signature: W. D. Moffat] EDITOR THE MENTOR ASSOCIATION ESTABLISHED FOR THE DEVELOPMENT OF A POPULAR INTEREST IN ART, LITERATURE, SCIENCE, HISTORY, NATURE, AND TRAVEL THE PLAN OF THE ASSOCIATION The purpose of The Mentor Association is to give its members, in an interesting and attractive way the information in various fields of knowledge which everybody wants and ought to have. The information is imparted by interesting reading matter, prepared under the direction of leading authorities, and by beautiful pictures, reproduced by the most highly perfected modern processes.