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Page:Popular Science Monthly Volume 18.djvu/589

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it. It is probably a natural form of Leyden jar very highly charged. The light of a flash, Professor Tait states, is of very much greater intensity than is commonly supposed, the apparent brightness being, on account of the exceeding small duration, less than one hundred thousandth part of what it would be if the lightning were permanent. This duration is not more than the millionth part of a second, and hence it is a mistake to suppose that you can see the direction in which the discharge is passing, whether from the clouds to the earth or from the earth to the clouds. Professor Tait insists upon the necessity of properly pointing and grounding lightning-rods to secure safety, and mentions several instances of the ignorance in this matter which seems to prevail among even well-educated people. In one case the point of the rod was actually covered with a glass insulator. The best ground connection is with the water-mains, but large masses of metal or other conductor in a moist soil answer when these are not at hand. Connection with a body of water inclosed in a masonry basin, such as a reservoir, is not a proper way of grounding the rod. A lightning-rod acts as a constant drain upon the charge in the clouds in its neighborhood, and when the rods are numerous over any area, as in a well-protected town, storm clouds will pass over without any lightning flashes taking place, though such form of discharge will occur before reaching the protected district and after passing it. The lecturer insisted that people should regard the use of rods as both a public and private duty. In regard to the sources of electricity in the atmosphere. Professor Tait has been led by experimental researches to infer that a separation of the opposite electricities occurs simply by the contact of particles of air and aqueous vapor, which, on the kinetic theory of gases, are in constant collision. In the same way zinc and copper become oppositely electrified when brought into contact. The precipitation of vapor-particles into cloud-particles and the agglomeration of these into rain-drops enormously increase the electrification, as the potential of a free charged sphere is proportional directly to the quantity of electricity on it and inversely to its radius. The separation of these highly charged particles of air and water is effected by gravity and the diffusion of gases which would cause the air-particles to escape from among the mass of precipitated vapor to the less highly electrified air above. Sir William Thomson has offered an explanation of the phenomena, based upon the fact that the lower air is usually negatively charged. Ascending currents carrying this air upward, the electricity, which was formerly spread out over a large area, may by convection become so much less diffused that it will be raised to a high enough potential to give a spark. However the electrification of the precipitated vapor occurs, there is no question about the fact that, clouds once formed, the particles are electrified. The solution of the problem of just how this is brought about must in all probability come through experiments made on a larger scale than any so far conducted in the laboratory.

 

Science in the Schools of France.—The modifications in the course of studies in the French public schools, recently decreed by the Superior Council, give to scientific teaching a more prominent place than has hitherto been allowed it, especially in the elementary classes. In the seventh class, the elements of the natural history of animals and plants are added to the history of soils and stones, and take the preference over it, as offering more interest to children and being of greater practical utility. In the sixth class, an hour is deducted from the ten devoted to Latin and added to those given to the sciences, which are allowed four hours a week. In the fifth class, where scientific instruction has been obviously deficient, the hours for Latin are reduced to five, and the sciences are given four hours. In the fourth class an hour is taken from Greek, and the hours for scientific instruction are increased to four. Scientific instruction will be continued in the third, second, and rhetorical classes without encroaching upon the other courses, an hour being taken from the study-hours for new subjects of natural history in the third class, for physics in the second class, and for subjects of physics which have not been previously entered upon by the pupils, in the rhetorical class. The Superior Council advises that the teach-