Page:Popular Science Monthly Volume 44.djvu/47

 The failure of the theory proposed in the case of the earth and moon is no less striking when applied to Mars, Jupiter, or any other planet. In every instance the position of the satellite assumed to afford permanent moonlight would be one of instability. This striking fact renders the oversight of Laplace the more remarkable. It may be stated, however, that by the arrangement of several moons about the same planet almost, if not entirely, perpetual moonlight might be possible. The system of Jupiter and his moons furnishes a clear illustration.

In conclusion, we have seen, then, that where one of the greatest mathematicians of all time suggested a change—a so-called improvement in the system of the world—the modification would have left us without tides, or, worse still, the earth in the system proposed would have lost control of her satellite, and we would not only have been deprived of moonlight, but also of the moon itself.



HE facility with which a high temperature may be obtained with electricity, and the heat controlled and located just where it is wanted, makes this agent peculiarly well adapted to the heating of metals for welding and forging purposes. This was early recognized by Prof. Elihu Thomson, to whom the development of the art is chiefly due, and who has devised a great variety of apparatus capable of performing all classes of work, from the simple welding of two wires to the making of large and complicated joints.

The principle involved is very simple. If a current be passed through a rod or wire, heat will be developed in it if the current be of sufficient volume. If this circuit, instead of being formed of a continuous conductor, be a broken one, such as would be furnished by two rods whose ends abut, the heat will be developed first at the surface of contact, as this is the point of greatest resistance, and then spread along the rods. And if, while the rods are in a heated condition, they be pressed together, they will become strongly united and form a perfect joint. On account of the radiation of heat from the surface and the cooling effect of the air, the rods become hotter at the center than at the surface, which is the reverse of what happens with a forge-heated bar, where the heating begins at the outside and gradually extends to the interior. This feature of the electric welding process has an important advantage in producing a firmer and more perfect joint, and 