second and so on—a well-known experiment, made every da}-for elementary classes. Nor is this all, for he interposes between his lodestone and a mass of iron thick boards, walls of pottery and marble, and even metals, and he finds that there is naught so solid as to do away with this force or to check it, save a plate of iron. All that can be added to this pregnant observation is that the plate of iron must be very thick in order to carry all the lines of force due to the magnet, and thus completely screen the space beyond.
But Gilbert is astonishing when he goes on to make thick boxes of gold, glass and marble, and suspending his needle within them, declares with excusable enthusiasm that regardless of the box which imprisons
the magnet, it turns to its predestined points of north and south. He even constructs a box of iron, places his magnet within, observes its behavior, and concludes that it turns north and south, and would do so were 'it shut up in iron vaults sufficiently roomy.' Our experiments show that if the sides of the box are thin, the needle will experience the directive force of the earth; but if they are sufficiently thick—thick as the walls of an ordinary safe—the inside of such a box will be completely screened; none of the earth's magnetic lines will get into it so that the needle will remain indifferently in any position in which it is placed. A few years ago, the physical laboratory of St. Johns College, Oxford, was screened from the obtrusive lines of neighboring dynamos
