Page:Popular Science Monthly Volume 59.djvu/359

Rh N. and S., remaining all the while at any place in the vessel of water in which it may be put. His words, though quaint, are exact: "It revolves on its iron center, and is not borne towards the rim of the vessel." He knew nothing of the mechanical couple in play; but he knew the fact; and with the instinct of a true philosopher, tested it in a variety of ways. With a most luminous insight into terrestrial magnetic phenomena, he observed that near the poles a compass needle tending, as it does, to dip greatly, must necessarily experience only a feeble horizontal directive force. To this he adds that 'at the poles there is no direction,' meaning thereby that a properly balanced compass needle

would remain indifferently in any azimuth in which it might be placed. We express the same by saying that the horizontal component of the earth's force vanishes at the poles.

Gilbert dwells at length on the inductive action of the earth. We have seen him hammering heated bars of iron and then allowing them to cool while lying in the magnetic meridian. He notes that they become magnetized, and does not fail to point out the polarity of each end. He likewise attributes to the influence of the earth the magnetic condition acquired by iron bars that have for a long time lain fixed in the north and south position as bars often are fixed in buildings and in windows, and he ingenuously adds: for great is the effect of