On account of the fact that the magnetic poles of the Earth do not coincide with the geographical north and south poles, a compass needle seldom points exactly north and south, and the angle between the magnetic meridian and the geographical meridian is called the _declination_. The discovery that the declination varies in different parts of the world was made by Columbus in 1492. For purposes of navigation it is obviously very important that the declination at all points of the Earth’s surface should be known, and special magnetic maps are prepared in which all places having the same declination are joined by a line. It is an interesting fact that the Earth’s magnetism is subject to variation. The declination and the dip slowly change through long periods of years, and there are also slight annual and even daily variations. At one time magnets were credited with extraordinary effects upon the human body. Small doses of lodestone, ground to powder and mixed with water, were supposed to prolong life, and Paracelsus, a famous alchemist and physician, born in Switzerland in 1493, believed in the potency of lodestone ointment for wounds made with steel weapons. Baron Reichenbach, 1788–1860, believed that he had discovered the existence of a peculiar physical force closely connected with magnetism, and he gave this force the name _Od_. It was supposed to exist everywhere, and, like magnetism, to have two poles, positive and negative; the left side of the body being od-positive, and the right side od-negative. Certain individuals, known as “sensitives,” were said to be specially open to its influence. These people stated that they saw strange flickering lights at the poles of magnets, and that they experienced peculiar sensations when a magnet was passed over them. Some of them indeed were unable to sleep on the left side, because the north pole of the Earth, being od-negative, had a bad effect on the od-negative left side. The pretended revelations of these “sensitives” created a great stir at the time, but now nobody believes in the existence of _Od_. Professor Tyndall was once invited to a seance, with the object of convincing him of the genuineness of spiritualism. He sat beside a young lady who claimed to have spiritualistic powers, and his record of his conversation with her is amusing. The Reichenbach craze was in full swing at the time, and Tyndall asked if the lady could see any of the weird lights supposed to be visible to “sensitives.” “_Medium._--Oh yes; but I see the light around all bodies. _I._--Even in perfect darkness? _Medium._--Yes; I see luminous atmospheres round all people. The atmosphere which surrounds Mr. R. C. would fill this room with light. _I._--You are aware of the effects ascribed by Baron Reichenbach to magnets? _Medium._--Yes; but a magnet makes me terribly ill.