NEWTON'S ELECTRICAL EXPERIMENTS.

445

The recorded electrical experiments made by Newton are few, and are separated by long intervals of time. The earliest one was made in 1675,' when he found that a telescope glass, a couple of inches in diameter, mounted in a ring so as to be held about a third of an inch above the table on which it was placed flatwise, would, when rubbed on its upper side, attract bits of paper, etc., lying beneath it; and that the paper would vibrate up and down between glass and table for some time after the rubbing ceased. The Royal Society, to which this was communicated, tried to repeat the experiment and failed. Newton then discovered that not only were better results secured by using a larger glass disposed barely a sixth of an inch distant from the table, but that the nature of the substance with which the glass was rubbed appeared to influence its excitation. This last seems to have impressed him, as well it might, for it was an entirely new observation. He says that he obtained twice as much excitement of the glass when he rubbed it with his gown as he got on rubbing it with a napkin; and he advises the Society not to use linen or soft woolen, but "stuff whose threads may rake the surface of the glass." The Society, curiously enough, obtained the best results by employing a "scrubbing brush made of short hogs' bristles," "the haft of a whalebone knife," and finally resorted to merely scraping the glass with the finger-nails. This experiment of Newton appears to be the first suggestion of the different effects attending the rubbing of the electric with dissimilar bodies, a subject which became of great importance through the subsequent brilliant research of Dufay.

The principal discovery in magnetism resulting from actual experiment which belongs to the early days of the Royal Society, is the first production of artificial magnets

1Horsley: Isaaci Newtoni, Opera. London, 1782, vol. iv., 373.

by Sellers in 1667.' It was of course old to magnetize iron needles by rubbing them with the lodestone; and that even a succession or chain of armatures could be rendered magnetic by induction from a single stone, both by actual contact and through simple location in the field, had been known for ages. Sellers, however, had been rubbing needles on the stone to find out the conditions under which they would become most strongly magnetized; and he made up his mind that the needle's strength or direction did not depend so much upon "fainter or stronger touches on the stone nor the multiplicity of strokes" as upon "the nature of the steel whereof the needle is made, and the temper that is given thereunto." So he tried all sorts of steel, and finding the magnetism apparently permanent in his needles, easily made the succeeding step-which was to regard the magnetized steel itself in the same light as the lodestone; or, in other words, as an artificial magnet which "shall take up a piece of iron of two ounces weight or more; and give also to a needle the virtue of conforming to the magnetic meridian without the help of a lodestone or anything. else that has received virtue therefrom."

As the century drew to its close, the growing commerce of England created an urgent demand for more definite knowledge concerning the variation of the compass. In 1580, William Burrowes determined the variation in London to be 11° 15' to the East. Edmund Gunter, the inventor of the scale and rule which bears his name, found that, in 1622, it had diminished some five degrees. Gellibrand, Gunter's successor in the Chair of Astronomy at Gresham College, observed that it had become reduced some two degrees more. In 1640, Henry Bond, a teacher of navigation in London, published his Seaman's Calendar, showing the progressive nature of this secular variation, and in 1668 issued a table predicting, though inaccurately, its changes in London for the next forty-eight

'Phil. Trans., No. 26, 478, 1667. Abridg., vol. i., 166.

years. But who actually discovered the secular variation is not certainly known. Bond attributes the honor to John Mair-other contemporary authority to Gellibrand, who at least has the preponderance of assent in his favor.'

The whole subject of compass variation, however, was thoroughly studied by Dr. Edmund Halley,' a mathematician and astronomer of great ability, who proposed the odd theory to account for it, that the earth has four magnetical poles, two near each geographical pole, and that the needle is governed by the pole to which it happens to be nearest. Unfortunately, however, the observed changes in the variation itself over certain periods of time interfered so greatly with this doctrine that it became evident to Halley that the notion of four fixed poles would not meet the observed conditions. Thereupon he evolved a still more striking supposition, to the effect that the earth really consists of two concentric magnetic shells, each having poles differently placed and not coincident with the geographical poles. Then as the poles on the inner shell "by a gradual and slow motion change their place in respect to the external, we may give a reasonable account of the four magnetic poles, as also of the changes in the needle's variations."

It is hard to believe that the imagination could exercise such control in the days of Newton. Yet the theory attracted considerable attention and had even great vitality, for in 1698, thirteen years after he had proposed it, Halley induced William III. to appoint him a captain in the Navy and give him command of a ship, in order to make long voyages for the express purpose of establishing the truth of his supposition. He made two voyages to various parts

1Dr. Wallis (Phil. Trans., 1702, No. 278, 1106), says that "at about the beginning of the reign of Charles I., Gellibrand caused the great concave dial in the Privy Garden at Whitehall, which is still remaining, to be erected in order to fix a true meridian line.

2 Phil. Trans., No. 28, p. 525, 1667; No. 148, p. 208, 1683; No. 195, p. 563, 1692.

of the Atlantic and Pacific Oceans, and came back not with the desired proof exactly, but with a useful chart exhibiting the variation of the needle in many parts of the world, and the general law of its phenomena1.

1 Brewster: Treatise on Magnetism. Edinburgh, 1836, p. 13.

NOTE. A curious illustration of the mixture of old and new ideas concerning magnetism which existed at the end of the seventeenth century is found in the title page of Dalance's "Traitté de l'Aiman," published in 1687, which is here reproduced in fac simile.

DALANCE'S TREATISE.

449

The lodestone, disposed in a bowl after the mode suggested by Neckam and Peregrinus, and marked with a longitudinal directing line, appears floating in front of the vessel, which the mariner, holding a rudder in one hand and a compass in the other, is about to board. The goddess, who appears to be advising him, points to the Great Bear, represented by the actual animal in the heavens, with the Pole Star situated at his tail, and also to a compass and a dipping needle, while in her left hand she has a sounding line. The idea evidently intended is that the divinity is advising the sailor to avail himself of all these means of guidance. There is also shown on the left a suspended armed lodestone, supporting at one pole a series of keys, and at the other a number of iron plates, this being possibly designed to indicate in some way the strength and consequent trustworthiness of the magnet.