On the other hand, all bodies may be considered more or less magnetic. The oxygen of the air is highly so compared with other gases. If the air were deprived of its oxygen magnetic poles would attract each other more strongly. The early electricians divided all bodies into two classes, paramagnetic and diamagnetic. Iron and steel, nickel and cobalt, are paramagnetic bodies. Bismuth is an example of a diamagnetic body. A bar of bismuth will place itself at right angles to the line between a north and a south pole, instead of in this line, as a bar of iron or steel would do.

In the case of electricity and magnetism the medium between conductors of electricity or between magnets plays an important part in the phenomena of what is called induction-in other words, the phenomena which result from the stresses called forth in the medium by the electro-magnetic waves. In gravitation, the medium, however, does not affect the attraction between the bodies. A plate of iron or a plate of paraffin between two attracting spheres does not affect their pull upon each other. There is also no evidence of a directive action of gravitation—that is, an elongated cylinder does not tend to place its longer axis vertical on the earth's surface.

Maxwell shows that "if we assume that the medium is in a state of stress, consisting of tension along the lines of force and pressure in all directions at right angles to the lines of force, the tension and the pressure being equal in numerical value and proportional to the square of the intensity of the field at the given point, the observed electrostatic and electro-magnetic forces will be completely determined. . . . A path is now open by which we may trace to the action of a medium all forces which, like the electric and magnetic forces, vary

inversely as the square of the distance, and are attractive between bodies of different names, and repulsive between bodies of the same names."

Gravitation differs from electrical and magnetic attractions in that there is no difference of sign, no positive and negative. We must therefore assume a different kind of stress. "We must suppose that there is a pressure in the direction of the lines of force, combined with a tension in all directions at right angles to the lines of force. Such a stress would no doubt account for the observed effects of gravitation."

A

B

FIG. 4.

Prof. C. A. Bjerknes,* in a very suggestive paper on Hydrodynamical Analogies,

has shown that magnetic and electrical attractions and repulsions can be imitated by giving fluids a pulsating or oscillating movement. By the term pulsation is meant a periodical change of volume; by that of oscillation, a change of length. Bjerknes' pulsator consists of a little tambour or drum (A, Fig. 4), covered on both ends with membrane. The drum, A, is connected by rubber tubing to an alternating air pump, which communicates air pulsations to the drum. The oscillator, B, consists of a sphere supported on a little lever; this lever can be set in vibration by the alternating air pump, communication being obtained by means of a tube. The pul

* Repertorium der Physik, xix, 1883.

Thus, on

sators and the oscillators were mounted on suitable balance arms, and were immersed in fluids. working the alternating pump

pulsating or oscillatory movements could be communicated to these fluids.

When two pulsators are brought near each other and when the pulsations are in the same phase there is an attraction (Fig. 5). On the other hand, if the pulsations

occurs.

N

N

FIG. 5.

S

are discordant—that is, in opposite phases-repulsion This is the exact opposite to what takes place in magnetism where like poles repel and unlike attract. Experiments were next tried with a combination of pulsators and oscillators. If an oscillating sphere was brought near a pulsator, attraction or repulsion could be obtained-attraction, if the oscillating sphere approached the pulsator at the instant of its dilatation, or repulsion if the sphere was removed from the pulsator in the time of its dilatation. Here we obviously have the means of drawing many interesting analogies between the motion of fluids and the phenomena of electricity and magnetism. Bjerknes shows that the phenomena of induction, temporary and permanent magnetism, and the phenomenon of the magnetic field can be imitated by his hydrodynamic apparatus.

Although analogies drawn from the action of fluids are highly interesting, they remain at the present time in the subject of electro-magnetism merely analogies. Fluids are gross substances, and their molecular grouping and attractions are of such magnitude that we can not reason safely from actions in them to actions in a

highly attenuated fluid or medium, such as that which is supposed to exist in the space about electrical conductors and magnets. Wave motions in the ether are supposed to take place around the atoms or molecules of bodies. These wave motions are handicapped, so to speak, by the presence of such bodies and by the waves which the vibrations of the bodies impress on the ether. Bjerknes's experiments have led many philosophers to speculate upon the possibility that the pulsation or oscillatory movements of an ether might account for the phenomena of gravitation and of attraction or repulsion in general.

Lord Kelvin, in an address on terrestrial magnetism, remarks, "I find it unimaginable but that terrestrial magnetism is due to the greatness and rotation of the earth"; and says further in regard to the hypothesis that magnetic disturbances are caused by the sun acting as a variable magnet: "In eight hours of a not very severe magnetic storm as much work must have been done by the sun in sending magnetic waves out in all directions through space as he actually does in four months of his regular heat and light. This result, it seems to me, is absolutely conclusive against the supposition that terrestrial magnetic storms are due to magnetic action of the sun; or to any kind of dynamical action taking place within the sun, or in connection with hurricanes in his atmosphere, or anywhere near the sun outside. It seems as if we may also be forced to conclude that the supposed connection between magnetic storms and sun spots is unreal, and that the seeming agreement between the periods has been a mere coincidence.” *

*Popular Lectures and Addresses, by Sir William Thomson (Lord Kelvin).

CHAPTER IV.

THE ELECTRIC CURRENT.

A DISTINGUISHED American lecturer once delivered a brilliant course of lectures on the Lost Arts, in which he showed that the ancients possessed a knowledge of many processes now lost to the world, and he intimated that the nation that could transport immense obelisks over great distances, and build the pyramids, might also have had a knowledge of science far beyond what historical investigation has revealed to us. It may seem unsafe to assert positively that the ancients knew nothing of electricity save what is manifested by the rubbing of amber; for modern excavations are continually increasing our knowledge of ancient life. Not the slightest evidence, however, exists that a knowledge of the art of covering a wire uniformly with silk, cotton, or other nonmetallic or insulating substance existed be- . fore the time of Benjamin Franklin.

Without insulated wire no progress was possible in electricity, for the electro-magnet, by means of which we transmit telegraph messages, propel cars, and, in short, accomplish most of the modern wonderful results in electricity, consists merely of a spool of cotton- or silk-covered wire placed upon an iron core. The rapid development of electrical science during the past thirty years is largely due to the facility with which a piece of