172 TEE CA USE OF MIR A GE. 



to the positive cloud. Hence in both instances high eleetric tension, or as 

 it is called, electric potentiality, prevails on every area of the earth's surface 

 covered centrally by either a high or low barometer. A careful consider- 

 ation of these facts at once suggests whether the electric condition of the 

 air in contact with and immediately over the intense electric point, is not 

 the cause both of the reflection of light where the objects are mirrored on 

 the sky, and of the aberration of light where objects below the horizon 

 loom up BO as t© seem above it. 



The mutual correlation, equivalence, and identity of the physical forces? 

 are well established, and with well-informed scientists are admitted facts. 

 But our knowledge as regards the behavior towards each Other and the ac- 

 tion between these forces is yet imperfect and in its infancy; but as far as 

 experiments have been made and observation has extended, the results 

 show that there is a mutual action and reaction between them; and that 

 one force will cause the influence of another to deviate from a straight line, 

 and forces it, measurably at least, to avoid collision between them. 



For illustration I might refer to the action of erystalt as regard light. 

 Crystals unquestionably are formed by electric action, the atoms of which 

 they are composed arranging themselves under the influence of opposite 

 polar forces. It is found that light is more readily and with less aberration 

 transmitted at right angles to the as^s of crystalization than along this axis. 

 When a crystal is suspended over a helix carrying an electric current, it 

 will place itself with the axis of crystalization longitudinally to the helix, 

 that is, across the coils of the helix, pointing in the direction that the helix 

 throws its magnetic force. If it be suspended over a straight wire carry- 

 ing such a current, it will place itself equatorially across the current, that 

 is, at right angles to it, again pointing in the direction the current throws 

 its magnetic force. These facts show that the crystal is affected by polar 

 forces exerting influences in opposite directions, and that light is less ob- 

 structed and less interfered with when it is transmitted at right angles to 

 the line along which these forces act, than along with or parallel to that 

 line. 



Herr Wiedmann's experiments show that crystals have greater con- 

 ductibility of electricity perpendicular to their axes than along those axes. 

 Electric currents always develop Magnetism at right angles, to the course 

 of the current. For illustration : suppose we draw figures of two circles, 

 and arrows along the line of the first circle showing that an electric cur- 

 rent flows along the circumference of the circle in the same direction that 

 we see the hands of a watch move when its face is turned towards us. Ar- 

 rows are likewise drawn around the second circle so as to show that the 

 current flows in the direction opposite to the movement of the hands of a 

 watch when its face is turned towards us. The electric current circulating 

 in the first figure will develop boreal or northern magnetism on the side we 

 are standing; and that of the second circle austral or southern magnetism 

 on the same side. In other words, when the progress of a circulating cur- 



