94 ANNUAL OF SCIENTIFIC DISCOVERY. 



rubbed against each other, are different in character, and if traces of 

 electricity do appear on rubbing together substances of similar kind, it 

 is owing to their being really in some way modified, either at the be- 

 ginning or while the friction is taking place. Chemical modifications 

 in this connection exert a much stronger influence than changes de- 

 pending on mechanical or physical causes. 



The discharging force which makes its appearance at the point of 

 contact between two conductors, and which is known as " electro-motory 

 power," although first observed in conductors, is not wholly confined 

 to them. On the contrary, it shows itself with equal constancy, wherever 

 two bodies, be they conductors or not, but of different kind, come into 

 contact, and it causes on one of these bodies the discharge of positive, 

 and on the other that of negative electricity. In the moment of the 

 separation of these bodies the fluida formed at their points of contact 

 are set free in the form of electricity. The direction taken by the 

 discharge is always the same for the same bodies, be they rubbed 

 against each other or only brought into contact. The fluida, which are 

 kept separated during actual contact, in the case of conductors and 

 while they are thus separated, may be carried off in opposite directions ; 

 on this principle depends the circulation of electricity in the Voltaic 

 pile. Bad conductors offer resistance to this carrying away of the 

 fluida ; on the other hand, they favor the accumulation of frictional elec- 

 tricity. The process of friction itself not only multiplies the points of 

 contact, but aids their process of separation to penetrate still deeper. 

 It, however, does not influence the direction of the discharge ; this de- 

 pends upon the electro-motory force which again corresponds with or 

 depends upon the difference between 'the bodies so treated. 



THE NATURE OF THE FORCES PRODUCING THE GREATER MAG- 



. NETIC DISTURBANCES. 



The following are the chief points of discourse on the above subject, 

 delivered before the Royal Institution, London, by Prof. Balfour Stew- 

 art, of the New Observatory : When a bar of steel is magnetized, it 

 has acquired a tendency to assume a definite relation to our earth. 

 Nothing in science is more mysterious than the cause of this. The 

 earth, like a great magnet, acts upon a magnetized needle with nearly 

 a directive force. At the present moment, a mariner's compass-needle 

 points in a direction of about twenty-one and a half degrees west of 

 true north, termed a declination to that extent, and at the same time 

 dips downwards, making an angle of about sixty-eight degrees with the 

 horizon. This declination and dip vary with time and place. But 

 there are .other changes which the magnet experiences when kept sus- 

 pended in the same place. 1. The secular change, viz., during a great 

 many years. 2. The annual variation. 3. The daily variation ; and 

 4, a change due to the moon. In addition to these are those curious 

 and unaccountable changes termed magnetic disturbances, or storms. 

 Atmospheric storms, even the greatest, are only local phenomena ; but 

 magnetic storms are cosmical, as has been shown by Gauss and Sabine, 

 and occur almost at the same moment all over the world. Hence 

 many colonial observatories have been established. Mr. Stewart having 

 explained the methods of observation, and referred to diagrams giving 

 results, showed that these magnetic disturbances are connected with 



