664 MAGNETISM AND THE COMPASS. 



meridian. By the Electric Fluid we produce Light, Heat, and the other 

 phenomena ; and the magnetic needle is a measurer, by its deflections, of 

 the most minute portions of Galvanism. From the magnet a spark can 

 be produced, absolutely similar in appearance and efl^ect to that of 

 Electricity and Galvanism ; and there is a positive and negative, or North 

 and South, pole to the magnet, and these attract or repel each other. 



There is one phenomenon connected with these sciences of very great 

 importance in practice, and that is, that of induction. A substance 

 electrified positively will induce a state of negative electricity, or will 

 cause a body that is within its influence to be negatively electrified ; the 

 North pole of a magnet will induce an opposite pole in that of another 

 piece of iron, in certain positions with respect to the magnetic meridian 

 and itself Thus, the iron employed in the construction of a ship, or 

 contained in its cargo, may all become, by induction, temporary magnets, 

 and have a most marked and important effect upon the Compass by 

 which it is steered ; and it is this cause which is too frequently overlooked 

 — that of Local Deviation — which has caused enormous errors in reckon- 

 ing, and consequently the loss of many vessels. As scientific detail is out 

 of our province, we must refer the reader to those works more expressly 

 treating on the subject. 



Terrestrial Magnetism. — The cause of the Magnetism of the earth, by 

 which the Direction, the Dip, and the Intensity of the Force of the 

 magnetic needle is controlled, is still involved in obscuritj', and no per- 

 fectly satisfactory system or theory has hitherto been framed to account 

 for the multifarious changes and phenomena of the Compass needle. 

 Among the more modern enquirers into the source of this most wonderful 

 principle, we may mention Professor Hansteen, Mr. Bain, Mr. Barlow, 

 Mr. Christie, Sir Edward Sabine, Mr. Gauss, Capt. Johnston, Mr. Archibald 

 Smith, Captain Evans, E.N., and many others. From their labours we 

 have arrived at a tolerably correct notion of the general effects of magnetic 

 phenomena ; and from these the laws by which they are governed have, 

 in some measure, been deduced. 



Now, the most reasonable supposition is, that the earth itself is a 

 magnet, or that magnetic currents exist on its surface in certain direc- 

 tions, causing the various deflections of the needle ; whether this Mag- 

 netism is induced from the sun, or other source, or whether the earth is 

 in a positive and permanently magnetic state, does not affect the present 

 question. From certain changes in the Compass, perhaps it might be 

 inferred, that the Magnetism is induced by temperature (heat) from the 

 sun ; or that the ferruginous matters, which enter so largely into the 

 composition of the earth, have received an inductive Magnetism from the 

 same source. In 1839, Gauss, among other important conclusions derived 

 from his researches, considered that the agents producing the magnetic 

 ■force of the earth are situated exclusively in the interior of the earth, and 

 that they are not situated in external space. 



In 1683 Dr. Edmund Halley published a theory of Magnetism, in which 

 occur the following ideas : — that the earth's Magnetism was caused by 

 four piles or points of attraction, two of them near each Pole; and that in 

 those parts of the world which lie nearly adjacent to any one of those 



