NATURAL PHILOSOPHY. 133 



magnetic force is Greater or less. It may travel in such a course that 

 if a magnetic needle were carried through the same course it would 

 be entirely unaffected magnetically, i. <?., it would be a matter of 

 absolute indifference to the needle whether it were moving or still. 

 Matters may be so arranged, that the wire, when still, shall have the 

 same diamagnetic force as the medium surrounding the magnet, and 

 so in no way cause disturbance of the lines of force passing through 

 both ; and yet when the wire moves, a current of electricity shall be 

 generated in it. The mere fact of motion cannot have produced this 

 current ; there must have been a state or condition around the mag- 

 net and sustained by it, within the range of which the wire was 

 placed ; and this state shows the physical constitution of the lines of 

 magnetic force. What this state is, or upon what it depends, cannot 

 as yet be declared. It may depend upon the ether, as a ray of light 

 does, and an association has already been shown between light and 

 magnetism. It may depend upon a state of tension, or a state of 

 vibration, or perhaps some other state analogous to the electric cur- 

 rent, to which the magnetic forces are so intimately related. Whether 

 it of necessity requires matter for its sustentation will depend upon 

 what is understood by the term matter. If that is to be confined to 

 ponderable or gravitating substances, then matter is not essential to 

 the physical lines of magnetic force, any more than to a ray of light 

 or heat ; but if, in the assumption of an ether, we admit it to be a 

 species of matter, then the lines of force may depend upon some 

 function of it. Experimentally, mere space is magnetic ; but then 

 the idea of such mere space must include that of the ether, when one 

 is talking on that belief; or if, hereafter, any other conception of the 

 state or condition of space rise up, it must be admitted into the view 

 of that, which just now, in relation to experiment, is called mere 

 space. On the other hand, it is, I think, an ascertained fact, that 

 ponderable matter is not essential to the existence of physical lines of 

 magnetic force. 



ON THE LOCATION OF COMPASSES IN IKON SHIPS. 



THE following paper was read at the meeting of the British Asso- 

 ciation, Belfast, by Capt. E. W. Johnson, of the iron ship of war, 

 Trident. He says : While the Trident was in the basin at Wool- 

 wich, it occurred to me to try whether a position could be discovered 

 where the influences of the ship's iron upon the compass were so 

 equalized, as to render the amount of deviation so small as to be of no 

 practical importance. The correct magnetic direction of the ship's 

 head- having been determined by a compass on the shore, and that 

 proving to be near to one of the points of maximum deviation, (the 

 standard compass on the quarter deck there indicating 20 westerly 

 deviation,) I moved the standard compass several feet further forward 

 in the centre line of the ship, and there found the westerly deviation 

 increased to 29. I now commenced to move the compass aft, six 

 or seven feet at a time, observing the deviation at each position, and 



