250 Messrs^ Bahbage and Herschel on the [OcTw 



In Mr. Barlow's experiments, the earth is our inducing mag- 

 net ; its two poles both act ou every particle of the revolving 

 shell employed in that gentleman's experiments, and their action, 

 when complej;e produces two poles, a north and a south, ati 

 opposite extremities of the diameter parallel to the dip. This 

 is the case when the shell is at rest. Let it now be set in motion 

 about any axis, anyhow incUned to the dip. If the communica- 

 tion and loss of magnetism were instantaneous, the places of 

 the poles (i. e. the points of maximum polarity) would be unaf- 

 fected by the rotation ; but as that is not the case, these points, 

 in virtue of the principles already stated, will shift their places, ^ 

 and decline from the direction of the dip in the same direction 

 as the shell's motion, that is to say, in the direction of a tangent 

 to a small circle, whose axis is the axis of rotation, and whose 

 circumference passes through the extremities of the diameter 

 parallel to the dip. The extent of this declination will depend 

 on the velocity of rotation and the diameter of this small circle, 

 and will be proportional to both, that is, to the velocity of rota- 

 tion multiplied into the sine of the angle made by the axis of; 

 rotation with the direction of the dip. It will, therefore, be a 

 maximum when the axis of rotation is perpendicular to the 

 magnetic meridian, and vanish when the shell is made to revolve 

 on an axis parallel to the line of dip. These consequences are 

 perfectly consonant to the results obtained by Mr. Barlow in 

 his paper ; and, in fact, the general result announced by him in 

 p. 449 of this volume, comes to the very same thing as above 

 stated ; for it is obvious, that the new axis of polarization there 

 spoken of, acting in combination with the original, or, as we 

 may call it, the primary axis developed in the quiescent state of 

 the shell, will exert a compound force on the needle, such as 

 would be exerted by a single equivalent axis situated inter- 

 mediately between them, but much nearer to the more intense 

 than to the more feeble one. The position of this equivalent 

 axis will necessarily be in the great circle passing through the 

 two component ones. Now the small circle described by the 

 point which was first the pole of the stronger or primary axis 

 about the axis of rotation is a tangent to this great circle, and 

 the equivalent axis (being but little removed from the primary 

 one, by reason of the small intensity of the other), will, there- 

 fore, have its pole situate indifferently in either circle. Or 

 conversely, the single axis produced in our view of the subject 

 being resolved into two ; one of which is that corresponding to 

 the quiescent state of the shell, and the other 90° removed from 

 it in the same place, this latter will be identical with Mr. Bar- 

 low's secondary axis. 



In what has been said, the velocity of rotation has been sup-, 

 posed commensurate to the velocity with which magnetism is. 

 propagated through the iron of the shell. But if we conceive- 



