CONTRIBUTIONS TO SCIENCE. 531 



Eeferring again to the case of the earth, it is demon- 

 strable from the fundamental laws and principles of dynamics 

 that if matter were conveyed from the equatorial regions to 

 the poles, and there deposited so as to lengthen the polar 

 axis at the expense of the equatorial diameter, the ' rate of 

 rotation of the earth would be increased and the length of 

 the day would be diminished ; while if the earth became 

 more oblate its velocity of rotation would diminish. In fact, 

 if any body be in rotation, and be unacted upon by external 

 forces, or if the forces acting upon it be such as not to affect 

 its rotation, and if the system be altered in shape by internal 

 stresses or otherwise, so that its moment of inertia about the 

 axis of rotation is increased, the angular velocity will be 

 diminished and, in the case of a sphere becoming an oblate 

 spheroid, the velocity at the circumference will also be 

 diminished, while if the moment of inertia be diminished, 

 the reverse effect takes place. 



Now Maxwell supposed that any medium which can 

 serve as the vehicle of magnetic force consists of a vast num- 

 ber of very small bodies or cells capable of rotation, and which 

 we may consider to be spherical or nearly so when in their 

 normal condition, until we have reason to believe them to be 

 of some other form. When magnetic force is transmitted by 

 the medium, these bodies are supposed to be set in rotation 

 about the lines of magnetic force as axis, and with a velocity 

 depending on the intensity of the force. For the sake of 

 fixing our ideas he supposed the rotation to be in the direc- 

 tion in which a right-handed screw would turn if it advanced 

 in the direction of the force. We thus have the magnetic 

 field filled with " molecular vortices," all rotating in the same 

 direction about the lines of magnetic force as axes. As we 

 have seen, these vortices will tend to contract in the direction 

 of their axes of rotation, and to expand at right angles to this 

 direction, so that if initially they are elastic spheres, they will 

 tend to become oblate spheroids like the earth. This tendency 

 will involve a tension in the medium along the lines of force, 

 these being the lines along which contraction tends to take 

 place, and this will be accompanied by an equal pressure in 



