TO PROFESSOR LEWIS CAMPBELL.                                                         Glenlair, Dalbeattie, Christmas 1876.

     . . . I hope that when this severe weather is past you will be able to derive benefit from a moderate use of Plato   and Sophocles.

     We intended to have gone round by Edinburgh, to pay Aunt Jane a visit; but we both had such bad colds that we   came home to nurse them, and are now snowed up, and enjoying the artificial heat of coals, peats, and sticks    judiciously intermingled.

     The demonstrator at the Cavendish Laboratory has been out of sorts all this term, and has had to go home about a    month ago, so we have not been in full force there. I hope he will be well in February, to absorb the energy of the   new B.A.'s set free from the Tripos and its attendant anxieties.

     As we get richer in apparatus, mathematical lectures give way to experimental, and the black board to the lamp     and scale. I have had a pupil quite innocent of mathematics who has learned to measure focal lengths of lenses,   and has found the electro-motive force from the water-pipes to the gas-pipes, and from either set of pipes to the   lightning-conductor.

     I have been making a mechanical model of an induction coil, in which the primary and secondary currents are    represented by the motion of wheels, and in which I can symbolise  all the effects of putting in more or less    of the iron core, or more or less resistance and Leyden jars in either circuit.

     I have also been making a clay model of Prof. W. Gibb's thermodynamic surface, representing the relations of the    solid liquid and gaseous states, and the different paths by which a body may get from the one to the other.—Your    Afft. friend,

                                                                       J. CLERK MAXWELL.