In a paper published in the Philosophical Transactions for 1884 (Part II., pp. 343 — 361), I have deduced from Maxwell’s equations for the electromagnetic field the mode in which the energy moves in the field. The result there obtained is that the energy moves at any point perpendicularly to the plane containing the directions of the electric and magnetic intensities, and in the direction in which a right-handed screw would move if turned round from the positive direction of the electric intensity to the positive direction of the magnetic intensity. The quantity crossing the plane per unit area per second is equal to the product of the two intensities multiplied by the sine of the included angle divided by 4 π . Hence it follows that the energy moves along the intersections of the two sets of level surfaces, electric and magnetic, where they both exist, their intersections giving, as it were, the lines of flow. In the particular case of a steady current in a wire where the electrical level surfaces cut the wire perpendicularly to the axis, it appears that the energy dissipated in the wire as heat comes in from the surrounding medium, entering perpendicularly to the surface.