In the “Philosophical Magazine” for November of last year I gave an account of a mode of exciting an induction coil by the direct application of one of M. de Meritens’ alternating machines, without the intervention of a contact-breaker or the use of a condenser. The experiments of Professor Dewar on the arc furnished by the machine itself, on its spectrum, and on its behaviour in respect of electrolysis described before the Royal Society (see “Proc. Roy. Soc,” vol. xxx, p. 170), have led me to think that an account of some of the peculiarities in the induced discharge, to which the machine gives rise, might be acceptable to the Society. And, first, as regards the secondary discharge in air. It was mentioned in the paper first quoted that the spark produced by this machine presented an unusually thick yellow flame, and that it was accompanied by a hissing noise different from that commonly heard with a coil excited by a battery. As the machine gives alternate currents, the secondary discharge presents sparks of equal strength in both directions, and the general appearance to the eye is symmetrical in respect of both terminals. The spark was observed in a revolving mirror, first in a vertical and secondly in a horizontal direction. The discharge, although apparently continuous, was immediately seen to be intermittent, with a period in unison with that of the machine. Tongues of flame, leading alternately from one terminal and from the other, crossed the field of view. The length of spark first used (vertically) was about half an inch. When the length was increased to about two inches, the discharge being vertical, flashes or bands of continuous light were seen to traverse the field of view in diagonals of low slope ( i. e ., nearly horizontally), showing that there were masses of heated matter passing from time to time at moderate velocity between the terminals. From the known period of the machine, and the number of the discharges crossed by these flashes in their passage from terminal to terminal, it was calculated that the time of passage was about ·03 of a second Occasionally there was a still brighter flash or meteor, which similarly traversed the field, but with a velocity apparently of about double that of the others.