goes on continually yielding under the action of a constant stress, we must assume, since we cannot imagine a breach of continuity in the phenomenon, that glass even when cold is also a "truly viscous fluid,” although the gain of strain per day may be almost infinitely small after the first few days. Similarly, as the true conductivity is very considerable in hot glass, we may conclude that there does exist true conductivity in cold glass, although the amount will probably be so small as to make its separation from surface conduction or other extraneous loss extremely difficult.

From the curves we have obtained of the charging of condensers, and assuming that there is no discontinuity, we must assume that even the first charging is itself a very rapid absorption, and since there is viscosity, even the very first charging must be accompanied with a generation of heat, that is, true conduction. Also since it is known that gases, like all other substances, are to a certain extent viscous, we cannot believe that air and other gaseous condensers show absolutely no absorptive phenomena, in fact, sufficiently accurate experiments have not yet been made on the subject.

We conclude, therefore, that the less the specific resistance of a substance the greater is its molecular plasticity, and the more plastic the substance is the greater will be the first charge; therefore from the stress and strain analogy it follows that the less the specific resistance of a substance the greater will generally be the specific inductive capacity, the result obtained experimentally at the commencement of this paper: according to what law, however, the one increases as the other diminishes, we are not at present in a position to state.

From all that precedes it follows that when the potential of a body surrounded by a dielectric is altered by induction, a portion of the electric energy is converted into heat, the amount being greater as the dielectric is more viscous. Consequently a charged body A, perfectly surrounded by a dielectric, may be discharged without contact with any conductor, by alternately bringing near and withdrawing a distant conductor B; for the capacity of the arrangement is alternately getting greater and less, therefore the potentials must be alternately growing less and greater, and since all alteration of potential is accompanied by an alteration of strain in the viscous substance composing the dielectric, the potential energy of the system must be gradually converted into heat, whereas from the almost infinite resistance of the dielectric, if the bodies were motionless, this result could never have been attained. Consequently since the particles of any body are in rapid motion, they must all tend to acquire the same potential, even if we imagine them separated by a dielectric of very great resistance, provided the relative motions are motions of translation. But if the movements are motions of rotation only, then this equalization of potential will only take place after a very great time, if it takes place

at all. This difference may, perhaps, explain why metals conduct so much better than glass, &c.

III. "Recent Experiments on Fog-Signals." By Dr. TYNDALL, F.R.S., Professor of Natural Philosophy in the Royal Institution. Received March 14, 1878.

Our most intense coast-lights, including the six-wick lamp, the Wigham gas-light, and the electric light, being intended to aid the mariner in heavy weather, may be regarded, in a certain sense, as fog-signals. But fog, when thick, is intractable to light; the sun cannot penetrate it, much less any terrestrial source of illumination. Hence the necessity of employing sound-signals in dense fogs. Bells, gongs, horns, guns, and syrens have been used for this purpose; but it is mainly, if not wholly, explosive signals that I have now to submit to the notice of the Society. During the long, laborious, and, I venture to think, memorable series of observations conducted under the auspices of the Elder Brethren of the Trinity House at the South Foreland in 1872 and 1873, it was proved that a short 54-inch howitzer, firing 3 lbs. of powder, yielded a louder report than a long 18-pounder firing the same charge. Here was a hint to be acted on by the Elder Brethren. The effectiveness of the sound depended on the shape of the gun, and as it could not be assumed that in the howitzer we had hit accidentally upon the best possible shape, arrangements were made with the War Office for the construction of a gun specially calculated to produce the loudest sound attainable from the combustion of 3 lbs. of powder. To prevent the unnecessary landward waste of the sound, the gun was furnished with a parabolic muzzle, intended to project the sound over the sea, where it was most needed. The construction of this gun was based on a searching series of experiments executed at Woolwich with small models, provided with muzzles of various kinds. The gun was constructed on the principle of the revolver, its various chambers being loaded and brought in rapid succession into the firing position. The performance of the gun proved the correctness of the principles on which its construction was based.

It had been a widely spread opinion among artillerists, that a bronze gun emits a specially loud report. I doubted from the outset whether this would help us, and in a letter dated 22nd April, 1874, ventured to express myself thus:-"The report of a gun, as affecting an observer close at hand, is made up of two factors-the sound due to the shock of the air by the violently expanding gas, and the sound derived from the vibrations of the gun, which, to some extent, rings like a bell. This latter, I apprehend, will disappear at considerable distances."