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§ 7. If, for a moment, we imagine the steady motion of the atom to be at a higher speed than the wave velocity of the condensational-rarefactional wave, two conical waves, of angles corresponding to the two wave velocities, will be steadily produced ; but we need not occupy ourselves at present with this case because the velocity of the condensational-rarefactional wave in ether is, we are compelled to believe, enormously great in comparison with the velocity of light.
§ 8. Let now a periodic force be applied to the atom so as to cause it to move to and fro continually, with simple harmonic motion. By the first sentence of § 5 we see that two sets of periodic waves, one equi-voluminal, the other irrota-tional, are continually produced. Without mathematical investigation we see that if, as in ether, the condensational-rarefactional wave velocity is very great in comparison with the equi-voluminal wave velocity, the energy taken by the condensational-rarefactional wave is exceedingly small in comparison with that taken by the equi-voluminal wave ; how small we can find easily enough by regular mathematical investigation. Thus we see how it is that the hypothesis o£ >§ 3 suffices for the answer suggested in that section to the question, How could matter act on ether so as to produce light ?
§ 9. But this, though of primary importance, is only a small part of the very general question pointed out in § 3 as needing answer. Another part, fundamental in the
u Rookh\ being becalmed in the Sound of Mull, I had an excellent u opportunity, with the assistance of Professor Helmholtz, and my u brother from Belfast [the late Professor James Thomson], of deter-£i mining by observation the minimum wave-veloeity with some approach u to accuracy. The fishing-line was hung at a distance of two or three u feet from the vessel’s side, so as to cut the water at a point not sensibly u disturbed by the motion of the vessel. The speed was determined by u throwing into the sea pieces of paper previously wetted, and observing u their times of transit across parallel planes, at a distance of 912 centi-il metres asunder, fixed relatively to the vessel by marks on the deck and u gunwale. By watching carefully the pattern of ripples and waves which u connected the ripples in front with the waves in rear, I had seen that “ it included a set of parallel waves slanting off obliquely on each side u and presenting appearances which proved them to be waves of the il critical length and corresponding minimum speed of propagation.” When the speed of the yacht fell to but little above the critical velocity, the front of the ripples was very nearly perpendicular to the line of motion, and when it just fell below the critical velocity the ripples disappeared altogether, and there was no perceptible disturbance on the surface of the water. The sea was u glassy ” ; though there was wind -enough to propel the S2hooner at speed varying between £ mile and 1 mile per hour.
undulatory theory of optics, is, How is it that the velocity of light is smaller in transparent ponderable matter thau in pure ether ? Attention was called to this particular question in my address, to the Royal Institution, of last April ; and a slight explanation of my proposal for answering it was given, and illustrated by a diagram. The validity of this proposal is confirmed by a somewhat elaborate discussion and mathematical investigation of the subject worked out since that time and communicated under the title, u On the Motion produced in an infinite Elastic Solid by the Motion through the Space occupied by it of a Body acting on it only by Attraction or Repulsion,5’ to the Royal Society of Edinburgh on July 16, and to the Congres International de Physique for its meeting at Paris in the beginning of August *.
§ 10. The other phenomena referred to in § 3 come naturally under the general dynamics ol: the undulatory theory of light, and the full explanation of them all is brought much nearer if we have a satisfactory fundamental relation between ether and matter, instead of the old intractable idea that atoms of matter displace ether from the space before them, when they are in motion relatively to the ether around them. May we then suppose that the hypothesis which I have suggested clears away the first of our two clouds ? It certainly would explain the “ aberration of light ” connected with the earth’s motion through ether in a thoroughly satisfactory manner. It would allow the earth to move with perfect freedom through space occupied by ether without displacing it. In passing through the earth the ether, an elastic solid, would not be lacerated as it would be according to Fresnel’s idea of porosity and ether moving through the pores as if it were a fluid. Ether would move relatively to ponderables with the perfect freedom wanted for what we know of aberration, instead of the imperfect freedom of air moving through a grove of trees suggested by Thomas Young. According to it, and for simplicity neglecting the comparatively very small component due to the earth’s rotation (only *46 of a kilometre per second at the equator where it is a maximum), and neglecting the imperfectly known motion of the solar system through space towards the constellation Hercules, discovered by Herschel f,
* Phil. Mag., Aug. 1900.
t The splendid spectroscopic method originated by Huggins thirty-three years ago, for measuring the component in the line of vision of the relative motion of the earth, and any visible star, has been carried on since that time with admirable perseverance and skill by other observers, who have from their results made estimates of the velocity and direction of the motion through space of the centre of inertia of the solar system.