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This result is general for all fluids, but if we would apply it to those only which are usually called elastic, we have, because in this case Ty" A - 'y,” A,

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which takes place in the reflected wave; and this is precisely the same value as that which belongs to light polarized perpendicularly to the plane of incidence; (Vide Airy's Tracts, p. 362.) We thus see, that not only the intensity of the reflected wave, but the change of phase also, when reflexion takes place at the surface of separation of two elastic media, is precisely the same as for light thus polarized.

As a = {3, we see that when there is no transmitted wave the intensity of the reflected wave is precisely equal to that of the incident one. This is what might be expected: it is, however, noticed here because a most illustrious analyst has obtained a different result. (Poisson, Memoires de l'Academie des Sciences, Tome X.) The result which this celebrated mathematician arrives at is, That at the moment the transmitted wave ceases to exist, the intensity of the reflected becomes precisely equal to that of the incident wave. On increasing the angle of incidence this intensity again diminishes, until it vanish at a certain angle. On still farther increasing this angle the intensity continues to increase, and again becomes equal to that of the incident wave, when the angle of incidence becomes a right angle.

It may not be altogether uninteresting to examine the nature of the disturbance excited in that medium which has ceased to transmit a wave in the regular way. For this purpose, we will resume the expression, p, - Be-" sin \, = B e-" sin (b y + c t):

or if we substitute for B, its value given by the last of the equations (10); and for as, its value from (11); this expression, in the case of ordinary elastic fluids where y” A = y, \,, will reduce to

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A being the length of the incident wave measured perpendicular to its own front, and 6 the angle of incidence. We thus see with what rapidity in the case of light, the disturbance diminishes as the depth a below the surface of separation of the two media increases; and also that the rate of diminution becomes less as 6 approaches the critical angle, and entirely ceases when 6 is exactly equal to this angle, and the transmission of a wave in the ordinary way becomes possible.

XIX. On a new Genus of Fossil Multilocular Shells, found in the Slate-Rocks of Cornwall. By D. T. ANSTED, B.A. of Jesus College; Fellow of the Society, and of the Geological Society.

[Read February 26, 1838.]

IT is not many years since the slate-rocks of Cornwall were described as contemporary in their formation with the Granite, and other igneous and altered rocks of that county. They were of course presumed at that time to be absolutely without trace of fossils; and when remains of organic life were first observed, the very possibility was questioned; but, after some doubt and sufficient inquiry, the fact was admitted, to the overthrow of the theory alluded to.

Since it has been granted that fossils may be expected in these beds, the search after them has not been unattended with success. Among others, Professor Sedgwick, during his geological researches in the South-west of England, has obtained many, in various states of preservation, which, with a few collected from the neighbourhood of Petherwyn, are now in the Woodwardian Museum; and it was during the temporary arrangement of these specimens, that I was struck with the occurrence of what seemed to me a new genus of multilocular shells, and induced to lay this paper before the Society.

The rest of the organic remains consist chiefly of fossil marine vegetables; many shells allied to terebratula; several species of orthoceratites; portions of the stems of radiated animals, and parts of

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