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86. Correlation of the Effects of Air Waves and Ether Waves. — The mutual approach of two molecules vibrating in the ether may be compared to the mutual approach of two (freely suspended) tuning-forks vibrating in air, or to the approach of a freely suspended piece of card or plate of any substance towards a tuning- fork vibrating in air.

The motion of the card or plate towards the vibrating fork being due to the excess of the normal air pressure at the back of the plate, over the reduced air pressure in front, due to the rarefaction produced by the stationary vibration of the air column intercepted between the vibrating prong and the plate; then when the motion of the plate towards the fork commences, the air molecules which rebound from the remote surface of the plate lose a portion of their velocity by transference to the plate. This loss of velocity is rapidly restored by the neighbouring air molecules in the inter- change of motion continually taking place between the molecules, and thus the loss of motion is carried off into the surrounding air in the form of a wave, and at a speed somewhat less than the normal speed (sixteen hundred feet per second) of the air molecules, or with the speed of a wave of sound.

87. Though the effect must be extremely feeble here, it is theoretically deducible that the energy of vibration of the fork must, during the approach of the plate (in analogy with the development of vibrating energy (heat) at the approach of molecules), be slightly augmented, due to the intervening vibrating air column being driven against the prong with which it vibrates in synchronism. The effect here must of necessity be extremely feeble, neither the rate of approach nor the energy of vibration in this case being at all com- parable with the case of molecules; also the effect would be probably more than masked by the dissipation of the vibrating energy of the fork into the surrounding air during the approach, for the vibration of the fork, unlike that of the molecule, is not continuously maintained by external sources. However, a physical cause must have its effect whether it be feeble or not.

It is, therefore, important to observe that the work done at the

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approach of the plate cannot be directly due to the vibrations of the fork, for if the movement of translation of the plate were derived from the vibrating energy of the fork, then the approach of the plate would have the effect of reducing the vibrating energy of the fork by an amount equivalent to the work done, whereas the approach of the plate has the exact contrary effect. The motion imparted to the plate must, therefore, be entirely derived from the surrounding air, whose molecules lose an amount of motion or energy equivalent to the translatory motion imparted to the plate, including the small extra amount of vibratory motion imparted to the fork during the approach of the plate.

The vibrations of the fork have the indirect object of maintaining the rarefaction of the air in front of the plate, or they serve the purpose of putting the air into a suitable physical condition such that an interchange of motion can take place between the molecules of air and the plate, the air molecules alone, and not the vibrating fork, being the source of energy in the approach of the plate, the same considerations evidently applying in the case of the mutual approach of two (freely suspended) vibrating forks.

88. So in the case of the approach of two molecules vibrating in the ether, the energy concerned in effecting the approach (combination) of the molecules can be solely derived from the surround- ing ether, and not from the vibrations of the molecules themselves. This is illustrated by the observed effects; for if the energy concerned in the approach of the molecules were derived or abstracted from the molecular vibrations, then the vibrating energy (heat) of the molecules would be necessarily reduced at their approach by an amount equivalent to the work done, whereas the exact contrary is the fact. The entire energy developed at the approach of the molecules, including both the translatory motion developed and the vibratory motion (heat) developed, must be therefore solely derived from the surrounding ether, which loses an amount of motion equivalent to the motion developed : the vibrations of the molecules merely serving the indirect purpose of putting the ether into a suitable physical condition, such that an interchange of motion can effect itself between its particles and the molecules; the ether, and not the molecular vibrations, being the source or motive agent of the molecular motion observed.

In the case of the combustion of a piece of coal, for example, the molecular motion developed is solely derived from the surrounding ether, and not from the molecular vibrations of the coal, the energy of these molecular vibrations (the heat) being greatly intensified in the process of combustion. The motion developed in the coal and molecules of air in the process of combustion is therefore solely derived from the motive agency of the surrounding ether : the normal molecular vibration of the coal (sustained by the dynamic action of the sun) merely serving the indirect purpose

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of putting the coal in a fit state, such that an interchange of motion can effect itself between its molecules and the ether, or a mass of coal is simply a piece of mechanism for utilizing the motion of the ether.

89. The above considerations serve to indicate that in addition to the well-known striking analogy which exists between the effects of ether waves and those of air waves, as exhibited in the phenomena of light and sound, a still further analogy exists as regards the effects due to a disturbance of the equilibrium of pressure of the medium by the action of the waves, as exhibited in the. movements ("attractions," &c.) of masses vibrating in air, and of molecules vibrating in the ether; the intensity of the effects in this latter case being greater in proportion as the intense ether pressure renders possible effects of a higher static value, and in proportion as the energy of the vibrations of molecules sustained by the powerful agency of the sun, incomparably surpasses the feeble vibrations mechanically produced in masses.