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 Cartularium Saxonicum; A.S. Laws (ed. Liebermann), pp. 220-224; Fabii Ethelwerdi Chron., Mon. Hist. Brit. 449-454. Secondary: Plummer, Saxon Chronicle, vol. ii. p. ci.; Napier and Stevenson, Crawford Charters, pp. 118-120;, s.v.

ÆTHELWULF, king of the West Saxons, succeeded his father Ecgberht in 839. It is recorded in the Saxon Chronicle for 823 that he was sent with Eahlstan, bishop of Sherborne, and the ealdorman Wulfheard to drive out Baldred, king of Kent, which was successfully accomplished. On the accession of Æthelwulf, Æthelstan, his son or brother, was made sub-king of Kent, Surrey, Sussex and Essex. Æthelwulf’s reign was chiefly occupied with struggles against the Danes. After the king’s defeat 843–844, the Somerset and Dorset levies won a victory at the mouth of the Parret, c. 850. In 851 Ceorl, with the men of Devon, defeated the Danes at Wigganburg, and Æthelstan of Kent was victorious at Sandwich, in spite of which they wintered in England that year for the first time. In 851 also Æthelwulf and Æthelbald won their great victory at Aclea, probably the modern Ockley. In 853 Æthelwulf subdued the North Welsh, in answer to the appeal of Burgred of Mercia, and gave him his daughter Æthelswith in marriage. 855 is the year of the Donation of Æthelwulf and of his journey to Rome with Alfred. On his way home he married Judith, daughter of Charles the Bald. According to Asser he was compelled to give up Wessex to his son Æthelbald on his return, and content himself with the eastern sub-kingdom. He died in 858.

AETHER, or (Gr. , probably from  , burn, though Plato in his Cratylus (410 ) derives the name from its perpetual motion— ), a material substance of a more subtle kind than visible bodies, supposed to exist in those parts of space which are apparently empty.

“The hypothesis of an aether has been maintained by different speculators for very different reasons. To those who maintained the existence of a plenum as a philosophical principle, nature’s abhorrence of a vacuum was a sufficient reason for imagining an all-surrounding aether, even though every other argument should be against it. To Descartes, who made extension the sole essential property of matter, and matter a necessary condition of extension, the bare existence of bodies apparently at a distance was a proof of the existence of a continuous medium between them. But besides these high metaphysical necessities for a medium, there were more mundane uses to be fulfilled by aethers. Aethers were invented for the planets to swim in, to constitute electric atmospheres and magnetic effluvia, to convey sensations from one part of our bodies to another, and so on, till all space had been filled three or four times over with aethers. It is only when we remember the extensive and mischievous influence on science which hypotheses about aethers used formerly to exercise, that we can appreciate the horror of aethers which sober-minded men had during the 18th century, and which, probably as a sort of hereditary prejudice, descended even to John Stuart Mill. The disciples of Newton maintained that in the fact of the mutual gravitation of the heavenly bodies, according to Newton’s law, they had a complete quantitative account of their motions; and they endeavoured to follow out the path which Newton had opened up by investigating and measuring the attractions and repulsions of electrified and magnetic bodies, and the cohesive forces in the interior of bodies, without attempting to account for these forces. Newton himself, however, endeavoured to account for gravitation by differences of pressure in an aether; but he did not publish his theory, ‘because he was not able from experiment and observation to give a satisfactory account of this medium, and the manner of its operation in producing the chief phenomena of nature.’ On the other hand, those who imagined aethers in order to explain phenomena could not specify the nature of the motion of these media, and could not prove that the media, as imagined by them, would produce the effects they were meant to explain. The only aether which has survived is that which was invented by Huygens to explain the propagation of light. The evidence for the existence of the luminiferous aether has accumulated as additional phenomena of light and other radiations have been discovered; and the properties of this medium, as deduced from the phenomena of light, have been found to be precisely those required to explain electromagnetic phenomena.”

This description, quoted from James Clerk Maxwell’s in the 9th edition of the Encyclopaedia Britannica, represents the historical position of the subject up till about 1860, when Maxwell began those constructive speculations in electrical theory, based on the influence of the physical views of Faraday and Lord Kelvin, which have in their subsequent development largely transformed theoretical physics into the science of the aether.

In the remainder of the article referred to, Maxwell reviews the evidence for the necessity of an aether, from the fact that light takes time to travel, while it cannot travel as a substance, for if so two interfering lights could not mask each other in the dark fringes (see .) Light is therefore an influence propagated as wave-motion, and moreover by transverse undulations, for the reasons brought out by Thomas Young and Augustin Fresnel; so that the aether is a medium which possesses elasticity of a type analogous to rigidity. It must be very different from ordinary matter as we know it, for waves travelling in matter constitute sound, which is propagated hundreds of thousands of times slower than light.

If we suppose that the aether differs from ordinary matter in degree but not in kind, we can obtain some idea of its quality from a knowledge of the velocity of radiation and of its possible intensity near the sun, in a manner applied long ago by Lord Kelvin (Trans. R. S. Edin. xxi. 1854). According to modern measurements the solar radiation imparts almost 3 gramme-calories of energy per minute per square centimetre at the distance of the earth, which is about 1·3×106 ergs per sec. per cm.2 The energy in sunlight per cubic cm. just outside the earth’s atmosphere is therefore about 4×10−5 ergs; applying the law of inverse squares the value near the sun’s surface would be 1·8 ergs. Let be the effective elasticity of the aether; then ＝c2, where  is its density, and c the velocity of light which is 3×1010 cm./sec. If ＝A cosn (t−x/c) is the linear vibration, the stress is d/dx; and the total energy, which is twice the kinetic energy (d/dt)2dx, is n22 per cm., which is thus equal to 1·8 ergs as above. Now ＝2c/n, so that if A/＝k, we have (2ck)2＝1·8, giving ＝10−22k−2 and ＝10−1k−2. Lord Kelvin assumed as a superior limit of k, the ratio of amplitude to wave-length, the value 10−2, which is a very safe limit. It follows that the density of the aether must exceed 10−18, and its elastic modulus must exceed 103, which is only about 10−8 of the modulus of rigidity of glass. It thus appears that if the amplitude of vibration could be as much as 10−2 of the wave-length, the aether would be an excessively rare medium with very slight elasticity; and yet it would be capable of transmitting the supply of solar energy on which all terrestrial activity depends. But on the modern theory, which includes the play of electrical phenomena as a function of the aether, there are other considerations which show that this number 10−2 is really an enormous overestimate; and it is not impossible that the co-efficient of ultimate inertia of the aether is greater than the co-efficient of inertia (of different kind) of any existing material substance.

The question of whether the aether is carried along by the earth’s motion has been considered from the early days of the undulatory theory of light. In reviving that theory at the beginning of the 19th century, Thomas Young stated his conviction that material media offered an open structure to the substance called aether, which passed through them without hindrance “like the wind through a grove of trees.” Any convection of that medium could be tested by the change of effective velocity of light, which would be revealed by a prism as was suggested by F. J. D. Arago. Before 1868 Maxwell conducted the experiment by sending light from the illuminated cross-wires of an observing telescope forward through the object-glass, and through a train of prisms, and then reflecting it back along the same path; any influence of convection would conspire in