November 7, 1895] 



NATURE 



MacCuUagh's Theory of the ^ther. 



Mr. Basset's criticisms in his letters in Nature of October 17 

 (p. 595) and October 24 (p. 618) call for some reply. I willingly 

 avail myself of the opportunity to attempt to make my meaning 

 clearer. 



(i) As regards the first letter, there seems to be some miscon- 

 ception. I have nowhere in the papers referred to given a proof, 

 such as he supposes, of the theorem which he calls in question, 

 viz. that a gyrostatic a;ther may be constructed which will function 

 according to MacCuUagh's optical scheme. That proposition is, 

 I take it, Lord Kelvin's ; and I simply gave references to his 

 treatment, which occurs, at any rate implicity, near the end of 

 the third volume of his " Collected Papers" (pp. 442, 466). 



The principal aim of the second of the papers referred to 

 (Phil. Trans. A, 1894) was, assuming the existence of a con- 

 tinuous medium with kinetic and elastic energies given by 

 MacCuUagh's expressions, to examine how far such a medium 

 would fulfil the functions that are required of the sether, as 

 ( i. ) the transmitter of radiation, (ii. ) the medium in which electric 

 actions consist and are transmitted, (iii. ) the underlying medium 

 in which ordinary matter may itself consist, in the form of per- 

 manent configurations of strain or motion possessing mobile 

 characteristics. In MacCuUagh's own time it was recognised, 

 by none more than by himself, that a medium like his was in no 

 way analogous to ordinary elastic matter. Moreover it was 

 held by many that it was an impossible constitution for any 

 medium at all. This latter conclusion I combatted by quoting 

 the fact that Lord Kelvin has actually shown how to make a 

 model, representing MacCuUagh's medium, by means of a 

 cellular structure composed of portions of ordinary matter in 

 spinning motion. This is the only way that I intended to intro- 

 duce the gyrostatic cether into the argument. For I hold it to be 

 more rational to take matter to be a structure of molecular type 

 in the primordial medium (which is not itself matter, but is a 

 continuutii with simpler fundamental properties than elastic 

 solid matter) than it would be to take the sether to be a mole- 

 cular or cellular structure built up out of ordinary matter. 



The functions required of the aether show that it must .be a 

 medium which can have kinetic energy involving inertia, and 

 also elastic energy of some kind when strained. According to 

 MacCuUagh's scheme, its elasticity would consist simply in 

 resistance to absolute rotation ; so that an element of volume of 

 the medium is taken to have relations to directions in space, of 

 the same general tyjie as the axis of a spinning gyrostat actually 

 possesses. The analysis of the interaction of this inertia and 

 this elasticity forms a dynamical theory of the medium, but the 

 dynamics is not the dynamics of ordinary matter. 



Mr. Basset easily arrives at inconsistencies by applying 

 MacCuUagh's energy formula directly to the structural gyrostatic 

 medium of Lord Kelvin. The reason is that the problem is one 

 invoK-ing ignored coordinates (in the phraseology of Thomson 

 and Tait's "Natural Philosophy ") corresponding to the latent 

 spinning motions of the imbedded gyrostats. Before the 

 principle of least action can be applied after the manner of an 

 ordinary continuous elastic medium, the actual energy function 

 of the gyrostatic medium must be modified in the well-known 

 manner, and it will thus assume a form. equivalent to Mac- 

 CuUagh's. It would, no doubt, be interesting and instructive, 

 as regards dynamical principles, to establish this in detail ; but 

 this is hardly the place to enter into a technical problem. 



(2) As regards Mr. Basset's second letter, on the reflection of 

 light from the surface of a magnet, the parallel which he draws 

 between one type of theory which I provisionally uphold, and 

 another which I reject, is, I think, not a real one. The latter 

 theory retains the dynamical equations and surface conditions 

 which belong to the luminiferous medium under ordinary 

 circumstances, merely adding on to the electric force a new part 

 of magneto-optic origin. This would hardly be open to 

 ()l)jection if it worked ; but it is adinitted that it does not work, 

 and in default of a specific reason being assigned for the 

 discrepancy the theory fails. It is as if a machine, whose mode 

 of working is thoroughly known under certain simple conditions, 

 were observed to be working steadily under more complicated 

 circumstances, while a mathematical analysis showed that it 

 ought to get jammed under these new conditions. The inference 

 would, I think, be that the machine has been reset, or some 

 change made in its constitution, which obviated the jamming. 

 Now the ordinary equations of the electric theory of light are, 

 presumably, deducible from the energy function of the medium 

 'by the principle of least action. When the substance that 



transmits the light is in an extraneous magnetic field, there is a 

 subsidiary term in the energy function which arises from this 

 field ; therefore the application of the principle of least action will 

 now give different equations of the medium, and different boundary 

 conditions, from those which ordinarily hold good. The state- 

 ment that the boundary conditions which held for non-magnetic 

 circumstances are not now maintained, is not to the point ; the 

 question is rather, whether the boundary conditions which are 

 appropriate to the actual formulation of the problem can all be 

 maintained, and if they can the theory is consistent. 



J. Larmor. 

 St. John's College, Cambridge, October 25. 



Lightning.— Chain Formation. 

 On September 9, 1895, I was cycling near Pitlochry, N.B. 

 The day had been extremely hot— 80° F. in the shade — and 

 as dusk came on it grew somewhat foggy, and flashes of distant 

 lightning became frequent. At ten o'clock there suddenly came 

 on a terrific thunder-storm. Crash succeeded crash, and the 

 lightning, of all colours, blazed almost continuously. Objects 

 fifteen miles off" could be seen as plainly, if not more so, than in 

 bright daylight. The rain soon turned the road into a torrent, 

 and my electric lamp failed to act properly. But the chief 

 peculiarity was the occurrence of eight strange flashes of a chain 

 formation, with large elliptical links, and of a golden-yellow 

 colour. These flashes were not rapid in their passage, as ordinary 

 lightning is wont to be ; but one of them took slightly over a 

 minute to pour from the clouds to the edge of the valley opposite 

 me. Two of these chains of living, burning gold passed between 

 adjacent clouds, while the remaining six came to earth, one in 

 the field just beside me. I then went off" to seek for shelter ; 

 but the storm continued till I a.m. 



William Crawford. 



Personal Injury from a Fire-ball. 



In compliance with a wish expressed by several scientific 

 friends, I place on record an instance of damage done by a 

 fire-ball or globular lightning. About five weeks ago, when I 

 was in Londonderry, the circumstances were related to me by 

 Mr. James Harvey, of Northland Road in that city. Mr. Harvey 

 was staying during the month of August at Culdaff", on the north 

 coast of Donegal ; and on the 24th of that month, at about 

 4 p.m., a little boy named Robert Alcorn, whose parents 

 occupied a house near Mr. Harvey's, was desired by his father 

 to go into the yard and drive away some fowls froin the door. 

 On going out of the house, the boy saw a large bright object in 

 the sky about the size of the table in his bedroom (I give his 

 own account, leaving out necessary considerations of distances, 

 &c. ), or apparently about six square feet in area. The object came 

 towards his house from the west, or north-west ; and when it 

 came close, it partly burst with a report like that of a gun. He 

 put his hands over his face to shield himself from " the spark," 

 and after the explosion the bulk of the ball appeared to con- 

 tinue its course towards the east, low down. When it burst, 

 however, it struck him, shattering the thumb and the first and 

 second fingers of the left hand, cutting, scratching, and black- 

 ening the right hand and left cheek, and shattering into frag- 

 ments several bone buttons on his coat. Very soon afterwards. 

 Dr. R. Young, of Culdaff", and Dr Newell, of Moville, attended 

 the boy, and amputated the fingers and a portion of the thumb. 



No one near the place saw the ball (except the boy, of course), 

 but the parents and several others heard the report, and the 

 boy's father rushed out immediately and caught his son as he 

 was falling. Mr. Harvey soon afterwards examined the place, 

 and could find no further trace of the fire-ball, except that a 

 piece of bark had been knocked off" a small tree within a few 

 feet of the place where the boy was struck. The local police 

 made exhaustive inquiry as regards the possibility of any one's 

 having fired a gun at the boy, or of his having had any explosive 

 in his possession ; but nothing of the kind transpired. 



It is well to add that at Redcastle (about eight miles away), 

 one of the residents saw, on the same day, a bright object in 

 the sky, which object he took to be a fire-ball. The day was 

 stormy, with hea%'y showers, but no thunder. 



M. Jamin relates (" Cours de Physique," tome premier, p. 470) 

 several instances of globular lightning, and from these I select 



NO. 1358, VOL. 53] 



