288 Lord Kelvin on 



sky seen in any direction, are described. This ratio they 

 denote by r. One specially notable result of Mr. Majorana's 

 is that " the value of r at the crater of Etna is about five 

 times greater than at Catania/' The barometric pressures- 

 were approximately 53*6 and 76 cms. of mercury. Thus the 

 atmosphere above Catania was only 1*42 times the atmosphere 

 above Etna, and yetit gave five times as much scattering of light 

 by its particles, and by the particles suspended in it. This at 

 once proves that a great part of the scattering must be due to 

 suspended particles ; and more of them than in proportion to 

 the density in the air below the level of Etna than in the air 

 above it. In Majorana's observations, it was found that 

 " except for regions close to the horizon, the luminosity of the 

 " sky had a sensibly constant value in all directions when 

 " viewed from the summit of Etna." This uniformity was 

 observed even for points in the neighbourhood of the sun, as 

 near to it as he could make the observation without direct 

 light from the sun getting into his instrument. I cannot but 

 think that this apparent uniformity was only partial. It is 

 quite certain that with sunlight shining down from above, and 

 with equal light everywhere shining up from earth or sea or 

 haze, illuminating the higher air, the intensities of the blue 

 light seen in different directions above the crater would be 

 largely different. This is proved by the following investiga- 

 tion ; which is merely an application of Rayleigh's theory to 

 the question before us. But from Majorana's narrative we 

 may at all events assume that, as when observing from 

 Catania, he also on Etna chose the least luminous part of the 

 sky (Phil. Mag., May 1901, p. 561), for the recorded results 

 (p. 562) of his observations. 



§ 63. The diagram, fig. 1 below, is an ideal representation 

 of a single molecule or particle, T, with sunlight falling on 

 it indicated by parallel lines, and so giving rise to scattered 

 light seen by an eye at E. We suppose the molecule or 

 particle to be so massive relatively to its bulk of ether that it 

 is practically unmoved by the ethereal vibration; and for 

 simplicity at present we suppose the ether to move freely 

 through the volume T, becoming denser without changing its 

 velocity when it enters this fixed volume, and less dense 

 when it leaves. In §§ 41, 42, of Lecture XV. above, and in 

 Appendix A, a definite supposition, attributing to ether no 

 other property than elasticity as of an utterly homogeneous 

 perfectly elastic solid, and the exercise of mutual force between 

 itself and ponderable matter occupying the same space, is 

 explained : according to which the ether within the atom will 



