the Weights of Atoms. 293 



sphere, due to the sun's direct light incident on them, we 

 have 



|=^sec/9gs [ r(Z ^~ J>) ]'fl.i(cos»g+l) ■ . . (19); 



provided we may, in the cases of application whatever they 

 may be, neglect the diminution of the direct sunlight in its 

 actual course through air, whether to the observer or to the 

 portion of the air of which he observes the luminosity, and 

 neglect the diminution of the scattered light from the air in 

 its course through air to the observer. This proviso we shall 

 see is practically fulfilled in Mr. Majorana's observations on 

 the crater of Etna for zenith distances of the sun not exceeding 

 60°, and in Mr. Sella's observation on Monte Rosa in which 

 the sun's zenith distance was 50°. But for Majorana's 

 recorded observation on Etna at 5.50 a.m. when the sun's 

 zenith distance was 81°*71, of which the secant is 6*927, there 

 may have been an important diminution of the sun's light 

 reaching the air vertically above the observer, and a consider- 

 ably more important diminution of his light as seen direct by 

 the observer. This would tend to make the sunlight reaching 

 the observer less strong relatively to the skylight than 

 according to (19) ; and might conceivably account for the 

 first number in col. 3 being smaller than the first number in 

 col. 4 of the Table of § 69 below ; but it seems to me more 

 probable that the smallness of the first two numbers in col. 3, 

 showing considerably greater luminosity of sky than accord- 

 ing to (19), may be partly or chiefly due to dust in the air 

 overhead, optically swelled by moisture in the early morning. 

 The largeness of the luminosity of the sky indicated by the 

 smallness of the last number in col. 3 (376), in comparison 

 with the last number of col. 4 (460), may conceivably be 

 explained by earthshine from air and volcanic ash and rock 

 and forest and. vineyard and sea below T the level of the crater 

 adding considerably to the illumination which the sky experi- 

 ences from above by direct sunlight. This addition would be 

 much greater at 11 a.m., when the sun's zenith distance was 

 29°'9, than at 9 a.m., when it was 44°*6. 



§ 69. The results of Majorana's observations from the crater 

 of Etna are shown in the following Table, of which the first 

 and third columns are quoted from the Philosophical Magazine 

 for May 1901, and the second column has been kindly given 

 to me in a letter by Mr. Majorana. The values of S/s shown 

 in column 4 are calculated from § 68 (19), with the factor of 



