xxxvi President'' s Address, [Aug. 29, 



to a full brick-red, and the effect was heightened by the presence of 

 an unusually brilliant white equatorial spot. The two spots completed 

 a revolution in different times, the white one taking about 5^ minutes 

 longer than the other ; and neither preserved a uniform period 

 throughout. In May 1883 the red spot had become almost invisible, 

 but in December it began to recover and in 1886 presented much the 

 same appearance as in 1882. The only conclusions as to it nature 

 that we can draw from the numerous observations are merely negative 

 ones ; it was not an eruption from a Jovian volcano ; it was not 

 attached to the surface of the planet ; it was not a portion of a glowing 

 disc seen through a rent in the enveloping vapours ; and it was not 

 self-luminous. 



Observations of the eclipses of Jupiter's satellites led Romer to the 

 discovery that light does not travel instantaneously, and the method 

 was employed by Glasenapp in 1874 to obtain a new determmation of 

 the velocity of light. But the ordinary observation of the time of 

 disappearance or reappearance of a satellite is an unsatisfactory one, 

 and a much better plan has been proposed and is being carried out 

 by Pickering. By means of a photometer he determines the instant 

 when the satellite has lost half its light, i.e. when it is half submerged 

 in Jupiter's shadow and finds he can fix this time with great accuracy. 

 These observations when carried out for 12 years will fuijnish a very 

 reliable value of the lijjht-equation and of the solar parallax. 



Saturn is probably in a still earlier stage of development than 

 Jupiter ; his specific gravity is so low that he would float in water 

 and as the density increases towards the centre it follows that the 

 surface cannot be solid or liquid ; hence we must regard a large 

 proportion of his globe as consisting of heated vapours, kept in con- 

 stant agitation by the process of cooling. An interesting question 

 suggests itself as to whether the wonderful ring has always main- 

 tained the same dimensions and the same distance from the planet. 

 Otto Struve made in 1851 a most careful determination of the dimen- 

 sions of the system and repeated them under very similar circum- 

 stances in 1882, There appeared to be a slight extension in width 

 of the rings both inward and outward, but so slight as hardly to be 

 separated from the possible errors of observation. Some change there 

 certainly has been, for in 1657 Huyghens described the interval 

 between the ring and the planet as somewhat greater than the width 

 of the ring, while at the present time tfeia interval is only about two-^ 

 thirds the width of the rings. 



