32 PROCEEDINGS OP SECTION A. 



Deimos and Pliobos ("fear and panic"). Phobos is only 3,760 

 miles and Deimos 12,500 miles from Mars' surface. The former 

 goes round the planet in a little over 7^ hours, and the latter in 

 about 30,t hours. The actual dimensions can only be estimated 

 from light measures, which give diameters of about nine and 

 eleven miles. 



Outside the orbit of Mars we come to that region of solar 

 dominion tenanted by the planetoids. You will remember that 

 at the beginning of this century no body was actually known 

 between the orbits of Mars and Jupiter, although it was already 

 suspected that some body or bodies did exist there. In 1801 

 Piazzi discovered the first of the minor planets, Vesta, and now 

 we have 277 on the list, the whole mass of which, it is computed, 

 would only form a body equal to 1 •3,000th part of the earth's 

 volume. Pickering, from photometric measvires, estimates the 

 diameter of the largest, Vesta, at 319 miles, wliile the smallest 

 have diameters probably not exceeding 10 miles. 



Jupiter, or as he is now often called, the giant planet, is a 

 favorite study for both the practical and physical astronomer. His 

 immense size, his systems of satellites, and the peculiar and 

 changing appearance of his sui-face tend to make this planet one 

 of the most attractive of the solar group. Probably the chief 

 interest is now centred on observational evidence of tlie physical 

 condition of the planet's surface. It has long been suspected that 

 some of the light we see on Jupiter is intrinsic or not reflected 

 sunlight, but recent investigation seems to show that while tlie 

 surface of the planet is apparently at a comparatively high 

 temperature, very little, if any, native light is emitted from him. 

 His visible surface is evidently vaporous and disturbed by 

 equatorial storms giving rise to the ragged and rippled edges of 

 the belts. His vapoury envelope appears to have a rotation 

 period differing from that of the solid body, for the periods 

 olitained by observing the successive passages of some markings 

 or spots are different from those given by other spots. Moreover, 

 the comparatively recent phenomenon of the red cloud or spot 

 afforded excellent opportunities for testing this question. This 

 remarkable appearance of an immense cloud-like mass of a distinct 

 brick red colour, which, I fancy, was first actually drawn on 

 paper by our esteemed President, Mr. H. C. Russell, in 1876, 

 continued to be visible on tlie planet's surface until 1885. By 

 consecutive observation of this red cloud in Jupiter's atmosphere, 

 it was found to complete its circuit in 9 hours 55 minutes and 36 

 seconds, while a white spot or cloud near it took some 5^ minutes 

 less, showing a relative motion one from another of nearly 260 

 miles an hour. Jupiter's actual rotation is assumed to be about 

 9 hours and 55 minutes. It seems then almost certain that 

 whatever may be the constitution of the Jovian surface, it is 

 unstable and .subject to a I'elative motion in itself. These observed 



