PKKSIDKNT's ADDKK.SS. 33 



facts, combined with the comparatively low density of its mass, 

 which is only a third moi'e tlian that of water, while the density 

 of our earth is nearly six times greater, lead us to the conclusion 

 that this planet is in a comparatively early stage of its history, 

 perhaps in a semi-thud state, destined in ages to come by cooling 

 and increasing density to a condition approaching that of our 

 earth. 



The most beautiful and wonderful of the planets is Saturn, 

 with its mysterious ring and its following of eight satellites. Its 

 immense distance, however, has so far presented an insurmount- 

 able obstacle to anything more than conjecture as to physical 

 condition or constitution. The mathematical questions as to 

 dimensions, distances, and motions of the several parts of this 

 group are now pretty well established within small limits, but the 

 constitution of that wonderful appendage the ring or of the 

 planet itself remains an unsolved problem. The density of Saturn 

 is extremely small, being only tlu-ee-quarters that of water, and 

 would float in that medium only three-quarters immersed ; this 

 being the case, we can scarcely conceive its being a solid body. 

 The rotation period, from observations in 1876 of a bright 

 equatorial spot by Asaph Hall at Washington, appears to be 

 about ten hours and a quarter, but if the visible surface of the 

 jilanet be in the same or perhaps in an earlier condition than that 

 of Jupiter, the actual rotation of the planet may be somewhat 

 diflierent. Definite markings on the body of the planet or on the 

 I'ing, by which the axial period may be measured, are of rare 

 occurrence, so that even on this point our knowledge is not very 

 definite. The ring is seen to be divided up into three principal 

 annuli, the two outer bright and solid looking, the inner darker, 

 with a guaze-like structure ; hence it is called the crape ring. 

 The constitution of this appendage has always been a puzzle to 

 astronomers, but, from dynamical and other physical considera- 

 tions, the most acceptable hypothesis appears to be that it is 

 composed of an aggregation of minute pai'ticles of matter 

 circulating about the central body in orbits proportioned to their 

 distances from it, and that the particles composing the inner ring 

 must, to account for what is seen, be of ditterent constitution to 

 those of the outer or brighter rings. The spectroscope gives 

 evidence of aqueous vapour in Saturn. 



Distance, that helps to keep Saturn's condition enshrouded in 

 mystery, casts a more impenetrable veil over our two outer 

 planets, Uranus and Neptune, concerning which recent astrono- 

 mical reseai'ch has added little to what was known a quarter of a 

 century ago. 



Sometimes lately, faint markings have been seen on Uranus, 



which have given a rough approximation to its period of rotation 



as about ten hours. On favourable occasions the little green disc 



has given decided indications of an equatorial Ijulging, as if its 



D 



