PHYSICAL CONDITION OF JUPITER AND SATURN. 233 



To conclude, the experiment I have just recounted shows that ex- 

 citation of a sensor nerve at any point of its length is propagated in 

 two directions, both centrifugally and centripetally. The same is 

 doubtless true of a motor nerve. Consequently, it is highly probable 

 that, as Vulpian has held, the nerves are simple conductors, differen- 

 tiated only by their functions, which latter depend upon the kind of 

 apparatus at their two extremities : for motor nerves there is a motor 

 nerve-cell and muscular fibre, and for sensor nerves a receiving nerve- 

 cell and an impressionable termination. La Nature. 



-- 



O^ THE PHYSICAL CONDITION OF JUPITEK AND 



SATURN. 



By DAVID TEOWBEIDGE, Esq. 



""VyOT many years ago the planets Jupiter and Saturn were re- 

 -L^ garded as solid bodies, their small relative density being sup- 

 posed to be due either to the peculiar arrangement of the materials 

 which compose them, or to the small specific gravity of the materials 

 themselves. Within a few years, however, the nebular hypothesis has 

 gained so strong a hold on the minds of astronomers, that the larger 

 planets are regarded in a very different light as respects their physical 

 condition from what they were a few years ago. It is now thouglit 

 that at least Jupiter and Saturn, and probably Uranus and Neptune, 

 have not yet cooled down from their heated and nebulous condition 

 sufficiently to be habitable globes, and that, owing to their relatively 

 high temperature, they have very extensive atmospheres. 



It is quite possible for us to guess at the extent and density of 

 their atmospheres, but, unless we subject our guesses to calculation, it 

 cannot be said that they are of much use to us, since the planets are 

 too far from us to allow direct observations to settle any points in dis- 

 pute. Let us, therefore, assume Jupiter and Saturn to be globes of 

 much higher temperature than the earth's, and that this temperature 

 varies from the surface upward inversely as the square of the distance 

 from the centre of the planet. Let us further assume that gases there 

 follow the same law of expansion as terrestrial gases, and that this law 

 holds for all temperatures above 32 Fahr. With these assumptions, 

 it is possible for us to subject onv guesses to calculation, and thus learn 

 how far they are admissible. 



As respects the telescopic appearance of those planets, it is plain 

 that we have but little evidence of an atmosphere outside the highest 

 cloud-layers. That their atmospheres extend above the highest clouds 

 is perhaps certain ; but if we attempt to explain the physical appear- 



