738 THE POPULAR SCIENCE MONTHLY. 



crosses the orbits of certain meteoric swarms, we have showers of 

 shooting stars, fortunately so small that their bombardment is unno- 

 ticed. 



Scarcely anything is known, or plausibly guessed, concerning the 

 condition and properties of nebulous matter. If, for example, the 



Fig. 1.— Spots on Jitpiteb observed and drawn bt Mr. E. L. Trouvelot.— Observation of 

 September 25, 1878, with the shadow of a satellite. 



spectrum of a nebula indicates hydrogen, we may be pretty sure it is 

 not in the state of the gas as it is known in our laboratories. The 

 recent discoveries of Crookes concerning the properties of matter a 

 million times more attenuated than common air lead to the hope that 

 fresh light may be thrown upon many astronomical questions ; but in 

 the mean time it is impossible to form more than a vague idea of the 

 condition of any star or planet that does not in its main features re- 

 semble our eai-th ; and this can be said only of Mars, on whose globe 

 we can discover what is probably land and w^hat is water, and see 

 white masses, which it is reasonable to believe are snow, form and 

 melt away as the planet's winter and summer affect them in turns. 



Our earth has long been in a state of slow, as distinguished from 

 that of rapid, change. The geologist finds the oldest rocks he can 

 discover affording indications that they were formed when the circum- 

 stances of the globe were sufficiently like what they are now for fair 

 comparison. The earth's surface may have been warmer, its atmos- 

 phere more moist, and it may have contained more carbonic acid than 

 we now find ; storms may have been more frequent and more violent, 

 but the assemblage of differences between what now is and what was 



