50 TERRESTRIAL ADAPTATIONS. 



If, on the other hand, the force of gravity were 

 much lessened, inconveniences of an opposite 

 kind would occur. The air would be too thin to 

 breathe ; the weight of our bodies, and of all the 

 substances surrounding us, would become too 

 slight to resist the perpetually occurring causes 

 of derangement and unsteadiness : we should feel 

 a want of ballast in our movements. 



It has sometimes been maintained by fanciful 

 theorists that the earth is merely a shell, and that 

 the central parts are hollow. All the reasons we 

 can collect appear to be in favour of its being a 

 solid mass, considerably denser than any known 

 rock. If this be so, and if we suppose the inte- 

 rior to be at any time scooped out, so as to leave 

 only such a shell as the above-mentioned specu- 

 lators have imagined, we should not be left in ig- 

 norance of the change, though the appearance of 

 the surface might remain the same. We should 

 discover the want of the usual force of gravity, 

 by the instability of all about us. Things would 

 not lie where we placed them, but would slide 

 away with the slightest push. We should have 

 a difficulty in standing or walking, something 

 like what we have on ship-board when the deck 

 is inclined ; and we should stagger helplessly 

 through an atmosphere thinner than that which 

 oppresses the respiration of the traveller on the 

 tops of the highest mountains. 



We see therefore that those dark and un- 



