LUMINIFEROUS MEDIUM, AND THE MECHANICAL VALUE OF SUNLIGHT. 61 



tary space, if rarefied according to Boyle's law always, and if the earth were at 

 rest in a space of constant temperature with an atmosphere of the actual density 

 at its surface.* Thus the mass of air in a cubic foot of distant space several times 

 the earth's radius off, on this hypothesis, would be j^o — p5 '•> while there can- 

 not, according to the preceding estimate, be in reality less than y^™ — tatt , which 

 is 9 x 10 m times as much, of matter in every cubic foot of space traversed by the 

 earth. 



* " Newton has calculated (Princ. iii., p. 512) that a globe of ordinary density at the earth's 

 surface, of one inch in diameter, if reduced to the density due to the altitude above the surface of 

 one radius of the earth, would occupy a sphere exceeding in radius the orbit of Saturn." — (HerscheV s 

 Astronomy, Note on § 559.) It would (on the hypothesis stated in the text) we may now say 

 occupy a sphere exceeding in radius millions of millions of times the distances of any stars of which 

 the parallaxes have been determined. A pound of the medium, in the space traversed by the earth, 

 cannot occupy more than the bulk of a cube 1000 miles in side. The earth itself, in moving through 

 it, cannot displace less than 250 pounds of matter. 



VOL. XXI. PART I. 



