the Luminiferous JEthev. 401 



beyond our atmosphere. The zodiacal light is supposed to be 

 an evidence of meteoric matter between the earth and sun. 

 The tails of comets are visible by some action of light upon 

 some kind of matter. Matter in space not exposed to the rays 

 of the sun will be at about the same temperature as the aether; 

 but if in the rays of the sun and destitute of an atmosphere at 

 the distance of the earth from the sun, its temperature w T ould 

 be very low r . If present laws can be extended so far, and the 

 earth were without an atmosphere, and the heat received 

 were not conducted away, it has been computed that the mean 

 temperature at the equator would be about —70° C. ( — 94° F.); 

 and at the poles —221° 0. *, or 114° F. above absolute zero. 

 The last result is obtained on the supposition that the poles 

 receive heat directly from the sun a part of the year ; it is 

 further shown that if the poles were never exposed to the rays 

 of the sun, the temperature would fall to that of the aether of 

 space. But the data is not uniform, and there is too large an 

 extension of empiricaTTormulas to satisfy one that the above 

 numerical results are reliable ; still they point more and more 

 strongly to a temperature not many degrees above absolute 

 zero. 



3. By the essential heat of the sether we mean the tempe- 

 rature which would be indicated by a thermometer graduated 

 from absolute zero in a room located in space beyond our 

 atmosphere whose walls were impervious to the passage of 

 external heat. It is the heat due to the self-agitated sether, 

 just as air has a temperature when not exposed to the rays of 

 the sun. If the sether be perfectly diathermanous to the sun's 

 rays, it will receive no heat on account of the heat of the sun 

 flowing through it, though it may be heated from other 

 sources. As direct evidence of an extremely low tempera- 

 ture of space, we cite the facts in regard to the meteorite which 

 fell at Dharmsalla, India, July 14, 1860 f. " The most re- 

 markable thing about it was, while the mass had been inflamed 

 and melted at the surface, the fragments gathered immediately 

 after the fall and held for an instant were so cold that the fin- 

 gers were chilled. This extraordinarj^ assertion, which is con- 

 tained in the report with no expression of doubt, indicates 

 that the mass of the meteorite retained in its interior the 

 intense cold of the interplanetary space, while the surface was 

 ignited in passing through the terrestrial atmosphere." Since 

 this body had been exposed to the rays of the sun, its tempe- 

 rature must have exceeded that of the space through w T hich it 



* Professional Papers of the Signal Service, U. S. A. : Washington, 

 D. C. 1884, No. xii. p. 54. 



f Comptes Rendus, 1861, tome liii. p. 1018. 



