168 AXNUAL REPORT SMITHSONIAN INSTITUTION, 193 5 



six mile-atmospheres of methane above its surface, the pressure, due 

 to this alone, would be about 500 mm and the limiting temperature 



— 165° C. A large excess of hydrogen might reduce this to —183°. 

 Solar radiation alone would maintain a mean temperature near 



— 220°. Whether the difference arises from the powerful "green- 

 house" effect of the methane itself, or from internal heat, cannot 

 yet be determined. It may be, however, that if the methane could 

 once be frozen out of Neptune's atmosphere, the surface temperature 

 would fall so much that it would stay frozen and leave the planet 

 with an atmosphere which, apart from the inevitable Eayleigh 

 scattering, exerted no influence upon visible light. 



The problem of planetary atmospheres, so perplexing a few years 

 ago, is now far advanced toward its solution. Toward its interpre- 

 tation many of the sciences have contributed — astronomy, physics, 

 chemistry, geology, biology, and technology. No one of them alone 

 could have resolved the difficulties. It may, therefore, be appro- 

 priate that the attention of so general a scientific gathering may 

 have been invited for a while to it, for it truly illustrates the old 

 motto, " In union there is strength." 



