408 SCIENTIFIC RECORD FOR 1885. 



about five years ago, and denominated the bolometer. The first of these 

 were made on the evening of November 12, 1880, chiefly with the view of 

 testing its sensitiveness, concentrating the hmar rays upon its face by 

 means of the 13-inch equatorial of the observatory, and a smaller convex 

 lens near its focus, when an average deflection of forty-two divisions of the 

 galvanometer scale was obtained. In June, 1883, the bolometer and its 

 adjuncts, having been much improved in the interval, measurements of the 

 lunar heat were resumed with apparatus better adapted to the purpose. 

 The heat radiated from the lunar surface may consist of three kinds in 

 different proportions : (1) Heat coming from the interior of the moon, 

 which will not vary with the phase ; (2) heat which falls from the sun on 

 the moon's surface, and is at once reflected regularly and iriegularly ; 

 (3) heat which, falling from the sun on the moon's surface, is absorbed, 

 raises the temperature of the surface, and is afterwards radiated as heat 

 of low refrangibility. The general conclusion of Professor Langley's ob- 

 servations is thus expressed by himself. "While we have found abun- 

 dant evidence of heat from the moon, every method we have tried, or 

 that has been tried by others, for determining the character of this 

 heat appears to us inconclusive; and without questioning that the 

 moon radiates heat earthward from its soil, we have not yet found any 

 experimental means of discriminating with such certainty between this 

 and reflected heat that it is not open to misinterpretation. Whether 

 we do so or not in the future will probably depend on our ability to 

 measure by some process which will inform us directly of the wave- 

 lengths of the heat observed." Combined with his experiments on solar 

 radiation, Professor Langley's investigations seem to })oint to the prob- 

 ability of a gaseous envelope to the moon, too small to make its presence 

 known in ordinary astronomical observations ; and it is well known that 

 Mr. Neison has advocated this view from other considerations in his 

 great work on the moon. (Athenceum.) 



Eclipse of the onoon, 1884, October 4. — In spite of the very unfavorable 

 atmospheric conditions in Central Europe, nearly forty observatories 

 participated in the observation of this eclipse and were able to furnish 

 valuable material, which will be made the subject of a special publica- 

 tion when the iiositions of all the occulted stars shall have been deter- 

 mined with the necessary precision. While awaiting the termination 

 of this great piece of work, Professor Struve gives a succinct resura6 of 

 the results due to the co-operation of so many workers in the Astro- 

 nomische Nachrichten, IsTos. 2640-46. 



Mars : The rotation period of Mars. — The seventh volume of the An- 

 nalfi of the Leyden Observatory contains a very thorough and painstak- 

 ing investigation by Professor Bakhuyzen of the rotation period of the 

 planet Mars. In ijrevious determinations one of two courses has usually 

 been adopted, either to compare drawings of Huygens or Elooke with 

 the most recent observations attainable, or to discuss some modern 

 series which seemed to promise to compensate for its restricted range 



