182 



KNOWLEDGE 



[Al-glst, 1901. 



discovery of this series of lines. If we accept this 

 formula, it would appear to be the true law connecting 

 their wave lengths, and would render them comparable 

 with those of other elements. The designation of the 

 lines is given in the first column of the follow ng table, 

 and the wave lengths derived by Mr. King in the second 

 column. The next two columns give th« value of », 

 taken from the sixth column of the table in Circular 

 No. 16, and the computed wave lengths taken from the 

 seventh column of the same table. A similar comparison 



1 



1 



with the formula ~ = 'J74t>l — 121 71*0 ;;;,, — 352010 

 is contained in the last two columns of the table. 



On the whole the observed values agi'ee more nearly 

 with the first formula than with the second. This is 

 remarkable, if it does not represent the true law, since 

 this formula contains no arbitrary constants. There 

 IS only one constant, and that is determined with great 

 ,'ucuracy from the ordinary series of hydrogen lines. The 

 second fonnula contains three arbitrary constants which 

 are selected so as to represent the observed value as 

 nearly as possible. A least square determination was 

 not considered necessary, since the outstanding differ- 

 ences from observation were evidently systematic, and 

 not accidental. The wave length of the line Ha:' differs 

 gi-eatly according to the two formvilas, but no means 

 as yet exist for determining radiations of such great 

 wave length in the stars. 



Cambridge, U.S.. Edward C. Pickering. 



February 11th, 1901. 



[The interest attached to the " second series " of 

 liydrogen lines arises from the fact that they were 

 identified as probably belonging to hydrogen by a purely 

 mathematical process. 



It is a remarkable circumstance that we are so much 

 indebted to a study of the celestial Ixjdies for our know- 

 ledge of the spectrum of hydrogen. Even the ordinary 

 series of lines in this spectrum was first recognised in its 

 entirety in the jjhotographic spectra of white stars, but 

 unlike these, the new series has not yet been reproduced 

 in terrestrial experiments. Carrying Prof. Pickering's 

 inquiry a step further, Rydberg has concluded that there 

 is still another series of hydrogen lines, of which the only 

 one in the visible spectrum is about wave-length 

 'J6S7.S; this also has not yet been artificially repro- 

 duced, but tlierc is evidence that it is represented by an 

 important line in the spectra of some of the Wolf-Rayet 

 stars and planetary nebulae. 



The Ihrce scries of lines of hydrogen thus differ from 

 tlie corresponding series in the case of many other 

 elements, inasmuch as they do not all appear simul- 

 taneously in the spectrum of the gas in laboratory 

 experiments. — En.] 



Hcttcr. 



[The Editors do not hold themselves responsible for the opinions 

 or statements of correspondents.] 



• 



LUNAR ATMOSPHERE AND OCEANS. 



TO THE EDITORS OF KNOWLEDGE. 



Sirs, — It has been considered (Proctor's " The Moon," 

 pp. 263-265) that the theoiy that the hypothetical 

 oceans and atmosphere formerly possessed by the moon 

 have been withdrawn to internal cavities is untenable — 

 though the existence of such cavities seems probable 

 enough — since they could not be large enough to contain 

 the volume of an atmosphere sufiicient in cjuantity 

 ever to possess any considerable density on the moon's 

 surface. Also, it seems very improbable that the 

 surface is at a sufficiently low temperature during the 

 day to allow the atmosphere to remain always in a 

 liquid or solid state on the surface. Now might we 

 not combine tliese two theories with advantage ? If all 

 the moon's heat has been lost, any cavities in its 

 structure must be at a temperature sufficiently low to 

 reduce all gases entering them to the solid state ; and 

 these cavities, a few miles from the surface, will not be 

 appreciably affected by solar heat. The cavities, which 

 must be supposed to be in connection with the surface 

 to some extent, would be full of air. When their 

 temperature became suflaciently low, this air would 

 liquefy, leaving an almost empty space. Into this space 

 there would be a constant slight influx of gas throughout 

 the day, the liquefaction, or, later, solidification of which 

 would leave room for fui-ther quantities of air. If the 

 temperature of the moon's mass has become equal to 

 that of surrounding space throughout, may we not 

 suppose that the whole, or very nearly the whole, of 

 the atmosphere, as well as the oceans, may not be within 

 the moon's mass, in the solid state, assuming, of couree, 

 that they have had an existence at all ? The small 

 cjuantities of heat introduced would be lost by con- 

 duction through the surrounding material, and could 

 never, of course, stop the process. 



J. O'May. 



BRiTfSFt 



\, 



/T\:_ ■ 



ornithologYca 



^^z^ 



ISk 



" V NOI£S::-_J 



Conducted hy Harry F. 'WiTnERBY, f.z.s., m.b.o.d. 



Red-fackd Variety av the Yellow Hammer. — At 

 the meeting of the British Ornithological Club, held on 

 June 19th, Mr. H. E. Dresser exhibited an aberrant fonn 

 of Eniberiza citriuella, with chestnut-red colour on the 

 throat like the Pine Bunting (Emheriza lencoceplmlit). 

 The specimen was olitained by Mi-. E. S. Montagu in 

 Cambridgeshire. Mr. Millais and Mr. Walter Rothschild 

 also exhibited British examples of this aberrant form. In 



