184 



Mr. G. H. Darwin on 



[Dec. 19, 



in the spectra of hydrogen vacua a notable difference in the lines 

 seen in the negative light, sometimes all and sometimes only one of 

 the recognised lines of hydrogen being visible in that, and in many 

 cases not visible in other parts of the tnbe. I had tried an experi- 

 ment with a hydrogen vacuum tube of Geissler ; but in that the 

 difference was but slight between the positive and negative lights, 

 though it was very great between the light in the narrow central part 

 of the tube and in the wide portions on each side of it, the crimson 

 light in the narrow tube giving a brilliant three-line spectrum, and the 

 blue light, both on the positive and negative side, giving a compara- 

 tively dim fluted spectrum of many bands. The difference between 

 the light of narrow and wide parts of the vacuum tubes has, I believe, 

 been noticed ; it is in this case the converse of the effects observed by 

 me in the air vacuum. 



II. " On the Precession of a Viscous Spheroid, and on the Re- 

 mote History of the Earth." By George H. Darwin, M.A., 

 Fellow of Trinity College, Cambridge. Communicated by 

 J. W. L. Glaisher, F.R.S. Received July 22, 1878. 



(Abstract.) 



This paper is a continuation of a previous one on the bodily tides of 

 homogeneous viscous spheroids (read on May 23rd), and it contains 

 the investigation of the rotation of such a body as modified by the 

 tides raised in it by external disturbing bodies. The earth is taken as 

 the type of the rotating body, and the sun and moon as types of the 

 disturbing ones ; this plan not only affords a useful vocabulary, but 

 permits an easy transition from questions of abstract dynamics to those 

 of direct applicability to the physical history of the earth. 



In the paper on tides it was shown that, if the disturbing influence 

 be expressed as a potential, which is expanded as a series of solid 

 harmonics, each multiplied by a simple time harmonic, then each such 

 term in the expansion corresponds with a tide in a viscous or im- 

 perfectly elastic sphere, which is independent of the tides corresponding 

 to all other terms. Also the height of every such tide is expressible 

 as a fraction of the corresponding equilibrium tide of a perfectly fluid 

 spheroid, and the tide is subject to a retardation which is a function 

 of the frequency of the generating term, and of the constants ex- 

 pressive of the physical constitution of the distorted spheroid. 



The case of the moon, supposed to move in a circular orbit in the 

 ecliptic, is treated first. The tide generating potential of that body 

 (of the type cos 2 — -|-*) has first to be expanded in the desired form ; 



* Terms of higher orders are shown to be negligeable. 



