466 Royal Societij :— 



laj-ers between the bine strata), must, I think, make these white strsvta 

 become ultimately, in reality, the surfaces of " sliding" or of " tear- 

 ing," or of chief differential motion, as according to Professor 

 Forbes's observations they seem to be. His first statement on the 

 subject, made as early as 1842, that " the blue veins seem to be per- 

 jicndicular to the lines of maximum ])ressiu'e," is, however, more in 

 accordance with their mechanical origin, according to the theory I 

 now suggest, than the supposition that they are caused by the tear- 

 ing action which is found to take place along them when formed. 

 It appears to me, therefore, that Dr. Tyudall's conclusion, that the 

 vesicular stratification is produced by pressure in surfaces perpendi- 

 cular to the directions of maximum pressure, is correct as regards the 

 mechanical origin of the veined structure ; while there seems every 

 reason, both from observation and from mechanical theory, to accept 

 the view given by Professor Forbes of their function in glacial motion. 



The mechanical theory I have indicated as the explanation of the 

 veined structure of glacial ice is especially applicable to account for 

 the stratification of the vesicles observed in ice originally clear, and 

 subjected to differential pressure, by Dr.Tyndall ; the formation of the 

 vesicles themselves being, as remarked in my last letter*, anticipated 

 by my brother's theory, published in the ' Proceedings ' for 

 May 1857. 



I believe the theory I have given above contains the true explanation 

 of one remarkable fact observed by Dr. Tyndall in connexion with the 

 beautiful set of phenomena which he discovered to be produced by 

 radiant heat, concentrated on an internal portion of a mass of clear 

 ice by a lens ; the fact, namely, that the planes in which the vesi- 

 cles extend are generally parallel to the sides when the mass of ice 

 operated on is a flat slab ; for the solid will yield to the " nega- 

 tive" internal pressure due to the contractility of the melting ice, 

 most easily in the direction perpendicular to the sides. The so-called 

 negative pressure is therefore least, or which is the same thing, the 

 positive pressure is greatest in this direction. Hence the vesicles of 

 melted ice, or of vapour caused by the contraction of melted ice, 

 must, as I have shown, tend to place themselves parallel to the sides 

 of the slab. 



The divisions of the vesicular layers into leaves like six-petaled 

 flowers is a phenomenon which does not seem to me as yet so easily 

 explained ; but I cannot see that any of the phenomena described by 

 Dr. Tyndall can be considered as having been proved to be due to 

 ice having mechanical properties of a uniaxal crystal. 



June 17.— The Lord Wrottesley, President, in the Chair. 

 The following communication was read : — 

 "On the Problem of Three Bodies." By Charles James Har- 

 grcave, LL.D., F.R.S. 



The author states that the principal object of this memoir is to 

 set forth two new methods of treating the dynamical equations by 

 processes of variation of elements, differing from the ordinary pro- 

 * See Proceedings for February 25, 1858. 



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