Prof. Challis on the Occultation of Saturn by the Moon. 315 



and the water subsequently to be refrozen to form the blue transpa- 

 rent laminae. 



Mr. Hopkins did not profess to maintain the entire adequacy of 

 the above explanation of the formation of the laminated structure, 

 though he could not but feel persuaded that it was founded in truth. 

 It seemed to explain very satisfactorily the conversion of n^ve, or opake 

 white ice, into transparent blue ice ; but he did not well understand 

 how transparent consolidated ice could be converted by pressure 

 into white opake ice. But still, at the bottom of an ice-fall, as that 

 of the glacier of the Rhone, the broken fragments were again united 

 into a continuous mass, and the laminated structure was reproduced 

 on a type entirely new, and conformable to the altered conditions of 

 the mass ; and, assuming a large portion of the ice descending the 

 fall to be in a sufficiently consolidated state, the process of recon- 

 struction must consist as much in the conversion of blue ice into 

 white, as of white ice into blue. Mr. Hopkins was not aware 

 whether this difficulty had been previously started, or, if so, what 

 ansv/er had been made to it. His object was more especially to 

 point out the essential distinctions between Professor Forbes's and 

 Dr. Tyndall's theories respecting the laminar structure. A disposi- 

 tion had recently manifested itself to confound the two theories, 

 whereas they were so fundamentally different, that the physical rea- 

 soning essentially involved in the one Avas totally inapplicable to 

 the other. The differences were such as could not be ignored, 

 if we would hope to arrive at a complete and final view of the 

 subject. 



The remarks in this lecture, Mr. Hopkins said, had been made 

 with a sincere desire of eliciting the truth, and not in the mere spirit 

 of advocacy of preconceived opinions ; nor would it, he conceived, be 

 inconsistent with this assertion if, in conclusion, he reminded those 

 who were interested in the subject, that though his own investiga- 

 tions nearly fifteen years ago respecting the internal pressures and 

 tensions of glacial masses were little noticed then, and had been little 

 mentioned more recently, no one had ever attempted to refute them ; 

 and now, on the contrary, all those observations and experiments of 

 Dr. Tyndall which related to this part of the subject, and were at 

 present generally received, were entirely confirmative of them. The 

 nature of the reasoning which has now been apphed to the subject, 

 whether founded on analogies with certain phaenomena of lamina- 

 tion, or on thermal principles, clearly proved the necessity of more 

 accurate conceptions of these internal pressures and tensions than 

 could ever be acquired from elucidatory experiments. 



May 30. — " On the Occultation of Saturn by the iVfoon on May 8, 

 1859." ]Jy Professor Challis. 



In observing this occultation. Professor Challis was prepared to 

 tike especial notice of the occurrence of any phaenomcnon like that 

 witnessed at the occultation of Jupiter on January 2, 1857, on which 

 occasion tlie disc of the planet at emergence was seen to be traversed 



