PHILOSOPHICAL SOCIETY OF WASHINGTON. 125 



layers. By introducing various gases and liquids between the jaws 

 of the slit, and by utilizing the influence of electrical or magnetic 

 stress, or by other devices, the true explanation may be possibly 

 ascertained. 



Mr. William B. Taylor further observed that the experiments 

 of Mr. Michelson were certainly very original and beautiful, and 

 the problem does not seem to be soluble by any known data. Mr. 

 Abbe's explanation seemed to be quite plausible. Between the 

 jaws of a slit only .001"" in diameter, the interval is probably 

 near the limits of mean molecular excursions, and the condition of 

 air or any gas under those circumstances would be a novel one to 

 contemplate. The exclusion of some wave? of light observed by 

 Mr. Michelson might be attributable to the disturbing behavior of 

 molecules so situated and restricted in their motions. 



Mr. J. W. OsBOKNE stated that when this paper was presented 

 to the National Academy of Sciences, Mr. Michelson had alluded 

 to an explanation of similar purport to that of Mr. Abbe, but had 

 rejected it as being untenable. 



The Chair announced to the Society the election to membership 

 of Dr. Jerome H. Kidder, U. S. N., and Ferdinand Augustus 

 Hassler. 



The first communication for the evening was from Mr. G. K. 

 Gilbert on 



THE drainage SYSTEM OF THE BLACK HILLS OF DAKOTA. 



[Abstract.] 



Mr. Gilbert stated that it had never been his privilege to see 

 the Black Hills ; it had, however, been his duty to prepare for 

 publication and edit the work of the lamented young geologist, the 

 late Henry Newton, which had been left by him in the form o-f 

 copious notes and unrevised manuscript. This work is an account 

 of the geology of the Black Hills derived from Mr. Newton's 

 studies while examining that district under the direction of the 

 Interior Department. 



The Black Hills form an isolated district of mountain and 

 plateau rising out of the great plains of Dakota, and are separated 

 from other mountain ranges by a wide interval, the nearest being 

 the Big Horn mountains to the westward. They are thus well 



