Stevenson.] li ^ [Uay 20, 



half of the star turned towards us during that time ; and during an inde- 

 finitely short exposure might be said to represent the condition of the 

 surface visible to us as it was during a certain minute time interval when 

 the waves set out from that surface. [Neglecting for the moment the 

 modification of this statement which the curvature of the star's surface 

 would render necessary owing to the fact that the light which proceeded 

 from the extreme outer edge would have a longer distance to travel than 

 that in the centre by a little more than the radius of the star, and there- 

 fore its arrival at the instrument might be later than that from the central 

 portion. ] 



But the broadening of this line into a surface by making a slight differ- 

 ence between the rate of the clock-work and the angular motion of the 

 earth, would represent this same elongated surface of the star at differ- 

 ent times. In other words the one axis would represent different parts of 

 the star at the same instant of time, and the other axis would represent the 

 same region (the hemisphere visible to us) at different periods of time. 



If the movements of the atmosphere of the star observed were as rapid 

 and extensive as those of our own sun, the consequence would be that we 

 would have a succession of different conditions of the star's atmosphere 

 placed in close juxtaposition, the whole series representing all the changes 

 that had occurred in the star's photosphere during the interval of expo- 

 sure. On this account it would seem that this method was not adapted 

 to do more than give the resulting average of these changes on a sensi- 

 tive plate of measurable breadth and would not permit the condition of 

 the photosphere at any one instant of time to be studied. 



It would be interesting to know what effect a similar procedure on the 

 disc of the sun would show, by juxtaposing a large number of instantane- 

 ous photographs of the disc as different parts of the latter were successively 

 brought over the slit of the spectroscope. 



Notes on the Surface Geology of South-tcest Virginia. By John J. Stevenson. 



{Read before the American Philosophical Society, May W, 1SS7.) 



New river, rising in North Carolina, flows across the Archtean area of 

 Virginia, and enters the " Great Valley" of that State in Wythe county. 

 It flows througli Wythe and Pulaski counties, separates the latter from 

 Montgomery, and flows through Giles county into West Virginia on its 

 way to the Ohio river at Point Pleasant. It drains the counties named, 

 with the addition of Bland. The most important forks of the Holston 

 river flow through Smyth and Washington counties of Virginia into Ten- 

 nessee ; while the Clinch river rising in Tazewell county of Virginia flows 

 through Russell and Scott counties and drains much of Wise. It is joined 

 in Tennessee by Powell river, which drains Lee county and part of Wise. 



