170 BULLETIN OF THE 



just described. The focal length required to be determined with 

 great accuracy, and this was readily effected. 



Another difficulty arose from the fact that the diameter of the 

 photographic picture on the negative was liable to variation, with 

 a varying length of exposure ; and the diameter of the image of 

 Venus is liable to an inverse variation of the same kind. If the 

 distance between the exterior boundaries of the sun and planet 

 were measured, this error would be liable to vitiate the result and, 

 hence, it was necessary to find the centers of the two images, and 

 measure the distances between these central points. Mr. Harkness 

 described the method by which this was satisfactorily accomplished. 



There were about twenty plates which gave anomalous results. 

 It was obvious after trial, that the difficulty was with the plates them- 

 selves and not due to the observers, since from any one plate a 

 number of observers obtained corresponding results. 



Mr. Harkness then spoke of the various methods employed to 

 ascertain the sun's parallax : 1st, by measuring the velocity of light, 

 and the time required for light to traverse known chords of the 

 earth's orbit; 2d, by measuring the aberration of light; 3d, by 

 measuring the parallax of the planet Mars ; and 4th, by the anal- 

 ysis of the motions of the moon ; all of which gave results in very 

 close agreement. 



The second communication was by Mr. G. K. Gilbert on 



THE ORIGIN OF THE TOPOGRAPHICAL FEATURES OF LAKE SHORES. 



This communication was reserved by the author. 

 After remarks by Mr. Antisell, the Society adjourned. 



202d Meeting. June 11, 1881. 



The President in the Chair. 

 Fifty-seven members and visitors present. 

 The minutes of the last meeting were read and adopted. 



The Chair announced to the Society that the General Committee 

 had resolved that at the conclusion of the present meeting the So- 

 ciety would stand adjourned until the second Saturday in October. 



The first communication of the evening was by Mr. J. J. Wood- 

 ward, the President of the Society, entitled 



