192 Remarks on the Dudley Observatory 



the station finally occupied was chosen, as most prohably 

 presenting the features requisite to the purpose. 



The point selected was the U. S. Coast Survey Primary 

 Station " ilelderberg " on the Helderberg range, about 

 fifteen miles west of Albany, in latii:ude north 42° 37' 

 37^55, longitude east of Washington 0^ 12"^ 9^77, 

 as determined by the U. S. Coast Survey, the elevation 

 of which above mean time is 1840 feet, as determined by 

 Mr. Johnson, the meteorologist of the party, from a series 

 of hypsometric observations extending over May 4th, 5th, 

 and 6th. 



In preparing for the transit proper, the diagram con- 

 tained in the pamphlet issued by the Superintendent of the 

 ITaval Observatory was at first used for examination, but 

 the relation of the sizes of the sun and planet was there so 

 much exaggerated, that it was thought advisable, in order 

 the more nearly to produce the exact representation of 

 features, to use a diagram giving the true relative dimen- 

 sions as well as the apparent angular magnitudes of the 

 two bodies. Accordingly such a diagram designed to be 

 viewed at a distance of 4500 feet, was constructed by Mr. 

 Wilson, having the four following phases : 



1. Indentation of Mercury on sun's limb, 0".26 



2. " " " " " 0''.50 



3. True internal contact 



4. Mercury wholly within sun's disc, least 



distance of adjacent limbs, 0''.25 



This diagram, arranged for ingress but equally suitable 

 for egress, was carefully studied with the telescopes to be 

 used on the expedition ; the line of sight passing for the 

 last few hundred feet within four or five feet of the ground, 

 thus introducing artificially the unsteadiness usually ac- 

 companying examinations of the sun's disc. 



