PHILOSOPHICAL SOCIETY OF WASHINGTON. 51 



Standard bar. Mr. Hilgard described the experiments which had 

 beea made for the determination of this dilatation, and exhibited 

 a diagram showing the great accordance of the results that had 

 been obtained for determining this essential element. The base- 

 line in question was nearly six miles in length ; and permanent 

 monuments having been placed at intervals of about one mile, 

 the^ three measurements give eighteen comparisons of measure' 

 which show in everj instance a very close agreement. The 

 following are the differences of the several measurements of the 

 entire base-line of 9339 meters from their mean, and the mean 

 temperatures at which they were obtained :_ 



Diff. ofjength . . _8.1 _o.3 +8.4 mm. 



Temperatures . . 51°. 4 440.7 90°. T Fahr. 



Showing a maximum divergence of the results of about one- 

 milhonth part of the whole length. Mr. Hilgard claimed in 

 conclusion, that no base-line of such length had before been 

 measured with the same degree of precision and with such 

 abundant evidence of the same. A full account of this opera- 

 tion will be published in the Coast Survey Report for 1873 



921) Meeting. October 9, 1875. 



Vice-President Taylor in the Chair. 

 Twenty-four members and visitors present. 

 Mr. E. S. HoLDEN exhibited and made remarks on 

 two drawings of nebula, made with the xxvi.-inch tele- 

 scope OF the U. S. naval observatory, by MR. L. TROUVELOT 

 OF CAMBRIDGE, MASS. ' 



(abstkact.) 



S ^'.J^ZV^'^Z ""^^ ^"^.'t.«d by the Superintendent of the U. 

 b. J. aval Observatory to visit the Naval Observatory, and to use 

 the XXV,. mch equatorial for the purpose of making draw nl of 



ft^Ts?t'9"w" n-.'l".'^^^^™^ ™ «"^bled fo remSnIy 

 f om Sept 21 to Oct. 1, 1875, and hence less time was bestowed 



o Tn^of the' 7r?^ ''' % P^t^^^^ satisfactory representadoa 

 of any of the objects viewed. Nevertheless, good pastel draw- 



