ON THE DETERMINATION OP TRANSATLANTIC LONGITUDES. 439 



graph. From it the station of Manomet, in the primary triangulation of the Co ~t 

 Survey, and the secondary station at West Duxbury are both visible. By a pi lim- 

 inary triangulation, resting on a measured base of 380.17 feet, the centre of tin 

 Transit-instrument was found to be 6161.73 feet east, and 4861.19 feet north, of the 

 secondary station at West Duxbury. Its latitude is 4_! J 2' 53".l N. This position had 

 the advantage of an open meridian line to the south of two thousand feet in length. 

 A granite block, with a copper bolt inserted in it, marks the precise spot over which 

 the centre of the Transit-instrument stood. 



* 



The observations for instrumental corrections, thread-intervals, and time were made 

 here, as at Brest, according to the chronographic method, and the instruments u 'd 

 were C. S. Transit No. 6, the Krille Sidereal Clock, and U. S. C. S. Chronograph No. 2. 

 The focal length of the Transit-instrument is forty-six inches, and its aperture two 

 inches and three quarters of an inch. The value of one division of level (B) is equal 

 to 0.86 of a second of arc, or 0.057 of a second of time. The correction to be applied 

 to the lamp-end of the axis for inequality of pivots, as deduced by observations on 

 three different nights, is 0.027 of a second of time, the lamp-cud being the small t. 



The performance of the Krille Clock is reported as tolerably satisfactory, though it 

 appeared to be «m<fer-compensated. The Chronograph No. 2 had been cleaned and 

 put in good order by the Messrs. Bond before it was taken to Duxbury, but Mr. 

 Goodfellow found that it was difficult to keep it in good adjustment. A fresh deter- 

 mination of the thread-intervals was made at Duxbury. When this instrument was 



v twisted 



unpacked, it was found that threads (C 4 ) and (C 6 ) had become hopele 

 together, so that they were useless throughout the campaign. 



The observations at Cambridge were under the charge of Professor Joseph Winlock. 

 Director of Harvard College Observatory, those for instrumental corrections and for 

 time being made with the Sidereal Clock and the Chronograph of the Observatory by 

 Mr. E. P. Austin, then an assistant at that place. Coast Survey Transit No. 5 Wll used. 

 Its position was twenty-eight feet west of the dome of the Observatory, or 0.025 of a 

 second of arc. The value of one division of the level is 0.96 of a second of I re, or 

 0.064 of a second of time. The lamp-pivot is the smallest, and requires a correction 

 of 0.014 of a second of time. No new observations were made for the determination 

 of the thread-intervals. The diaphragm of the Transit-instrument at each of the three 

 stations, Cambridge, Duxbury, and Brest, consist* of twenty-five threads, arranged m 



five tallies. . , 



Whenever the weather permitted, a large set of observations were made before and 



after exchanging longitude signals, in order to ascertain the instrumental corrects, 



