14 THE TRANSATLANTIC LONGITUDE. 



service in computations of this kind, as well as in numerous otlier astronomical 

 observations and reductions. To both these gentlemen I desire to make acknow- 

 ledgment for their valuable services in the office as well as in the field. 



The nature of the undertaking had, of course, thus far precluded any deter- 

 mination of personal equation between the observers. This was provided for with 

 as little delay as possible. My plan had contemplated the entire elimination of 

 this disturbing element at Heart's Content, since it would affect the longitudes 

 of Calais and Valencia equally, but with opposite signs. It proved that this pre- 

 caution had been overlooked, and that the time had been determined by Mr. Dean 

 during the exchanges of signals with Europe, and by Mr. Goodfellow during those 

 with the United States; but, as will be seen, this proved of no practical import- 

 ance. During a long series of years the personal equation between these two 

 gentlemen, as determined several times annually, was inappreciable ; and so, too, it 

 proved in the comparisons made after their return from the present expedition. 

 At the earliest practicable date extensive observations were made for the determi- 

 nation of the personal equations between each pair of observers. The results of 

 these will be given in their place. 



It may, perhaps, be well to add a few words concerning the instruments used, 

 which were the regular apparatus of the telegraphic party of the Coast Survey, 

 consisting at each station of a transit-instrument, a chronograph, and a circuit- 

 breaking clock. 



The transit-instruments have an aperture of about 7 centimeters, and a focal 

 length of about 116 centimeters. Each is provided with a reversing apparatus 

 attached to an iron stand, and capable of reversing the instrument with ease in about 

 twenty seconds ; so that it is not difficult to observe a star, in one position of the 

 axis, within 30 or 35 seconds after observing it in the other. The illuminating 

 lamps are placed on brackets unconnected with the instrument, and as far from it 

 as possible. The reticule carries five " tallies" or sets, of five spider-lines each, at 

 intervals of about 2| equatorial seconds of time, the several tallies being separated 

 from each other by twice this distance. The tallies are denoted by letters of the 

 alphabet from B to F inclusive, and the individual threads by subjacent numbers, 

 the numeration beginning with the " Lamp End" or end at which the illumination is 

 admitted to the field, so that when this end is west, a star at its upper cvilmination 

 traverses the threads in the direct order of their numeration from Bi to F,,. The 

 instruments are provided with diagonal eye-pieces of magnifying power not far from 

 100, and signal keys are permanently fixed on each side in convenient positions. 

 The chronographs at Valencia and Heart's Content were " Spring Governors" by 

 Messrs. Bond & Son; that at Calais was a " Kerrison's Regulator," Avith modifica- 

 tions by Mr. Saxton. Upon all of them one pen, which is constantly tracing a 

 line upon a revolving cylinder, records the signals both of the clock and of the 

 observer by offsets from this normal line. 



The experience of eighteen years has shown that the greater simplicity of the 

 apparatus, when provided with but a single electro-magnet and recording pen, far 

 overbalances in the longitude work of the survey any inconveniences arising from 

 a possible confusion of the clock-signals with those given by the observer. The off^ 



