REPORT OF THE SECRETARY. 7 



the Transatlantic Longitude. The principal results of this investigation 

 were communicated by Dr. Gould to the meeting of the National 

 Academy of Sciences, during its session in 18G7, and the report in full 

 was presented to the Institution for publication, with the consent of the 

 Superintendent of the Coast Survey, Professor Peirce, in February 

 1869. It relates to the determination of the difference of longitude 

 between England and America, by means of the electro-magnetic tele- 

 graph. This method was first practically applied in the Coast Survey 

 of the United States,. between places in this -country, and received its 

 full development in this great national work before it began to be used 

 elsewhere. Previous to the introduction of this method, three others 

 had been employed on the Survey. First, that of observations of the 

 time of the culmination of the moon at the two places between which 

 the difference of longitude, or, in other words, of time, was to be deter- 

 mined; second, that of observing the times of beginning and ending of 

 eclipses of the sun and of occultations of known stars by the passage 

 over them of the moon ; and third, that of transporting a large number 

 of chronometers between England and America a number of times in 

 succession and obtaining the average difference of time as indicated by 

 the whole series. The determinations of the transatlantic longitude 

 which have been obtained by these methods have been generally re- 

 ferred to the Capitol at Washington and the Observatory at Greenwich, 

 England, as standard points. From the several methods just men- 

 tioned, and also from that of the electro-magnetic telegraph, the fol- 

 lowing results have been obtained : 



From eclipses and occultations the difference in time 

 between the dome of the Capitol and the Greenwich 

 Observatory is 5 h 8 m 14 S .S6 



From moon culminations the difference is 5 h S" 1 10 s .12 



From the transportation of chronometers the difference 

 is 5 h 8 m 12 s .30 



From the transmission of electricity through the cable, 

 and the use of the electro-magnetic telegraph, the dif- 

 ference was found to be 5 h 8 m 12 s .45 



By the last method the difference of time between the ends of the 

 cable was probably determined within the fraction of a second, but as 

 the signals could not be sent directly between Greenwich and Washing- 

 ton, a somewhat greater departure from the actual difference must be 

 allowed. This difference is, however, very small, not exceeding, per- 

 haps, a single second. 



The process of ascertaining the difference of time, or in other words, 

 the difference of longitude of two points, by means of the telegraph, 

 consists in transmitting a series of signals either way through the con- 

 ductor, and in observing the exact time of the appearance of the signal 

 at one station, while its time of starting is registered at the other. If 



