REPORT OF THE SECRETARY. 19 



tlie Transatlantic Longitude. The principal results of this investigation 

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

 Academy of Sciences, during its session in 1SG7, and the rejiort in full 

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

 Superintendent of the Coast Survey, Professor Peirce, in February 

 18G0. 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 

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

 had been employed on the Siu-vey. 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 

 eclii)ses 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^ 8"^ 14«.86 



From moon culminations the difference is 5^ 8"" 10M2 



From the transportation of chronometers the difference 

 is 5^ 8=^ 12^30 



From the transmission of electricity through the cable, 

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

 ference was found to be 5'^ 8™ 12M5 



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

 cable was j)robably determined within the fraction of a second, but as 

 the signals could not be sent directly between Greenwich and 'Washiug- 

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

 allowed. This difference is, however, verj- small, not exceeding, j)er- 

 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 



