10 THE TRANSATLANTIC LONGITUDE. 



immediate vicinity. Upon the mirror is thrown a beam of light through a slit in 

 front of a bright kerosene lamp, and the deflections of the needle are noted by the 

 movements of the reflected beam, which is received upon a strip of white paper. 

 The exquisite delicacy of this galvanometer, as well as the electrical excellence of 

 the telegraph cables, may readily be appreciated after the beautiful experiment in 

 which the electricians at Valencia and Newfoundland conversed with each other on 

 a circuit not far from 700 myriameters (4320 statute miles) in length, formed of the 

 two cables joined at the ends, using a battery composed of a percussion gun-cap, a 

 morsel of zinc, and a drop of acidulated water. 



The absence of any means for the automatic registration of signals received, 

 presented, of course, a very serious obstacle in the way of an accurate longitude 

 determination, inasmuch as the loss of time in noting the signals was not only very 

 considerable, but quite uncertain ; but the programme of operations which I had 

 prepared before leaving home was based upon the assumption that the use of self- 

 registering electro-magnetic signals would not be acceptable to the Tekgraph 

 Company. All objections to these were, however, waived in our favor by Mr. 

 Latimer Clark in the most cordial manner, and considerable time was expended on 

 two evenings in endeavoring to obtain satisfactory signals which should be self- 

 registering. Unfortunately, these efforts were unsuccessful. The cable could not 

 be discharged with sufficient rapidity for the purpose when the charge was suffi- 

 ciently strong to actuate our most sensitive electro-magnet. A permanent deflec- 

 tion only was observed at Newfoundland, while the Valencia clock was breaking 

 the circuit during an eighth part of every second; nor did any modification^n the 

 character of the battery render these interruptions of continuity perceptible at the 

 other extremity of the cable. 



I had previously designed availing myself of an ingenious suggestion of Dr. Gibbs, 

 by which the heat from the lamp should be concentrated and reflected, together 

 with the light, by the mirror-galvanometer; being then received on a very delicate 

 thermo-electric pile, which should thus record upon the chronograph the time of the 

 signals. But too little time was available for the purpose, and although Mr. Far- 

 mer, whom I had requested to prepare some apparatus based on this principle, 

 made sufficient progress with his experiments to show the practicability of the 

 suggestion, he was obliged to abandon all hopes of constructing any satisfactory 

 instrument in season to be available for our purposes. 



Thus it became necessary to fall back upon the original programme which had 

 been prepared before leaving Boston, and furnished to Messrs. Dean and Davidson. 

 This was as follows: — 



PROGRAMME FOR TRANSATLANTIC LONGITUDE CAMPAIGN. 



Tliis campaign will consist of two parts, "Heart's Content — Calais," and "Valencia — Heart's 

 Content." 



Star-signals being impracticable in each case, the only determinations of longitude will be by 

 comparisons of clocks between the stations ; consequently no precautions should be omitted which 

 can in any way increase the precision of the clock-corrections and rates. Only stars of the Ameri- 

 can Ephemeris should be employed ; levels should be continually read during the observations ; all 



