20 



REPORT OF THE SECRETARY. 



the transmissioD of the signal were instantaneous, the difference of time 

 observed would be the exact difference of longitude ; but if any time 

 elapses between the making of the signal at one station and its appear- 

 ance at the otlier, the difference of time will be greater than the true 

 time when the signal is sent eastward, and less than it when sent west- 

 ward. If tlie velocity of transmission be the same in both directions, 

 the true difference of longitude will be obtained by taking the mean of 

 two sets of differences, the one giving the longitude as much too great 

 as the other gives it too small. 



From exi^eriments on the velocity of transmission of electrical im- 

 pulses through long conductors, it is probable that the first part, as it 

 were, of the electrical wave reaches the distant station in an inappre- 

 ciable moment of time, but that in order to overcome the inertia, and 

 give perceptible motion to the signal apparatus, an accumulation of 

 power is required. The time necessary to this accumulation will 

 depend on the weight, and other causes of resistance in the apparatus, 

 and also on the intensity of the electrical current 5 hence, the two instru- 

 ments ought to have i^recisely the same degree of sensitiveness, and the 

 battery daring the continuance of the exi)eriment should retain the same 

 electro-motive power. 



The method of telegraphing, employed with the cable, is that devised 

 by Sir William Thomson, of Glasgow, and is founded on the ap])lica- 

 tion of the principle of reflection first applied to instruments of pre- 

 cision by our countryman, Joseph Saxton, of the Coast Survey. It 

 consists of a mirror of about half an inch in diameter, to the back of 

 which is attached a small magnetic needle, the joint weight of the two 

 being less than one grain, which is suspended, by a single fiber from 

 the cocoon, in the center of a coil of many spires of fine wire, forming 

 part of the galvanic circuit. Upon this mirror is thrown a beam of 

 light through a slit in front of a bright kerosene lamp, and the deflec- 

 tions of the needle are noted by the movements of the reflected beam 

 received upon a slip of white paper. The exquisite delicacy of this gal- 

 vanometer, as well as the conducting j)ower of the telegraphic cable, 

 may be appreciated from the result of an exijeriment in which signals 

 were sent from Ireland to Newfoundland, a distance of 2,160 miles, by 

 means of a battery composed of an ordinary percussion gun cap, in 

 which was inserted a morsel of zinc and a drop of acidulated water. 



For determining the time in these observations, a small transit instru- 

 ment, a chronograph, and an astronomical clock were required at each 

 station, for the accommodation of which a temporary observatory was 

 erected. The observations on the American side were in charge of Mr. 

 G. W. Dean, with the assistance of Mr. E. Goodfellow, while those on 

 the coast of Ireland, as well as the general du-ection of the enterprise, 

 were under the charge of Dr. Gould, assisted by Mr. A. T. Mosman. 

 The use of the cable was freely granted by the Anglo-American Com- 

 pany, and Dr. Gould received from the astronomer royal, Dr. Airy, and 



