1 00 LECTURES 



carried repeatedly between the two stations on the Cunard steam- 

 ships.* 



" The differences of longitude between Cambridge and the prin- 

 cipal stations of the survey in other sections are determined by the aid 

 of the electric telegraph, wherever this has been established. In this 

 method, which is by far the most accurate for determining difference 

 of longitude, the Coast Survey has taken the lead, and has brought it 

 to a state of perfection which subsequent operations of a similar nature 

 executed in Europe have not yet reached. 



"The idea of comparing the local time of different places by means 

 of the electric telegraph is sufficiently obvious, and dates from the 

 conception of the telegraph itself; but the refined methods by which 

 the intervention of human senses and operations, and the consequent 

 liabilities to error, are, in the greatest possible degree, avoided, and 

 by which the time of transmission is measured and eliminated from 

 the longitude, have been the result of careful study and lona; expe- 

 rience. The method of recording observations of time on a chrono- 

 graphic register, by means of a galvanic circuit, known in Europe as 

 the American method, originated in the Coast Survey with the first 

 attempts to determine longitude by means of the electro-magnetic 

 telegraph. The chronogrophic record is made on a cylinder, revolving 

 with nearly uniform velocity, covered with a sheet of paper, u]wn 

 which a pen traces a line, interrupted or deflected for an instant 

 through the agency of an electro-magnet_, every time the pendulum 

 of the clock passes the vertical, and in doing so interrupts a galvanic 

 circuit. Either cylinder or pen are at the same time slowly moving 

 lengthwise, so that the line formed is a long spiral, which is thus 

 graduated into spaces corresponding to seconds of time, and described 

 with uniform velocity. When any instant of time is to be recorded, 

 the observer strikes a finger-key, which also breaks the galvanic 

 circuit and causes a similar mark to be made on the record, the position 

 of which, in reference to the adjacent seconds marks, can be read off 

 with great precision. In the chronographs employed in the Coast 

 Survey a second is generally represented by from one-half to three- 

 quarters of an inch, the cylinder being regulated so as to make one 

 revolution in half a minute. 



"By an ingenious arrangement in the clock the break for every sixty 

 seconds or minute is omitted, and every five minutes two breaks are 

 omitted. By this means a whole sheet may be read off without any 

 other note than the time of beginning and ending. 



"The method of determining longitudes by means of the electric 

 telegraph is substantially and in brief as follows : A transit instru- 

 ment, astronomical clock, and chronograph are mounted at each 

 station. Alter suitable observations for instrumental corrections at 

 each station, which are recorded only at the place of observation, the 

 clock at the eastern station is first put in connexion with the circui 

 so as to write on the chronographs at both stations. 



••■• The preceding account of the details of the operations of the Coast Survey has been 

 furnished by Professor Trowbridge, and what follows is copied from the report of a com- 

 mittee of the American Association for the Advancement of Science. 



