Annual Report on the U. 8. Coast Survey. 313 
tude of Washington, Hudson and Dorchester, with those of the 
European observations of Cambridge, Greenwich, Edinburgh, 
Hamburgh, Copenhagen, Kremsmunster and Cracow. The cor- 
tesponding observations in this country and Europe, in the year 
1839, furnish 428 differences of longitude. 
Use of the Magnetic Telegraph for Differences of Longi- 
tude.—The employment of this method of ascertaining longitude 
during the past year, has proved entirely successful between 
Washington, Philadelphia and Jersey City, opposite New York. 
Lieut. M. F. Maury again directed the codperation of the National 
Observatory ; the operations at Philadelphia were under the di- 
tection of Professor Kendall, and those at New York under Prof. 
Loomis. The details of the observations were arranged by As- 
sistant S. C. Walker of the coast survey. The principle of this 
method consists in transmitting signals from one station to an- 
other, and these noted by a time-keeper well regulated to the time 
of the place; the difference in the times gives the difference in 
longitude expressed in time. 
“The signals are given at one of the stations by pressing a 
key, which causes a closing of the circuit. This closing it is in- 
tended shall be simultaneous with the ticking of a clock or chro- 
hometer at the station. The circuit being closed, if the electrical 
wave or current takes a sensible time to propagate itself, or to pass 
‘fom one station to another, the absolute time of the signals reach- 
ing the second or receiving station, is sensibly different from that 
of making the signal at the first or giving station. A coil about 
the poles of a horse-shoe magnet of soft iron, forms part of the 
Circuit through which the electrical effect is transmitted. Under 
its influence the soft iron becomes magnetic, attracting the soft 
on bar (armature or keeper) delicately poised at a determined 
distance from the poles of the magnet; the movement of this 
*per sets in action a local battery, which gives sufficient power 
to make the dots and lines constituting the Morse telegraphic sig- 
nals.* The click of the keeper of this temporary magnet is com- 
pared at the receiving station, with that of a clock or chronome- 
ter, thus marking the time at which the signal, made at a known 
time at the giving station, is received. From this explanation, it 
appears that there is liability to error: 1. In the clock times at 
the different stations. ‘These are easily examined, and the most 
probable times assigned at each station, the personal equation for 
clock correction being determined and allowed for, they may be 
treated as if only very small errors existed. 2. The time of stri- 
king the trigger or key, to close the circuit, may not coincide with 
the clock beat, The error, if any, from this source is determin- 
able by experiment; and careful experiment failed to detect a 
Dts aa i 
* See a figure of this apparatus, in this volume, p. 56. 
