Telegraphic Determinations of Longitude. 11 
tained with an instrument of this kind over the Atlantic cables, 
by the current proceeding from a battery composed of a single 
copper percussion cap with a small scrap of zinc and a drop of 
acidulated water. The use of the mirror is to make visible the 
movements of the magnet. The coil is mounted upon a standard 
so as to be about eight inches above the table. At the distance of 
eighteen inches or two feet is placed a lamp. This is surrounded 
by a screen which cuts off all the light, except that which passes 
through a tube directed towards the mirror. Lenses in the tube 
focus the light on the mirror and thence it is reflected to a vertical 
white surface, a sheet of paper for instance, at a suitable distance 
and appears as a small and brilliant spot. A movement of the 
magnet causes a horizontal deflection of this spot to the right or 
left depending upon the direction of the current passing through 
the coil. As these movements can be produced at will by means 
of the key at the sending station, it is only necessary to apply to 
them the dots and dashes of the Morse alphabet, to have a very 
ready and perfect means of communication. To the uninitiated 
spectator the facility with which the practiced operator translates 
these apparently meaningless movements is remarkable. If the 
cable is long and not in good condition the signals are sometimes 
almost imperceptible, while any slight jar of the table or appa- 
ratus will produce a large and irregular effect. Earth currents 
also will cause vibrations hard to distinguish from the signals, and 
if, as sometimes happens, the battery is connected in the wrong 
way, the signals will be reversed. In spite of these drawbacks 
the skillful operator reads off the message and rarely makes an 
error. This instrument is still in use on some of the cable lines, 
but on most of them it has been replaced by a recording instru- 
ment, also the invention of Sir Wm. Thompson, which is almost 
as sensitive, and of which I will speak later on. The key used in 
connection with these instruments, both the mirror and recorder, 
is arranged with two levers, so connected that pressing one of 
them causes a current to be sent over the line in one direction, 
while the other sends it in the opposite. 
The method adopted for comparing chronometers by means of 
these instruments was as follows :—Everything being ready tor 
the exchange of signals, the observer at one station seated him- 
self, where he could see the face of the chronometer, with his 
hand on the cable key. At ten seconds before the beginning of 
a minute as shown by the second hand, he pressed his key several 
