OPTICAL TELEGRAPHY. 
21 
position correctly or approximately. The complete revolution 
of the mirror serves as in Leseurre’s system to sweep the 
horizon with a luminous beam, which attracts the attention of 
the corresponding station. 
At about four yards in front of the apparatus is a staff which 
acts as a mark between the centre of the heliograph and the 
corresponding station. On this staff two pieces, H, h 1 , slide 
up and down. One of these is placed in a line between the 
mirror and the corresponding station. The other one 
carries a horizontal wooden cross-piece, H 1 , about a foot 
long. When the apparatus is not in use, the ray of sun¬ 
light reflected by the mirror falls upon the latter. Bat 
w T hen the Morse key is touched, the ray is carried towards 
the upper slide, h, in the line of communication. The 
operator, on seeing the upper slide illuminated every time he 
presses the key, is certain that his signals reach the corre¬ 
sponding station exactly. In England the signals sent by this 
apparatus can be seen at the distance of 50 miles ; but in 
India and in similar climates they carry to a distance of 70 
or 100 miles ; and the rotundity of the earth appears to set 
the only limit to their range. The adjustments which have 
to be made in the position of the mirror, in consequence of 
the sun’s daily motion, are easily effected by means of appro¬ 
priate regulators, even while the signals are being transmitted. 
It will be observed that the upper slide stops the luminous 
rays from reaching the distant station when the Morse key is 
pressed. In this system, therefore, the signals are given by 
stopping the light; that is to say, the solar rays reflected by 
the mirror continuously indicate to the distant station the 
position of the other ; and it is when the light disappears 
that a signal is being made. In this consists almost all the 
novelty of the system ; the application of the Morse key to a 
system of correspondence by rays of light dates farther back, 
and was adopted as early as 1863 by the squadron engaged 
in laying the submarine cable in the Persian Gulf. MM. 
Lissajous and Ternant had also in 1870 proposed a system 
of optical telegraphy by light shown by means of a Morse 
key, and the application by Mr. Henry C. Mance of this 
instrument dates at farthest only from 1877. 
The heliograph apparatus of Mr. Mance weighs only 6 lb., 
