The New Student's Reference Work/Heliograph

He′liograph, an instrument chiefly used in military operations in signaling messages from one station to another by means of a small mirror, which reflects to a distant point the rays of the sun cast upon it. The instrument is simple and portable, and when used is made to send messages to a distant point by the use of long and short flashes, thus spelling out words and sentences by the use of the dot-and-dash or Morse alphabet. The essential difference between messages by the heliograph and by the ordinary telegraph “sounder” is that in the first case the message is read by sight and across the distance at the station sending the same, and in the latter by sound and at the station to which it is sent.