Page:The Burton Holmes lectures; (IA burtonholmeslect04holm).pdf/213

 THE TOWER OF THE TELEGRAPH

THE "OPTICAL TELEGRAPH" STATION

A chain of towers, situated at intervals of from twenty to thirty miles, stretches southward from Biskra as far as Wargla, the last garrisoned oasis of the French. In each tower are two huge magic-lanterns, one pointing northward, the other southward. The lenses of these lanterns are provided with shutters, which open and close in response to a key on which the operator plays as on a telegraphic instrument. Thus at night news is flashed across the desert from tower to tower by intermitting rays of light. We climb the ladder-like