the line. A single individual, thus equipped, can "tap" a telegraph-line, in the daytime, by receiving the message in the ordinary way; and at night (when, of course, it would be easier to approach the line) by listening to the clicking of the armature against the electro-magnet of the instrument. But all these dangers are only of a partial or temporary character. By carefully patrolling and testing the line, it cannot be interrupted for any length of time without the damage being observed and repaired. By adopting a secret arrangement that there shall be a certain number of letters in the two or three words at the beginning or end of every message, a dispatch sent by an enemy can, in most cases, be detected; and again, by employing a cipher alphabet, it will be difficult for any one who taps the line to obtain information from the messages which fall into his hands.
From this brief sketch of the structure and uses of the field telegraph, the reader will understand what an important part it plays in modern warfare. On the march it directs the movements of advancing columns, on the battle-field it flashes orders and information with the speed of thought to right, centre, and left, of the immense lines extended over mile after mile of country; in beleaguered cities it places the whole defense from moment to moment under the eyes of those intrusted with its direction, and it is of no less value in the attack. It is not too much to say that, without this wondrous power, it would be almost impossible to direct the movements of the thousands on thousands of men, and guns, and horses, which form the vast armies of Continental Europe. It has effected a revolution in military science, none the less important because it is hidden from the general view, and seldom attracts the attention of even the ubiquitous special correspondent. Armed with all the weapons which inventive genius and mechanical skill can devise, the modern commander has the lightning also to do his work, and the electric current gliding on its secret path through the wide network of cable and wire tells him what is passing each hour in the remotest parts of the theatre of war, and transmits the mandates which decide the fate of nations.—Popular Science Review.
| HINTS ON THE STEREOSCOPE. |
By FRANKLIN C. HILL.
THE following diagrams will help to explain the principle on which this instrument acts. The stereoscope-glasses are halves of the same lens, placed with their outer edges toward each other (L L), Fig. 1. Rays of light (R R) from the objects (0 0), striking the oblique surfaces of the lens, are refracted outward, toward the focus, and thus reach the eyes (E E) in an oblique direction, appearing to come from
