In 1871 Mr. Hall put in operation the first automatic electric block system, on the New York & Harlem Railroad, between the Grand Central Station and Mott Haven Junction. It was normal "safety." The wheels of a passing train striking a lever completed a circuit, which put the signal to danger, after the train, and held it so until the succeeding signal went to danger, when a separate circuit was completed, which released the former signal, allowing it to return to clear. The disadvantage in having the wheels of a train strike a lever to complete the circuit led Mr. F. L. Pope to experiment. After a successful attempt in transmitting an electric circuit through an ordinary track with fishplate joints, he made a signal test at East Cambridge, Mass. A section of track was insulated from the rest, with a wire circuit, including a battery and electromagnet for operating the signal, fastened at either end to the opposite rails. The metal wheels and axles completed the circuit, throwing the signal to danger against following trains. A detent served to keep the circuit closed until the next signal was reached, when a separate circuit released the detent, permitting the signal to clear. In 1879 this system was put in service, and, with some alterations, still remains in some localities. Following the original manual semaphore and the controlled manual system of operation came the pneumatic and electric systems, for localities which required a great number of signal movements. With the development of motors and batteries capable of economic operation, automatic signals of the semaphore type have been successfully and widely installed. In the semaphore system numerous failures have occurred, due to the formation of ice and sleet upon the blades. This has led to the introduction of the so-called "upper quadrant" operation; that is, the motion of the signal being from horizontal to an upwardly inclined position and back. On account of the widespread prevalence of electric lighting and the building up of the territory adjacent to railroads, changes in the color indication of night signals have been adopted, generally in such localities using green instead of white for the safety indication. INTERLOCKING.