In an autograph letter addressed to the author by Professor Henry, secretary of the Smithsonian Institution, under date of October 14, 1875, the Professor discusses Robinson's peculiar method of using batteries in signaling by which he obtained the above wonderful durability of 180 days or more without renewal, and pronounced the results obtained "very remarkable." His discussion of the subject is somewhat suggestive of the principles of the storage battery. Switches In 1876, 7 and 8, Mr. Robinson made a number of installations on the Boston and Providence, Old Colony and the Boston, Lowell and Nashua railroads. On the latter road, at Wilmington Junction, he equipped two parallel sections of the double track, including six switches, in this short space, five of them connected with one of the blocks. These sections were arranged as regular closed circuit blocks, operative under the moving trains. The switches were also connected up in such a way that every switch had to be closed and locked for the main line or the danger signal would be exposed against approaching trains. This installation was made in 1876. The switch connection applied to these switches is shown in Fig. 5 and a general plan of the same is illustrated in Fig. 6. Both of these figures are reproductions from Robinson's aforesaid British patent of 1879. [Illustration: Fig. 5.] [Illustration: Fig. 6.] It will be observed that when the switch is on the main line the wires 7, 8 are connected by the plug 6 on the switch connection, thus completing a working circuit through the rails and around the switch, but when the switch is placed for a siding the wires 7 and 9 are connected by the plug 5, thus short circuiting the current from the magnet M, thus producing the same effect as would the presence of a train on the section. It is always better to short circuit the current than trust to the mere opening of circuit since short circuiting is sure to produce instantaneous results. It will be observed, however, that in the above case the movement of the switch connection both opens the rail circuit and short circuits the current from the relay. It may be here stated that Mr. Robinson equipped three switches in one closed circuit block, in the manner described above, on the Philadelphia and Erie railroad in 1873. [Illustration: Fig. 7.] Fig. 7, from Robinson's English patent of 1879, aforesaid, shows the switch G arranged to operate the signal by hand from an office, station or telegraph post by the roadside, as heretofore described. Drawbridges