DAVY'S ELECTRICAL TELEGRAPH. 27? 



like Venetian blinds, and each has a piece of cast iron at the opposite 

 end to counterpoise the weight, and make it move freely into all 

 positions. Now it is easy to conceive, that by repeating all those 

 positions with both arms extended together instead of one singly, various 

 combinations may be made, sufficient to express all the remaining letters, 

 and some other necessary signals. 



The first figure in the cut exhibits the whole telegraph from ; ts 

 foundation to its top, together with a section of the roof of the Admiralty 

 in which it is fixed. The second figure shows the internal machinsry 

 on an enlarged scale; the third exhibits the seven different positions 

 which the two arms (the only parts used in communicating information) 

 are made to take, and the extreme simplicity of the contrivance becomes 

 the more manifest from observing that the arms are always extended in 

 right lines. The remaining thirty figures, numbered from one to fin 

 (which latter indicates that the communication is completed), it will be 

 seen are all combinations of some two of the seven previously explained, 

 and for facility of reference are numbered to correspond ; the figure 

 twenty-seven for example, consisting of those marked 2-7 in the smaller 

 engraving. The figure pausa, indicates the completion of a sentence. 

 Several telegraphic dictionaries have been compiled, but Sir Home 

 Popham's is the one in general use in the navy ; it consists of thirteen 

 thousand words and sentences. Our cut exhibits all the signals used at 

 the Admiralty; but their various interpretations are frequently changed, 

 and known only to the officers engaged in that service. 



DAVY'S ELECTRICAL TELEGRAPH. 



This beautiful invention is one of the first and most useful applications 

 of that most extraordinary and subtle fluid, e^ctricity. 



If a piece of zinc, and another of copper, be Jaced near and opposite each 

 other without touching, in a vessel containing acid, such as oil of vitriol, 

 diluted with water, the two metals will acquire opposite states of electri- 

 city. If now a metalic wire of some length be connected by solder, or 

 any very perfect metallic contract with each metal, a current of what is 

 called voltaic electricity, or galvanism, will constantly flow along each 

 wire, from one end of it to the other. 



This electric current possesses magnetic properties, and if the conducting 

 wire be brought close over the needle of a mariner's compass, which 

 points north and south, it will turn it out of its direction, and cause it to 

 point east and west, or vice versa. The wire connecting the zinc and 



T 



