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THE POPULAR SCIENCE MONTHLY, 



built as lightly as possible, and weighs when loaded only 14 cwt., and 

 is easily drawn by two horses. On the outside are two insulated brass 

 conductors i (Fig. 2), to which wires can be attached. Inside the 

 windows is a shelf with a drawer f?, on which the instrument t can be 

 placed when in use, and opposite to this is a seat or bench s for the 



Fig. 1. 



Fig, 2. 



Prussian Station-Wagon. 



Section of Prussian Station-Wagon. 



operators, on which a man can sleep at night. Under the seat is a re- 

 cess in which a spare instrument t is kept, while the batteries are ar- 

 ranged in a box h under the shelf. When a message is to be sent from 

 this movable station the wagon is stopped, and the line-wire is attached 

 to the insulated conductor ^. This is connected with the instrument 

 and battery, and in order to complete the circuit the battery is placed 

 in electrical communication with the second insulated conductor, to 

 which another wire is attached which joins it to the earth-conductor 

 or earth-stake {piquet d terre) (Fig. 5). Thus the course of the cur- 

 rent, when transmitting a message, is from the battery to the instru- 

 ment, and by the first insulated conductor e (Fig. 2) to the line of 

 wire, the earth-plate of the receiving-station returning it to the earth- 

 conductor, driven into the ground near the wagon, and thus back by 

 the second insulated conductor to the battery. 



The instruments are of the Morse pattern, constructed so as to fit 

 in a very small space, and recording the signals with ink. The bat- 

 tery (of which there are two in each station-wagon) is a simple form 

 of M. Marie Davy's sulphate-of-mercury battery. It consists of ten 

 elements, one of which is shown in section in Fig. 8 ; c is a charcoal 

 A'essel, containing sulphate of mercury moistened with water to the 

 consistency of paste, and in this the zinc plate z is suspended by means 

 of the India-rubber cover I. The whole is placed in the India-rubber 

 vessel z, and a copper collar y is added, to which a connecting wire 

 can be attached. This battery has the advantage of being very port- 

 able, while the India-rubber cover prevents the charge of sulphate of 

 mercury from being spilt by the motion of the wagon. 



The line may be either an aerial or a ground wire, or a combination 



