72 



THE POPULAR SCIENCE MONTHLY. 



Post-office, a vast increase of business might be done with but little 

 more expense. Accordingly, to gain the increased business they re- 

 duced the rates one-half, and succeeded but not in a pecuniary sense. 

 Prof. Jevons ascribes this disappointing result to the great cost of 

 erecting and maintaining the lines ; to their small carrying capacity 

 when compared with that of a railroad-train; and to the number of 

 hands and heads which each telegraphic message has to pass through 

 before reaching its destination, and which must all be paid. But the 

 progress of the last five years, made principally in this country, has 

 demonstrated that these difficulties are not insuperable. 



In order of time, the first important step toward this end was the 

 Duplex Telegraph of Mr. Joseph Stearns, of Boston, Massachusetts. Its 

 object is to allow of two operators using the same wire to send mes- 

 sages in opposite directions simultaneously. To persons having only 

 a general acquaintance with the ordinary working of the telegraph, 

 this at first seemed impossible ; and, when it was accomplished, it was 

 held by many some scientific men among the number to furnish 

 an indubitable proof of the theory that the electric waves, or currents, 

 or whatever they might be held to be, necessarily passed each other 

 in contrary directions over the wire. That they do not will be evident 

 from the subjoined explanation. 



It must be remembered that the galvanic battery gives birth to a 

 force which returns in a circuit to where it was generated, and accel- 

 erates the liberation of more force, being like a steam-en o-ine em- 

 ployed partly in fanning its own fire. This circuit can be performed 

 much more easily through great lengths of some substances, such as 

 the earth and metals, than through very small spaces of others, as the 

 air and the dilute acid of the battery. Galvanic electricity is, there- 

 fore, strictly confined in a sort of mill-round ; or it may best, for our 

 present purposes, be represented by water flowing through such a 

 system of water-courses as is shown in the annexed cut. "We will 



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a 



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it 



Fig. 1. 



suppose them to include a reservoir and a secondary circuit at each 

 end. Let the reservoirs A and B have water pumped into them by 

 force-pumps, and distributed by them to both the main and secondary 

 circuits, in equal quantities and in the direction of the arrows, so as 

 to maintain the water-wheels X and W in the same positions. The 

 highest points in the system must be supposed to be at the front of the 

 reservoirs, and the lowest at the back of them. 



If an additional volume of water come from J, being equally divided 



