All these machines are in simultaneous operation; that is to say, four messages are being sent from New York to Chicago, and four messages are being sent from Chicago to New York, all at the same time and over a single wire, and the entire process is automatic.  The method by which eight messages can be sent over a single wire at the same time without interfering with one another cannot readily be described in simple terms.  It may give some comprehension of the underlying principle to say that the heart of the mechanism is in two disks at each end of the line, which are divided into groups of segments insulated from each other, each group being connected to one of the sending or receiving machines, respectively.  A rotating contact brush connected to the line wire passes over the disk, so that, as it comes into contact with each segment, the line wire is connected in turn with the channel leading to the corresponding operating unit.  The brushes revolve in absolute unison of time and position.  To use the same illustration as before, the brush on the Chicago disk and the brush on the New York disk not only move at exactly the same speed, but at any given moment the two brushes are in exactly the same position with regard to the respective group of segments of both disks.  If we now conceive of these brushes passing over the successive segments of the disks at a very great rate of speed, it may be understood that the effect is that the electrical impulses are distributed, each receiving machine receiving only those produced by the corresponding sending machine at the other end.  In other words, each of the sets of receiving and sending apparatus really gets the use of the line for a fraction of the time during each revolution of the brushes of the distributer or disk mechanism.  The multiplex automatic circuits are being extended all over the country and are proving extremely valuable in handling the constantly growing volume of telegraph traffic.

What has thus been achieved in developing the technical side of telegraph operation must be attributed in part to that impulse toward improvement which is constantly at work everywhere and is the most potent factor in the progress of all industries, but in large measure it is the reflex of the growing—­and recently very rapidly growing—­demands which are made upon the telegraph service.  Emphasis is placed on the larger ratio of growth in this demand in recent years because it is peculiarly symptomatic of a noticeably wider realization of the advantages which the telegraph offers as an effective medium for business and social correspondence than has heretofore been in evidence.  It means that we have graduated from that state of mind which saw in the telegraph something to be resorted to only under the stress of emergency, which caused many good people to associate a telegram with trouble and bad news and sudden calamity.  There are still some dear old ladies who, on receipt of a telegram, make a rapid mental survey of the entire