It may be safely said that no one of the wonder-working agencies of the nineteenth century, of an importance in any degree equal to that of the Electric Telegraph, is so little understood in its practical details by the world at large. Its results come before us daily, to satisfy our morning and evening appetite for news; but how few have a clear knowledge of even the simplest rules which govern its operation, to say nothing of the vast and complicated system by which these results are made so universal! The general intelligence, at present, doubtless outruns the dull apprehension of the typical Hibernian, who, in earlier telegraphic times, wasted the better part of a day in watching for the passage of a veritable letter over the wires; but even now,--after twenty years of Electric Telegraphy, during which the progress of the magic wire has been so rapid that it has already reached an extent of nearly sixty thousand miles in the United States alone,--even now the ideas of men in general as to the _modus operandi_ of this great agency are, to say the least, extremely vague. Even the chronic and pamphlet-producing quarrel between the managers of our telegraphic system and their Briarean antagonist, the daily-newspaper-press, fails to convey to our general sense anything beyond the impression that the most gigantic benefits may be so abused as to tempt us into an occasional wish that they had never existed. One reason of this general ignorance has been the absence of any text-book or manual on the subject, giving a clear and thorough exposition of its mysteries. The present is the first American work which takes the subject in hand from the beginning and carries it through the entire process which leads to the results we have spoken of. Its author brings to his work the best possible qualification,--a long familiarity with the subject in the every-day details of its development. His Introduction informs the reader that he has been engaged for thirteen years in the business of practical telegraphing. He is thus sure of his ground, from the best of sources, personal experience.