_Fourth._ The _record of intelligence is made in a permanent manner and in such a form_ that it can be at once bound up in _volumes_ convenient for reference, if desired. _Fifth._ _Communications are secret_ to all but the persons for whom they are intended. [11] It now occupies a space 10 inches long, 8 inches high, and 5 wide. These are the chief advantages of the Electro Magnetic Telegraph over other kinds of telegraphs, and which must give it the preference, provided the expense and other circumstances are reasonably favorable. The newness of the whole plan makes it not so easy to estimate the expense, but an _approach_ to a correct estimate can be made. The principal expense will be the first cost of the wire or metallic conductors, (consisting of four lengths,) and the securing them against injury. The cost of a single copper wire ¹/₁₆ of an inch diameter, (and it should not be of less dimensions,) for 400 miles, was recently estimated in Scotland to be about £1,000 sterling, including the solderings of the wire together; that is, about $6 per mile for one wire, or $24 per mile for the four wires. I have recently contracted for twenty miles of copper wire, No. 18, at 40 cents per pound. Each pound, it is estimated, contains 93 feet, which gives a result coinciding with the Scotch estimate, if $1.60 per mile be added for solderings. The preparation of the wire for being laid, (if in the ground,) comprehends the _clothing of the wires_ with an insulating or non-conducting substance; the _encasing them in wood_, _clay_, _stone_, _iron_, or _other metal_; and the _trenching_ of the earth to receive them. In this part of the business I have no experience to guide me, the whole being altogether new. I can, therefore, only make at present a rough estimate. Iron tubes enclosing the wires, and filled in with pitch and resin, would probably be the most eligible mode of securing the conductors from injury, while, at the same time, it would be the most costly. Iron tubes of 1½ inch diameter, I learn, can be obtained at Baltimore, at 28 cents per foot. The _trenching_ will not be more than three cents for 2 feet, or about $75 per mile. This estimate is for a trench 3 feet deep and 1½ wide. There is no _grading_; the trench may follow the track of any road, over the highest hills or lowest valleys. Across rivers, with bridges, the circuit may easily be carried, enclosed beneath the bridge. Where the stream, is wide, and no bridge, the circuit, enclosed in lead, may be sunk to the bottom.