This sheathing with sheet copper is not necessary. It is quite sufficient to enclose the building with a network of a good conducting substance. For instance, if a copper wire, say No. 4, B. W. G. (0.238 inches diameter), were carried round the foundation of the house, up each of the corners and gables, and along the ridges, this would probably be a sufficient protection for an ordinary building against any thunderstorm in this climate. The copper wire may be built into the wall to prevent theft, but should be connected to any outside metal, such as lead or zinc on the roof, and to metal rain-water pipes. In the case of a powder-mill it might be advisable to make the network closer by carrying one or two additional wires over the roof and down the walls to the wires at the foundations. If there are water or gas pipes which enter the building from without, these must be connected with the system of conducting wires; but if there are no such metallic connections with distant points, it is not necessary to take any pains to facilitate the escape of the electricity into the earth. But it is not advisable to put up a tail pointed conductor. STEAM MACHINERY AND PRIVATEERING. Mr. Barnaby, a prominent English naval constructor, has written a memorandum on the British mercantile marine as an adjunct to the navy in time of war. He points out that privateering has been made obsolete, not merely by popular feeling, but also by the progress of the arts. A privateer, he thinks, must be prepared to meet regular ships of war of about the same strength. This the introduction of steam machinery has made impossible. War ships are built for security, merchant steamers for economical work, and the different objects have necessitated different arrangements. In a word, the machinery of war ships is carefully disposed below the water line, that of marine vessels is usually above the water line. The latter would therefore be much more subject to injury from shot than the other. This state of things excludes from service as privateers all but the swiftest vessels, and Mr. Barnaby thinks that the use of the merchant marine "would be confined to ships that could save themselves by their speed if they met a ship of war, whether armored or not," and that only those which can steam eleven and a half or twelve knots an hour can be considered serviceable for privateering. This limits the number of vessels available for this service to 400 or 500, and the common idea that England can, in case of war, "cover the sea" with her ships is proved to be untrue. Even these vessels could not be used as privateers except against certain nations. The Government would be compelled to buy them, and this would cost, he estimates, a hundred to a hundred and fifty million dollars. This addition to the regular fleet he thinks would enable England to "close up every hostile port, and the slow steamers and the helpless sailing