"The distance from Chagres to New York has recently been run in seven days. The same speed would carry a steamboat from Panama to San Francisco in ten days. Allow three days to convey freight across the Isthmus, on a railway, and both passengers and freight will be conveyed from New York to San Francisco in twenty days. "This celerity of movement would secure for American produce the entire market of California. Sailing vessels may be successfully employed between our Atlantic and gulf ports and the terminus of the railway on this side of the Isthmus; and _propellers_ from Panama to San Francisco. These latter vessels will be found peculiarly suited to that trade; they can use their steam through the calms of the Bay of Panama, and against head-winds and currents going north, and their sails with favorable winds and currents coming south. "These modes of conveyance, in connection with the railroad across the Isthmus, would be sufficiently expeditious and economical to turn the tide of commerce, between the Atlantic and Pacific States of the Union, into that channel. The tendency of our commerce on the Pacific to promote the employment of ocean steamers is of much importance as connected with the defence of our extensive line of coast from latitude 32° to 49°, the protection of the whale fishery, and other branches of trade on that ocean. The establishment of a line of heavy steamers to China would promote all these objects; increase our intercourse with that country, and probably be the means of opening communications with Japan. Money wisely employed in promoting these objects, it is believed, would add more to the power and prosperity of the country than its expenditure on any _general system_ of fortification at the present prices of labor and materials. There is one point, however, of such vast importance that no time should be lost in taking the necessary steps to render it perfectly impregnable--that is, the entrance to the harbor of San Francisco. On the strength of the works which may be erected to defend that passage will depend the safety of California in time of war with a maritime power. Permit a hostile fleet to cast anchor in the harbor of San Francisco, and the country would be virtually conquered. "The coast has not been surveyed, nor has its outline been correctly ascertained. There are many rocks above and below the water-line, and small islands not mentioned or indicated on any chart, which render navigation near the land, especially at night, extremely dangerous. "An accurate survey of the coast, to commence at the most important points, the construction of lighthouses, and the placing of buoys in proper positions, are objects of much importance, and, it is not doubted, will attract the early attention of Government."