SIR: In compliance with the instructions of the commanding general of the department to make a "reconnoisance of the coast of California, with reference to the location of works of military defence," I reported in my last upon so much as relates to the upper province, and I now submit a few remarks on the military defence of the peninsula of Lower California. I. _General description of the coast and harbors._--The principal ports of the coast of Lower California, visited by whaling and merchant vessels, are San Quintin, Magdalena, San Jose, La Paz, Escondido (near Loreto), and Mulige.--There are some other points which vessels occasionally touch for supplies and at trade, but they are comparatively of little commercial or military importance. The port of San Quintin, in latitude thirty degrees twenty-three minutes, is represented as affording a secure anchorage for vessels of every description, and to be sufficiently commodious for the reception of a numerous fleet. The extensive bay of Magdalena has acquired considerable notoriety from its being resorted to every winter by large numbers of whaling vessels. Its size gives it the character of an inland sea, its waters being navigable for the distance of more than a hundred miles. It furnishes several places of safe and commodious anchorage. The bay of San Jose, near Cape Saint Lucas, is much frequented by coasting vessels, and occasionally visited by whalers and men-of-war. Being the outlet of a fertile valley, extending some forty or fifty miles into the interior, it is probably the best place in the peninsula for supplying shipping with water and fresh provisions. It is, however, a mere roadstead, affording no protection whatever during the season of southeasters.