The British brig-of-war Spy anchored in the harbor of Monterey this evening. She is from San Blas, with dispatches for Admiral Seymour. Her officers are perfectly silent as to news from the United States and Mexico. She leaves in a few hours for the Collingwood at the Sandwich Islands. She has, undoubtedly, news of moment, but will not reveal it. WEDNESDAY, AUG. 12. The U. S. ship Warren, under Commander Hull, arrived this afternoon in thirty days from Mazatlan, bringing the eventful intelligence that war had been declared between the United States and Mexico. The mysterious silence of the officers of the Spy is now explained. But their secrecy has availed them for only twenty-four hours. The war news produced a profound sensation here. The whole population were instantly thrown into groups in the corridors and at the corners of the streets. The hum of voices continued late into the night. It was an extinguisher on the hopes of those who had looked to Mexico for aid, or who had clung to the expectation that the American government would repudiate our possession of California, and order the squadron withdrawn. They now relinquish all idea of a return to their old political connection, and appear resigned to their fate, which seems inevitable. These disappointed families compose but a part of the population; another portion has become thoroughly wearied with revolutions, and are prepared to countenance almost any government that promises stability. THURSDAY, AUG. 13. The Warren sailed this morning for San Pedro, to convey the war intelligence to Com. Stockton. It will throw a new aspect upon his operations in California. Expediency gives place to moral necessity. We have now a double motive for exertion—national honor, which looks at home, and an enlarged philanthropy, which looks here. It is of but little moment what the ultimate action of our government may be in reference to California. It cannot change her destiny. She is severed forever from Mexico. Should our government attempt to throw her back on that country, she will not stay thrown back. The rebound will carry her further off than ever. She is on a wave which will not ebb till this generation have mouldered in their graves. FRIDAY, AUG. 14. Sixty of a tribe of wild Indians, who live in the mountains, about two hundred miles distant, made a descent a few days since upon a farm within thirty miles of Monterey, and carried off a hundred horses. Twenty of the tribe, with the chief, remained behind to secure further booty. Intelligence of this having reached Capt. Mervin, he dispatched a mounted force, apprehended them in their ambush, and brought them to Monterey, and delivered them over to our court for trial.