The Secretary of the Treasury, by a circular letter addressed to collectors of the customs on the 7th day of October last, a copy of which is herewith transmitted, exercised all the power with which he was invested by law. In pursuance of the act of the 14th of August last, extending the benefit of our post-office laws to the people of California, the Postmaster-General has appointed two agents, who have proceeded, the one to California and the other to Oregon, with authority to make the necessary arrangements for carrying its provisions into effect. The monthly line of mail steamers from Panama to Astoria has been required to "stop and deliver and take mails at San Diego, Monterey, and San Francisco." These mail steamers, connected by the Isthmus of Panama with the line of mail steamers on the Atlantic between New York and Chagres, will establish a regular mail communication with California. It is our solemn duty to provide with the least practicable delay for New Mexico and California regularly organized Territorial governments. The causes of the failure to do this at the last session of Congress are well known and deeply to be regretted. With the opening prospects of increased prosperity and national greatness which the acquisition of these rich and extensive territorial possessions affords, how irrational it would be to forego or to reject these advantages by the agitation of a domestic question which is coeval with the existence of our Government itself, and to endanger by internal strifes, geographical divisions, and heated contests for political power, or for any other cause, the harmony of the glorious Union of our confederated States--that Union which binds us together as one people, and which for sixty years has been our shield and protection against every danger. In the eyes of the world and of posterity how trivial and insignificant will be all our internal divisions and struggles compared with the preservation of this Union of the States in all its vigor and with all its countless blessings! No patriot would foment and excite geographical and sectional divisions. No lover of his country would deliberately calculate the value of the Union. Future generations would look in amazement upon the folly of such a course. Other nations at the present day would look upon it with astonishment, and such of them as desire to maintain and perpetuate thrones and monarchical or aristocratical principles will view it with exultation and delight, because in it they will see the elements of faction, which they hope must ultimately overturn our system. Ours is the great example of a prosperous and free self-governed republic, commanding the admiration and the imitation of all the lovers of freedom throughout the world. How solemn, therefore, is the duty, how impressive the call upon us and upon all parts of our country, to cultivate a patriotic spirit of harmony, of good-fellowship, of compromise