[Inclosures.]

HEADQUARTERS DEPARTMENT OF THE PACIFIC,
San Francisco, Cal., January 28, 1862.

Brigadier General LORENZO THOMAS,

Adjutant-General U. S. Army, Washington, D. C.:

From the latest and most reliable information I am more strongly than ever impressed with the importance of an early and prompt occupation of Guaymas.

G. WRIGHT,

Brigadier-General, U. S. Army, Commanding.

Copy of letter from Thomas Robinson, Esq.

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Immediately on the receipt of this please call upon General Wright, and state to him, in my name, that by express received by me to-day from Arizona, and from most reliable source, I have the following statement:

The Southern troops, under Colonel Baylor, Military Governor, are expected at Tucson within ten days, numbering 900 men. It is said they will immediately make a strong and continued campaign on the Apaches. Reports say (which, I fear, is very probable) they march into Sonora. In the mean while their headquarters will be at Calabazas, on the line. Agents to purchase flour, corn, &c., are actually in the Territory under escort. Contracts for hay have already been given out. Brigadier-General Sibley, C. S. Army, with 3,000 men, takes command in Arizona and New Mexico, and will immediately attack the United States forces in New Mexico. The Southern soldiers are full of fight. Only the other day 100 crossed the Jornada del Muerto, drove in the pickets of the United States forces, and made 40 prisoners.

These same reports were afloat here three days since, when I arrived, and to-day are confirmed. There is no doubt in my mind as to the desires and intentions of these Southern forces. What the devil do they care for Arizona, without 100 souls in it, and nothing worth having there? They wish to march into Sonora, as is intimated from many sources, and take quiet possession, for we are not at present in condition to resist, having just passed through a very sore trial, although with success. If they once get possession of this State and its ports, the North may just as well give up the complete line through from Gulf of Mexico to Gulf of California, and it will require a supreme effort then to rout them.

This is no newspaper talk, but something certain, and the only way to avoid a most serious and difficult position is for the United States Government to send, without a moment's delay, the necessary forces to act.

Let me request of you to urge upon General Wright the necessity of this step. Let 1,000 men, properly equipped, be sent immediately to Guaymas, officered by gentleman of prudence and judgment, and I will see that they get through immediately to Arizona. The Governor and people will be too happy to see such a friend coming to their rescue. If necessary, let the general telegraph to Washington for the necessary powers, but he must act promptly, and I will guarantee his full success, and by adopting these measures he will have acquired a victory which will be more than galling to the South.

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I will take the contract for transporting troops and equipments to Arizona and furnishing everything necessary. You are aware the Congress of Mexico has given a cordial permit for transit of troops