SIR: When I wrote my last letter I had not seen the capitulation of Monterey, at least the official report. It appears, that the terms were conditional, and allow no more time than is absolutely requisite for Gen. Taylor to make his arrangements for ulterior movements. The city was taken by a forced march, with only a part of the troops. The attempt to cast censure, by indirection, on this meritorious officer, betrays a jealousy of his growing popularity. The capture of Monterey is of immense importance, as well on account of its being one of the keys of Mexico, as on account of the vast amount of public property, in arms and munitions captured, which Mexico is in no condition to replace. That city was a stronghold under the Spaniards before the Mexican revolution, and contained a great quantity of cannon, transported with immense difficulty and expense, from the city of Mexico. It was in fact, the metropolis of the provinces of the Rio Grande; and if a new confederacy should be formed by those States, it would be the seat of the federal Government. In a few weeks, the sickly season will be over; the surviving sick will be again fit for duty, the commissariat will be enabled to complete its supplies, and transportation for the army, considerable reinforcements will reach the seat of war, and then we may expect an onward movement. General Taylor having now gained a safe footing, must go to work to consolidate his occupation, or if you will, his conquest of the country between the Rio Grande, the mountains, and the Gulf. This extensive tract of country, as large as Italy, and resembling it in scenery, climate, and soil, is now cut off by our arms and by its natural boundary, from the rest of Mexico. It ought to be completely occupied, by fortifying the principal points on the Rio Grande, the mountain passes of the Sierra Madre, and also by the occupation of the chief towns of New Leon, Coawilla, and Tamaulipas. Tampico, of course, must be taken by a combined land and naval attack. It has a shallow sea coast from Tampico to the mouth of the Rio Grande, of four hundred miles, whilst it stretches east and west, between the mountains and the river, nearly double that distance. Here is an extent of surface equal to that from the Potomac to the Savannah and east of the Appalachian chain. It is capable of containing four millions of souls, and in the hands of Americans, would be rapidly filled up by them and European emigrants. The climate is said to be delightful, and much more temperate than might be expected from its latitude. I have rated its population at five hundred thousand; this estimate may be too high; but it is possible, that the common estimate may be too low. The following is that given by a recent pamphlet, including Chewawa: