The Tehuantepec Surveying Expedition has returned to New Orleans. Surveys, which show the practicability of the railroad route, are complete. A few parties have been left on the ground to survey a line for the construction of a carriage road. The Coatzacoatlcos River is reported navigable, for twenty-five miles above its mouth, for ships drawing eleven feet of water. The climate is believed to be healthy. The Mexican government having evinced some unfriendliness to the Tehuantepec project, the interference of the United States has been solicited, but declined. The balance of the fourth installment of the Mexican Indemnity, under the Treaty of Guadalupe Hidalgo, was paid at the U.S. Treasury on the 28th of June--amounting to $1,815,400. The whole amount of the installment is $3,360,000. The Court Martial convened at Washington on the 23d June, for the trial of General Talcott, chief of the ordnance department, has closed its labors by the conviction of the accused of all the charges preferred against him, and his dismissal from the service. The charges were: a violation of the 132d article of the regulations for the government of the Ordnance Department; wilful disobedience of orders and instructions from the Secretary of War in relation to a contract for supplies; and conduct unbecoming an officer and a gentleman, among other things, in making a declaration which was positively and wilfully false, and intended to deceive the Secretary of War. Preparations for the next presidential canvass are being commenced in many of the States. General Scott has received the nomination of two state conventions--that of Ohio, and that of Pennsylvania--besides having been nominated at public meetings in Delaware, Indiana, and other places. Mr. Woodbury has been nominated in New Hampshire, and meetings of various degrees of importance have expressed preferences for other candidates in various parts of the country. The crops of all sorts are represented as being in a very prosperous condition throughout all sections: of wheat and potatoes more abundant than ever before, and of cotton and rice very much better than the drought in the early part of the season promised. The Extra Session of the New-York legislature adjourned on the 11th of July, after passing several important bills. That for the enlargement of the Erie Canal is a measure of great moment to the industry and commerce of the state. It provides for the complete enlargement of the Erie Canal within four years, thus securing the immense business which would else seek other avenues to the seaboard, and endowing the state with a large revenue independent of taxes. Chief Justice Bronson, whose political relations give to his opinions in this case a peculiar value, has published an elaborate vindication of the bill's constitutionality. The legislature of New Hampshire adjourned on the 5th of July. The legislature of Connecticut has also adjourned, having elected no Senator in the place of Mr. Baldwin.