The squadron, after having rendezvoused at Key West and Ship Island, arrived without any material detention, at the South West Pass of the Mississippi. A want of acquaintance with the changes in the bar, occasioned probably by the sinking of four or five rafts, flatboats, and an old dry dock by the enemy, resulted in some delays, but the whole squadron at length, with the exception of the frigate Colorado, got safely over, and anchored twelve miles up the river at the head of the passes. The efficiency of mortars, elevated permanently at forty-five degrees, depends chiefly upon an accurate knowledge of the distance to the object to be fired upon. This distance determines the quantity of powder necessary for the discharge, and the length of the fuses to be employed. Captain Porter understood the impossibility of judging and estimating distances and bearings correctly, particularly when the objects are for the most part hidden from view, as was the case with the forts on the wooded and crooked Mississippi, and had therefore requested of the department the aid of a party from the U. S. coast survey, and the writer of these notes had been detailed by Prof. A. D. Bache, the superintendent of that work. One acting assistant, two sub-assistants, and one aid were attached to the party, and the steam gunboat Sachem was placed at their disposal. This vessel arrived in the Mississippi on the 11th of April. Captain Porter at once requested Mr. Gerdes to furnish a reliable survey of several miles of the river, below and including the fortifications. In this service a number of gunboats belonging to the fleet and to the mortar flotilla accompanied the Sachem, partly to afford protection, and partly to draw the enemy's attention from the operations of the surveyors. Mr. Gerdes commenced work with his party on the 13th of April, and continuing for five consecutive days, made a reliable map of the river and its shores from the 'Jump' to and including Forts Jackson and St. Philip, with their outworks and water batteries; the hulks, supporting the chain across the river, and every singular and distinguishable object along its banks. The survey was made by triangulation carried forward simultaneously on both sides of the river. Two coast survey signals were found, the 'Jump telegraph post,' and 'Salt-work's chimney top,' of which the geodetic relations were known, and the work was founded upon a base line connecting these two points. Sub-assistant Oltmanns, and Mr. Bowie as aid, were detailed for the west shore, Mr. Gerdes and acting assistant Harris taking the eastern side, while sub-assistant Halter observed angles from permanent stations. The angular measurements were made with all kinds of instruments found suitable to the locality. Only a few of the stations were on solid ground, nearly all the shore being overflowed. Frequently the members of the party were compelled to mount their instruments on the chimney tops of dilapidated houses.