The Mississippi Valley Sanitary Fair.

The most extensive preparations are being made by the citizens of St. Louis for the Mississippi Sanitary Fair to be held in that city commencing on the 17th of May next. A site has been procured for a building to be erected for the purpose of the Fair, on Twelfth street between Olive street and Wabash Avenue. From an article in the Missouri Democrat we take the following description of the building which has been designed by Mr. Wm. Rumbold, architect, aided by Messrs. Ha[unknown]and Brown:

The dimensions of the building are as follows: Length, 500 feet; breadth, 114 feet, with two transepts extending up and down Locust street respectively 65 by 35 feet: height of the main building, 30 feet. The roof will be raised in the manner of steamboat cabins and sleeping cars. The general shape may be described as a parallelogram with an octagonal center, which rises above the main roof, and whose wings form a Greek cross. This central figure encloses the space for the floral and horticultural exhibition. The whole building is to have twenty-six doors, with twelve feet entrances, all of which may be opened in case of an accident, to allow the exit of the crowd. The main entrances will be from the Olive street and Washington avenue sides, forming three handsome archways, where ticket receivers may be stationed. The exits are four in number, and located on the right and left of the transepts — i.e. on both sides of Locust street. The building will absorb nearly 260,000 feet of lumber, which has been ordered already.

Whatever liberality and enterprise can do will be done by the patriotic and public spirited citizens of St. Louis, and we expect to see the "Mississippi Valley Sanitary Fair" prove a great success.