The Projectile which passed through Gen. Beauregard's headquarters at the battle of Manassas Plains, on the 21st of July, has been presented by him to the Mayor of Richmond. Mr. Mayo designs to present it to his Excellency Gov. Letcher, who is gathering a lot of war curiosities. The projectile is about ten inches in length, three in breadth, and was filled with some kind of explosive material, which, however, did not "go off" till removed by rebel fingers.