Our advices from Maryland are of the highest importance. They show that the rebel army it retreating from that State much more expeditiously than they came into it, and that they are not making their escape unscathed. Gen. Burnside passed through Frederick on Saturday morning, and the whole National army pushed on after the retreating enemy by every available road, and even crossed the fields. The reception of Gen. Burnside in Frederick was a perfect ovation, the people turning out with the wildest enthusiasm ; and when Gen. McClellan entered the place, the streets and sidewalks were perfectly blocked with the enthusiastic masses, including a large proportion of ladies. The firing heard on Friday was an artillery duel between Gen. Pleasanton and the rear guard of the rebels, who were so hard pushed that they were forced to fall back to Middletown, from which place they were also driven in the afternoon. On Saturday our cavalry had two affairs with the enemy, and so pushed them that they were forced to burn half a mile of their wagon trains. This, in addition to a train of a hundred ammunition and subsistence wagons, captured by Gen. Franklin, will prove quite a serious loss tor the rebels. Our troops were in fine spirits, and much elated at the cordial reception extended to them by the people of Maryland. All danger of an invasion of Pennsylvania may now be considered past, as the rebels will no doubt have enough to do to escape the death they have courted. Gov. Curtin, however, does not relax his preparations, but is determined to put the State upon a thorough war footing.