_November 8._--The Harris Light broke camp at Leesburg early in the morning, and advanced to White Plains, where we encountered and defeated a detachment of Rebel cavalry, driving them towards the mountains. Continuing our journey through this pleasant valley between the Blue Ridge and the Bull Run mountains, we soon joined our main army, whose headquarters were at Warrenton. This is the most beautiful village in this region of country, situated on the crest of fruitful hills, and elegantly laid out. It is the shire-town of Fauquier County. Here a few days were consumed in effecting the alterations incident upon a change of commander, and on the fourteenth the Army of the Potomac was constituted into three grand divisions, to be commanded respectively by Generals Sumner, Franklin, and Hooker. The following day Warrenton was abandoned, and the army swept down towards the Rappahannock. The sight was a grand one. On our march, orders were received from President Lincoln enjoining a stricter observance of the Sabbath in the army and navy, than had been done before. As a general thing the Sabbath had not been regarded as any more than any other day. Indeed, very few men in the rank and file kept any calendar of time, and seldom knew the date or day. This was occasionally the case even with officers. The only possible way of keeping pace with flying time in the army, is by writing a diary. But even when it was known that the Sabbath had been reached, no regard was taken of its sacred character. One of the causes of our disaster at the first battle of Bull Run was supposed by many to be, that we had desecrated the holy Sabbath by our attack. However true or false such a view may have been, the order we received to-day from Washington was universally felt to be opportune. Two days' march brought our advance to Falmouth, and on the twenty-first General Patrick, our provost-marshal general, was directed to repair to Fredericksburg under a flag of truce, and request the surrender of the city. The authorities replied, that while its buildings and streets would no longer be used by Rebel sharp-shooters to annoy our forces across the river, its occupation by Yankee troops would be resisted to the last. Had the means of crossing the river been at hand, General Burnside would have made hostile demonstrations at once; but through some misunderstanding between himself and General Halleck, at Washington, the pontoons were not in readiness.