From Martinsburg.
Baltimore, July 8.
--A letter was received here to-day, dated at Martinsburg, July 6th, in which it is stated that Capt. Doubleday and the Rhode Island battery had arrived--Gen. Patterson's entire army is encamped there.
No forward movement had taken place.
The same latter says that Gen. Johnston had been reinforced by 7,000 men from Manassas, and had taken a position for a fight.