Sighted WW II barrage balloon at 1500' AGL (no lights) which accelerated rapidly as I approached in my light aircraft.

I was southbound in my light aircraft from Jefferson County Airport during some pleasure and proficiency flying.  The weather was clear, very light to no wind; my altitude was approximately 1500 AGL, airspeed 90 knots.  Appproximately 5-6 nm south of Jefferson County Airport (10-11 nm south of the town of Port Townsend), directly ahead approximately 1 nm ahead and at about my altitude I saw what I immediately thought was a WW II barrage balloon, like those used in the Battle of Britain--it was so like that that no other thought or association came to my mind. It was brown or olive drab in color and seemed too small to be a modern blimp. The "balloon" was stationary, as if tethered in the fields below, though I saw no tethering lines; it had no airfoils or control surfaces except the fin-like empennage typical of barrage ballons and blimps, and it had no engine, propellor or other visible source of power. I saw no lights anytime during the episode.  It was pointed in a westerly direction.  My immediate thought was to wonder why anyone would tether a barrage balloon in at this point.  I altered my heading to pass behind (east of) the balloon and take a closer look. When I was approximately 1/4 nm from it, the balloon suddenly accelerated at a very high rate of speed to the west, toward the Olympic Mountains; I turned to follow it;  in a few seconds it had gone from my sight.  There was no visible exhaust or other indication of power or motion.  I have been a certified flight instructor for approximately 30 years and was in instructor in the Strategic Air Command.  I have seen interesting sights while flying but I can remember no other sight that I could not explain to my self.  I recently learned of your web sight, which is one reason for the timing of this report.  My memory of the episode is vivid; I have identified the date in my pilot log book.