A fairly large, gray, metallic object, cone-shaped with a flat, circular bottom spotted in the sky over Willow Grove.

I don't remember the exact date, but it was a weekend sometime back in the spring of 1992 or 1993.  It was early afternoon, and I was driving with my girl-friend (now wife) northeast on Fitzwatertown Road in Willow Grove.  A fairly large, gray, metallic object, cone-shaped with a flat, circular bottom was moving at a slow pace (approx 30 mph) in a north-westerly direction, towards Willow Grove Naval Air Station.  I can only assume the object was solid, as the flat, circular bottom was dark, possibly cast in shadow.  There were no discernable markings at all. The sky was somewhat overcast and cloudy gray.  I saw it out of the driver's side front windshield for approximately 5 seconds, as it passed from right to left across my path.  It was flying rather low over the trees (possibly 200 ft high). Its altitude never varied, nor did its speed. It made no sudden movement, did not display any lights, nor did it make any discernable sound.  It seemed to be moving with purpose and direction.  My wife did not see it as she was reading something in the car at the time.  I didn't point it out to her either, as I wasn't sure what I was seeing, and had to keep an eye on the road as well, since I was still driving.  I can recognize most military aircraft in the US armed forces, from an A-10 Warthog to a P-3 Orion (both of which fly out of the Willow Grove Air Station).  I can say with certainty, that this was not a fixed-wing, nor rotor aircraft.  It did not look like a balloon or blimp of any sort I've seen in my lifetime.  I can only give you an idea of its size, by saying at arm's length it was about the size of a quarter.  I've never seen anything like it before or since.