Around July 18, 1977 about 3:30PM in the afternoon. A very nice sunny day with only patchy cumulus clouds.

Driving home on US Rt.40 near Brier Hill Pennsylvania I witnessed a fairly large saucer shaped craft just hanging about three fourths out of a cloud that was just a little bigger than the craft. The ceiling I would have estimated to be about 3 to 5 thousand feet. 
I couldn't exactly estimate the size but, there was a farmhouse below and the craft looked about the same size as the house about 50ft across. Given the altitude - the craft must have been a couple times the size of the house. I saw the craft for 15 seconds or more. 
I viewed it as I traveled towards it going east to west on Rt. 40. Almost wreaked my car, ran off the road before I could stop. When I stoped and looked back up the craft was just disappearing into the cloud. 
The craft was flat black in color and was a classic flying saucer shape. Like two pie pans welded together as in the drawing enclosed. The ouside edge was sharp looking and the sun reflected off it like the paint was worn off and the metal was showing. 
The upper surface and the lower surface were flat with a concentric ring which was only slightly raised. If this thing was in the night sky you would not see it, beautiful design, very simple and clean. The surface was smooth and there were no visible antennas or windows showing. Although I saw the upper surface and the lower surface as it went into the cloud, I did not see any lines for hatches or doors for gear etc. 
There was no noise comming out of the cloud and it just seemed to drift with the other clouds in the area. I watched the cloud for a few minutes and then went home in amazement. Though about going back to the area with a camera but, I couldn't be sure of which cloud contained the craft, pictures of clouds wouldn't carry much weight. I travel quite a bit and always think about this big flying saucer when we are flying through clouds.
Witness indicates that date is approximate.  PD