Dear wwh,

Your supposition is correct. You are referring to the 'cowl', usually painted white, which has a wind vane and a hot air vent. The structure in the picture is quite a large oast house. For large scale commercial production they were arranged in batteries and did not look attractive, but there are still many single oasts, smaller than the twin shown in the picture, and these can look very picturesque when converted into cottages. Sadly, the industry is virtually dead now.

The best source of information I have found on hops and oasts, including a cross section through an oast house is at:

http://www.digiserve.com/peter/oast.htm