Chimney pot tops and corners. After google-searching I found that chimney toppers are called cowls. You can have a directional wind cowl, an updraft cowl, a capping cowl or a clamping cowl. There are probably even more. I have been observing these cowls in the neighbourhood recently. On our house we have two sorts. One homemade and stationary and the other two are stainless steel and seem to be for anti-downdraft purposes.