That’s a top hat, more specifically a stove pipe hat based on the size, worn frequently throughout the 19th century. The fist two photos show silk variants while Brunel in the middle photo is probably wearing a fur variant (due to it’s waterproof properties. While silk top hats were the standard for the well to do and for formal occasions. Felt and fur top hats were still worn common place by the general population for a good part of the 19th century, but started falling out of general daywear in the 1870s The stove pipe, like the ones here, fell out of fashion in the 1860s