In a casing of a rotary machine, cast components are sealingly mounted together by means of seals accommodated in (semi-) enclosed cavities. A ring seal includes an annular base and an annular skirt extending axially from the base. To ameliorate manufacturing tolerances, the seal defines circumferentially spaced, intermittent cavities to allow seal material to be deformed into the cavities as required. Land areas between cavities have cross-sectional surfaces increasing progressively from a free surface of small area to cause compressive force required for deformation to increase exponentially with an exponent larger than 1. The cavities may be peripherally spaced, being open at a radially outer surface of the seal. The seal may be of elastic material which may be incompressible. The skirt may taper away from the base. The base may have a projecting sealing lip. The skirt may have, at a free end, a slanted, peripheral lip.