Rectangular tanks are tested as follows: 3/15in., 101b.; Jin., 51b. per square inch. The corresponding values for cylindrical tanks are 401b. and 251b, per square inch. The cylindrical shape is almost invariably used when the pressure exceeds about 12 1b. per square inch. The resisting powers of all tanks that are not spherical or cylindrical are increased by the use of internal stay-rods.