129) The most economical section of a rectangular channel is one which has hydraulic mean depth or hydraulic radius equal to

130) The ratio of velocity of fluid in an undisturbed stream to the velocity of sound wave is known as Mach number.

131) All the gases are considered to have compressible flow and all the liquids are considered to have incompressible flow.

132) When a cylindrical vessel containing liquid is revolved, the surface of the liquid takes the shape of

133) The hydraulic mean depth or the hydraulic radius is the ratio of