The transformation of snow to ice and the occlusion of gases

Power steering apparatus comprising a changeover valve adapted to be operated in one direction or the other by turning of a steering shaft so that a supply oil passage connected to an operational oil pressure source is selectively connected to a right or left chamber of a power cylinder. At least one external oil pressure reaction chamber is arranged to be reduced in volume by the operation in either direction of the changeover valve to resist displacement thereof, the oil pressure reaction chamber being connected through a first oil passage to a control pressure source for delivery of oil at a pressure corresponding to vehicle speed. The first oil passage is connected through a second oil passage to the supply oil passage and a control valve is mounted in the second oil passage to be opened when the pressure within the first oil passage is higher by a predetermined value than the pressure within the supply oil passage. A throttle can be connected in series with the control valve in the second oil passage and a spring loaded relief valve can be provided in a by-pass passage connected in parallel to the second oil passage.