Abstract: Mobile apparatus for spreading a constant volume of liquid per unit area of surface in spite of variations in speed of travel, has a spreading liquid circuit which includes a pressure regulator for controlling the liquid pressure in response to the pressure in an auxiliary liquid circuit. The pressure in the auxiliary liquid circuit is controlled by a sensing wheel which is carried on a pivotally mounted arm so that the sensing wheel can engage either with a wheel of the vehicle on which the apparatus is mounted or with the ground. The sensing wheel provides a pump for the auxiliary circuit and the flow rate of the pump is controlled by an orifice which can be partially occluded by a cam-like shutter.