Rotary atomisers, initially of brush and disc design, but subsequently superceded by a rotating cage device , have for some while been used with fixed-wing aircraft for producing relatively finely dispersed homogeneous liquid sprays. Common to all these types is rotation in the vertical plane with axial liquid feed intended to give uniform discharge from the rotor periphery. Atomisation, for a given liquid and a fixed rotor diameter, is a function both of the rotor speed and also of the airspeed of the aircraft. In combination these two factors determine the resultant shear force producing break-up of the discharging liquid filaments. With fixed-wing aircraft, having a minimum forward speed of, say, 60 mph, it is ultimately this velocity which determines a theoretical upper limit to the size of droplet which may be produced, but in practice drop size also varies with throughput, and, in connection with anti-Locust spray work, Sayer has found that the normal droplet spectra may be considerably coarsened by dispensing with the orifice feed restrictors and non-drip valves, thus allowing the rotor feed rate to rise to three to four times the design figure.