The study of fluid flow in such gaseous systems as combustion chambers and furnaces is of value in the design and subsequent operation of the equipment. Useful information may be obtained from flow visualization in these systems under constant density conditions, even though in practice considerable density changes occur. Hydraulic models are then found to have distinct advantages over air models.Apparatus has been developed by means of which the flow of water may be studied in transparent models of the simulated air systems. The flow is rendered visible by plastic tracers, of similar density to water, which are illuminated by continuous filament lamps, long-duration electronic flash lamps or vapour discharge lamps, according to the information required. Selected parts of the flow field may be observed by means of a narrow, flat beam of light directed through the appropriate section of the model. Semi-quantitative records of flow components may be derived from photographs obtained with the flash illumination. Individual tracers may be observed by selective illumination involving an ultra-violet light source and fluorescent tracers.The subject is treated from a wholly practical point of view. Details of the apparatus and techniques are given, with examples of some of the types of information obtainable, and the elementary requirements for similarity with aerodynamic systems are stated.