The apparatus is shown in Fig.1, the principal component being the length of smooth-walled circular brass pipe, into which air is drawn at the left and from which it is discharged by the fan at the right. At entry the air passes through a parabolic nozzle, in which the pressures are observed by an axial probe. The pressures at one point in the nozzle and several points in the pipe are observed by means of series of piezometer rings in the wall. The pressures in the pipe cover three regimes: the pressure recovery after the nozzle, a setting length, and a length in which the flow of air is turbulent but steady for observation of pipe friction. Following this part, a perspex block is provided in which is mounted a total head tube which, with the aid of a micromoter screw, may be traversed across the tube. Between this block and the fan it is possible to insert for investigation venturi tubes and orifices of various forms, which their pressure tappings. The pressure tappings are all led to an inclined water manometer.