The present report is the first of a research series in which the author intends to investigate the phenomena of flows in the jet-apparatus for dredging and to obtain reliable data to design an apparatus of high efficiency. A new calculation method for the characteristies of the Jet-apparatuses has been developed. also a method for the comparison of the efficiencies of Jet-apparatuses for dredging is proposed in this report.
　In the begining of this report, the effect of changing the diameter of mixing chamber and nozzle to the characteristies of the apparatuses was investigated for the most basic type of circular section.
　Using a simple designed model of the Jet-apparatuses, the velocity and pressure at every point in the mixing chamber were measured, and the measured values were compared with the calculated values of the model Jet-apparatuses by this method.