This article discusses the phenomenon of free movement of buffle bodies in a fluid. The basis of hydraulic design of many technical devices (septic tanks, aeration and flotation chambers, hydro-and pneumatic transport, etc.) is the deposition rate (or floating) of the various bodies in the fluid. The solution of objective is obtained by the methods of mathematical modeling only at very low Reynolds numbers. Physical modeling of the phenomena is based on the results of fixed bodies flowed by oncoming external flow study. But from the standpoint of modern physical modeling methods the body, which moves freely, and the fixed body, streamlined at the same rate, represent two different phenomena: at the first variant speed is not known beforehand, at the second - it is originally given. Consequently, at the first variant there is no criterion between similarity numbers. The paper shows how the criteria are obtained from similarity numbers. Decisions are based on the similarity theory.The numbers, similarity criteria and criterial equations are determined. Proposed dependences have generalizing properties, if the conditions belong to the same class. In case of equal numbers of similarity, such phenomena are described by the same dependencies. They include: deposition of heavy particles in the air and water environments, the water droplets in the air, floating bubbles in the water, etc.