Sir Geoffrey Taylor discusses the propulsion of microscopic organisms in viscous fluids. Large objects use inertia to move through air or water, but microscopic organisms rely on viscous forces instead. Taylor analyzes how waves of lateral displacement along a thin layer can cause the layer to move forward, with the speed being proportional to the square of the amplitude and inversely proportional to the square of the wavelength. This mechanism explains how a propulsive tail can move a body through a viscous fluid without relying on inertial reaction. Energy dissipation and stress in the tail are calculated, showing that viscous forces dominate over inertial forces in microorganisms. Taylor also examines the interaction between the tails of two nearby microscopic organisms, finding that when waves are in phase, less energy is dissipated, while when waves are out of phase, there is a strong interaction due to viscous stresses. The study concludes that the tails of spermatozoa often move in unison when they are close and aligned, demonstrating the role of viscous forces in their propulsion. The analysis highlights the importance of viscous forces in the movement of microscopic organisms, contrasting with the dominant role of inertia in larger organisms.Sir Geoffrey Taylor discusses the propulsion of microscopic organisms in viscous fluids. Large objects use inertia to move through air or water, but microscopic organisms rely on viscous forces instead. Taylor analyzes how waves of lateral displacement along a thin layer can cause the layer to move forward, with the speed being proportional to the square of the amplitude and inversely proportional to the square of the wavelength. This mechanism explains how a propulsive tail can move a body through a viscous fluid without relying on inertial reaction. Energy dissipation and stress in the tail are calculated, showing that viscous forces dominate over inertial forces in microorganisms. Taylor also examines the interaction between the tails of two nearby microscopic organisms, finding that when waves are in phase, less energy is dissipated, while when waves are out of phase, there is a strong interaction due to viscous stresses. The study concludes that the tails of spermatozoa often move in unison when they are close and aligned, demonstrating the role of viscous forces in their propulsion. The analysis highlights the importance of viscous forces in the movement of microscopic organisms, contrasting with the dominant role of inertia in larger organisms.