On the motion of spheres in a fluid at low Reynolds numbers. (4th Report. Two equal-sized spheres moving in parallel with each other).

The velocities, angular velocities and hydrodynamic forces experienced by equalsized spheres freely-falling in parallel with each other in glycerol are measured using the`falling sphere'technique at low Reynolds numbers. The width of the test tank with a square cross section was 100 times as large as the diameter of the sphere. The width of the test tank with a square cross section was 100 times as large as the diameter of the sphere. The measurements, with an estimated uncertainty of ±1.4%, show that two equal-sized spheres moving in parallel with each other interact hydrodynamically at a distance within six times the diameter of the sphere at Reynolds numbers below 1.55, and that the drag force experienced by one of the two spheres is well approximated by the formula proposed here at low Reynolds numbers.