The resistance offered by bodies to motion through fluids has for many years been a problem of the first importance, and one which has received a correspondingly large degree of attention both from scientific investigators and those commercially interested in the subject. A considerable amount of mathematical investigation has been carried out; but a detailed study of fluid motion has only proved possible in the case of a perfect fluid, i.e., one in which viscosity is not present. Assumptions have been made as to the quantities upon which the motion of the fluid and the body depends, and formulæ have been prepared expressing the laws connecting force and speed for dynamically similar bodies.

In the present note attention will be directed chiefly to the case where the only properties of the fluid upon which the motion depends are the viscosity and density, though brief mention may be made of instances where the acceleration due to gravity is also important.