Differential, in the higher Geometry, is an infinitely small quantity, or part of quantity, so small as to be less than any assignable one, and is thus denominated because it is frequently considered as the difference of two quantities, and as such, is the foundation of the differential calculus. The term differential is also applied, in Mechanics, to various machines for imparting to a body the difference of motion of two other bodies moving in contrary directions; an example of this is shown in the Chinese crane, under the article Crane. For further instances, see Pulley Differential, Screw Differential, and Wheel Differential.