Leibnitz first proposed to find a curve, down which a body descending by the force of gravity, shall make equal approaches to the horizon in equal portions of time. This curve, as it has been found by Bernoulli and others, is the second cubical parabola placed with its vertex uppermost, and which the descending body must enter with a certain determinate velocity. The question was rendered general by Varignon for any law of gravity, by which a body may approach towards a given point by equal spaces in equal times. Maupertuis also resolved the problem in the case of a body descending in a medium whose resistance is as the square of the velocity.