Dictionary.net

See Method of Curves.

When one quantity undergoes a series of changes depending on the progress of another quantity, this dependence can be expressed to the eye by means of a curve. Suppose it were required to register the variations in the height of a barometer throughout the twenty-four hours of a day. A sheet of paper can be places on a cylinder in a vertical position, and made to revolve uniformly by clockwork ; if a pencil point pressed against the paper rises and falls with the mercury in the barometer, it will plainly trace out a curve on the paper. Now, suppose the paper to be unwarped, a horizontal line on it, if properly divided, will show the progress of the time throughout the day, and vertical lines drawn from the horizontal line to the curve will show the corresponding heights of the barometer. The variations in the heights of the barometer are thus completely represented by this method, which is one instance of the Method of curves. Indicator curves, adiabatic lines, cotidal lines, etc., are other instances of a method which admits of application in every branch of physics.