The expression of relationships between variables is a highly important part of mathematics. For our purpose the relationship is usually expressed in algebraic form as one or more equations, but in many instances, the variation is such that this is not possible. For example, in recording air pressure, the variables time and pressure may not be simply related algebraically, but could be recorded graphically by a pen which at any instant is capable of moving along one line. This record could be made on a moving strip of paper over which the pen, at any instant, is constrained to move either in a straight line or along the arc of a circle. Such a strip of paper is usually provided with suitable reference lines forming a grid, one set of lines giving all possible positions of the pen for a given instant, a second set giving lines of constant pressure for the period of time.
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