In the book "Calculus Made Easy" by Silvanus Thompson, in the Chapter 5 ("Next Stage. What To Do With Constants"), Thompson says (about the graph of an equation $ y = 7x^2 $ and the graph of $ \frac {dy}{dx} $, that:
Carefully compare the two figures, and verify by inspection that the height of the ordinate of the derived curve, Fig. 6a, is proportional to the slope of the original curve, (See here about slopes of curves.) Figure 6, at the corresponding value of x. To the left of the origin, where the original curve slopes negatively (that is, downward from left to right) the corresponding ordinates of the derived curve are negative.
What does he mean by "height of the ordinate of the derived figure"? The relevant section of the book can be found here: Calculus Made Easy: Chapter 5.
Ordinate is an old-fashioned way of saying $y$-axis or $y$-value.
The $x$-value is called abscissa.