The eighteenth article is this, “_in any parallelogram, if the two sides that contain the angle be moved to their opposite sides, the one uniformly, the other uniformly accelerated; the side that is moved uniformly, by its concourse through all its longitude, hath the same effect which it would have if the other motion were also uniform, and the line described were a mean proportional between the whole length, and the half of the same_.” To the proposition you object first, “_that it is all one whether the other motion be uniform or not, because the effect of each of their motions, is but to carry the body to the opposite side_.” But do you think that whatsoever be the motions, the body shall be carried by their concourse always to the same point of the opposite side? If not, then the effect is not all one when a motion is made by the concourse of two motions uniform and accelerated, and when it is made by the concourse of two uniform or of two accelerated motions. Secondly, you say that these words, _and the line described were a mean proportional between the whole length, and the half of the same_, have no sense, or that you are deceived. True. For you are deceived; or rather you have not understanding enough distinctly to conceive variety of motions though distinctly expressed. For when a line is gone over with motion uniformly accelerated, you cannot understand how a mean proportional can be taken between it and its half; or if you can, you cannot conceive that that mean can be gone over with uniform motion in the same time that the whole line was run over by motion uniformly accelerated. Yet these are things conceivable, and your want of understanding must be made my fault.