[140] In i. De Cælo.
[141] It is from this cylindric spiral that the screw is formed.
The present very obscure passage, may be explained by the following figure. Let A B C, be a right angle, and D E the line to be moved, which is bisected in G. Now, conceive it to be moved along the lines A B, B C, in such a manner, that the point D may always remain in A B, and the point E in B C. Then, when the line D E, is in the situations d e, δ ε, the point G, shall be in g, γ, and these points G, g, γ, shall be in a circle. And any other point F in the line D E, will, at the same time, describe an ellipsis; the greater axis being in the line A B, when the point F is between D and G; and in the line B C, when the point F is between G and E.
[143] That is, the soul of the world.
[144] In Timæo.
[145] The ellipsis.
[146] The cissoid. For the properties of this curve, see Dr. Wallis’s treatise on the cycloid, p. 81.
[147] The conchoid.