<s id="A18-1.12.01">[12] How one has to enlarge or reduce regular plane or solid figures in a certain ratio, we have now explained.</s>
<s id="A18-1.12.02">Now it is, however, also very necessary to devise a method for the irregular plane and solid figures, by means of which the same procedure is possible for us.</s>
<s id="A18-1.12.03">But first we want to say in advance some things that are suited to facilitate its understanding; we shall then let the proof of that follow.</s>
<s id="A18-1.12.04">It is said that plane and solid figures, be they regular or irregular, are congruent if one can describe on one of them such a figure of straight lines, that it is equal and similar to the one that is described on the other one; and it is said that figures are similar to each other if one can describe in one of them figures of straight lines in a manner that one can describe in the other one [figures] similar to them.</s>
