The Book of Mechanics

...diligence, because although those things which are our own are inserted into the words of Archimedes, if anyone should prefer to read only the words of Archimedes, he will be able to attain that rightly; for we have not omitted even one word of Archimedes: nay rather, we have arranged them so that they retain their full sense and can be read continuously, as if nothing had been inserted between them. This, indeed, we judged would not be useless to students, who will have Archimedes alone without our additions; but with our additions, they will have the demonstrations of Archimedes continued and explained. For the sake of Huberio's doctrine, we have added very many scholia, in which we have revealed the order and the craftsmanship of the author throughout; and we have demonstrated many lemmas necessary for the demonstrations of Archimedes, and have added several other things very useful for the explanation and the subject matter. So that the dicta of Archimedes may shine more brightly, before we approach the explanation of his words, it seemed opportune to us to first declare some things regarding what he assumes as known in these books of Archimedes. Then, the scope and intention of Archimedes is proposed for consideration; likewise the division of the books, and other things of this kind, which will bring the greatest ease to understanding the mind of Archimedes.

Since, therefore, he assumes that we have an exquisite knowledge of the center of gravity, it was fitting to provide its definition: for its easier knowledge, however, we thought it must be noted first of all that four centers are found. Namely, the center of the universe, the center of magnitude, the center of figure, and the center of gravity, which center of gravity is rightly defined by Pappus of Alexandria in the eighth book of the Mathematical Collections in this manner.

DEFINITION OF THE CENTER OF GRAVITY.

The center of gravity of every body is a certain point, placed within, from which, if a heavy object is conceived by the mind to be suspended, while it is moved, it rests and preserves the position which it had at the beginning, and does not turn about in that movement.