Christiani Hugenii Zelemii, dum viveret, toparchae opuscula postuma, quae continent dioptricam. Commentarios de vitris figurandis. Dissertationem de corona & parheliis. Tractatum de motu. De vi centrifuga. Descriptionem automati planetarii

To this succeeds the Treatise on Motion, in which the rules of motion are demonstrated, which the Author had already communicated to the literary world thirty years ago and more, in the French Journal of March 18, 1669.

Because of the affinity of the subject matter, this is followed by the little book on Centrifugal Force, which we found in the Author's papers, but by no means arranged in a convenient order. Therefore, we judged it best and most in keeping with the Author's intention to follow, as much as possible, the order of the Theorems on Centrifugal Force that he had already published many years ago at the end of the Horologium Oscillatorium Oscillating Clock; although, to do this, it was necessary to derive certain of those theorems, such as VII, IX, X, and XI (as they are numbered here), from the foundations laid by the illustrious Huygens. But even so, it was not possible to obtain exactly the same number of propositions, so that we would not deny the public anything the Author had written about these matters.

While we were writing this, the French Journal of the year 1702 happened to fall into our hands. As we were glancing through it, we noticed that on May 23, 1701, demonstrations of these 13 theorems were introduced, and at the same time, our Author was accused of error in that he posits the time of two minimum oscillations of a pendulum to be equal to the minimum circuit of the same pendulum moved in a conical motion. This is because the time of two such oscillations is equal to four times the time of a perpendicular fall from twice the length of the string. This cannot be true: for the time that is four times the perpendicular fall from twice the length of the string bears a ratio to the time of the fall from half the string as 8 to 1, which involves no ratio of the circumference of a circle to its diameter; yet the time of the lateral oscillation of a pendulum required by the time of the perpendicular fall from half the string's length, as demonstrated by Huygens in Prop. 25, part 2 of the Horologium Oscillatorium, does require such a ratio.

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