Physico-Mathematical Thoughts

provided the waters are deep enough, anyone will be able to navigate, and indeed even complete a journey around the entire circumference of the earth.

XXI. We also add certain books on Harmonics The mathematical study of music and sound., which briefly and clearly encompass the practice and theory of music. From these, a reader can learn musical composition in a very short time and compose bicinia, tricinia, or even quadricinia These are musical compositions for two, three, or four voices respectively..

XXII. Although the things proposed in the following books may seem dry and more difficult than what can be applied to moral behavior by just anyone, I trust that if they are read carefully, nothing will provide a greater supply of new thoughts or more urgent prompts for piety to learned and diligent preachers. This applies whether one considers the difficulty in examining weights and the gravity Mersenne refers to physical weight, but uses it as a metaphor for the seriousness or "weight" of human choices. of bodies, which is analogous to judging human actions; or Pneumatics and Hydraulics The study of air and water pressure., from which they may draw what makes their listeners leap toward eternal life and become vigorous in every good work; or Ballistics, through which they may show that men are like Arrows, which God directs in different ways toward this or that target, but all toward his own glory. Whether in Mechanics or the entire Synopsis, things will be found that can be adapted to sermons in a thousand ways.

XXIII. Besides this general Preface, other prefaces are read at the beginning of each book. These always contain something new to supplement or correct what was said in the following books. Therefore, let the following Preface to the book on Weights be read, as well as the others placed before the Hydraulics, Mechanics, Synopsis, Conics, Optics, and other treatises.

XXIV. I also possess many inquiries received through letters which could bring much utility or honest pleasure to students. While I prepare those, I would also like to advise everyone who has preserved things in their desks that are worthy of the light and useful for promoting sciences or arts: do not withhold them from the Republic of Letters original: "litterarum Reipublicæ". This refers to the international community of scholars who shared knowledge during the 17th century.. I will not subscribe to the opinion of those who keep various treatises of more hidden disciplines to themselves so that they allow no one else to be aware of them. Indeed, they do not listen to that most holy man in the first book of On Christian Doctrine, chapter 1: For any thing that is not diminished by being shared is not possessed as it should be, as long as it is kept and not given away. original: "Omnis enim res quæ dando non deficit, dum habetur, & non datur, nondum habetur quomodo habenda est." Mersenne is quoting Saint Augustine.

XXV. Regarding Ballistics, you can compare the movement of thunder, which is often forgotten, with the movement of cannonballs, although it is difficult to know whether it is faster than a cannonball. I do not speak here of the speed of lightning or light, since it is not a body, but a certain instantaneous transmission of motion. It is like the motion someone impresses on the end of a staff: in the same moment, the other end moves.