In Two New Sciences, Galileo writes:

I take a perfectly round brass ball about the size of a walnut and project it along the surface of a metallic mirror held in a nearly upright position, so that the ball in its motion will press slightly upon the mirror and trace out a fine sharp parabolic line...The above experiment furnishes clear and tangible evidence that the path of a projectile is a parabola... In the execution of this method, it is advisable to slightly heat and moisten the ball by rolling in the hand in order that its trace upon the mirror may be more distinct.

I am confused about three points:

  1. What exactly does this experiment describe?
  2. Does the experiment as described work?
  3. Did Galileo actually perform this experiment?

My best understanding is as follows: Galileo took some flat, polished, rectangular metal surface, and placed it at an incline on the ground. He then takes a brass ball, slightly warm and moist, and rolls it up the ramp. He rolls it not straight up the ramp, but slightly to the side. This causes the ball to follow a parabolic path against the mirror. As it does so, it leaves some kind of mark of its path on the metal surface.

Will this experiment actually work? I don't know enough about mirrors in Galileo's time to be sure what kind of mirror he would use. Also, I'm not sure why the brass ball would leave a mark on the mirror - would something like galvanic corrosion apply in this case? Would it depend on the type of metal used in the mirror?

Most accounts I find describe Galileo using a different method to discover that the path of a projectile is a parabola; for example, see this article by Stillman Drake. I have done a small amount of research on Google/Google Scholar to see if any researcher has commented on the "mirror method"; thus far, I have not found anything (though my searches are limited to English).

So, did Galileo even perform this experiment? Even if he didn't, would the experiment still work?

  • $\begingroup$ You re right; the procedure described by Galileo in the Second Day of Dialogue (1638) regards the way to draw a parabola. But it is not an experiment : Galileo already knows that the path of a projectile is parabolic. As you said with ref to Drake's article, the experiments described into earlier (1608) manuscripts are attempts at verifying the law of parabolic path through suitable experiments. We may conjecture that the drawing methods is sort of "reverse engineered" from experiments with path of rolling balls. $\endgroup$ – Mauro ALLEGRANZA May 10 '19 at 14:09

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