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Reading how Galileo measured time in the experiment with inclined plane, it says on Wikipedia, that:

Galileo accurately measured these short periods of time by creating a pulsilogon. This was a machine created to measure time using a pendulum.[15] The pendulum was synchronized to the human pulse. He used this to measure the time at which the weighted balls passed marks that he had made on the inclined plane.

However, I am unable to find any drawing or reference to what it actually looked like.

According to other source I found, Galileo actually used a guitar and by listening to string rhythms/frequencies he could measure subseconds intervals.

Does anyone have a reference to how it really was?

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According to the medical journal article The History of Instrumental Precision in Medicine, referring to the pulsilogon :

Unclaimed by Galileo, it was attributed to Paolo Sarpi, and clearly enough was appropriated at a later date by that notable genius, Sanctorius, who also, like Galileo, called it the pulsilogon. We have no drawing of Galileo’s pulsilogon, but it must have been identical with the simpler form as shown in Sanctorius.

The Bulletin of the Scientific Instrument Society article Recreating the Pulsilogium of Sanctorius details a recent attempt to reconstruct the various types of pulsilogia attributed to Sanctorius and includes the following etching: Sanctorius pulsilogia

However, the same article includes the following comment on the relation between Galileo's instrument and that of Sanctorius:

All the scholars who dealt with Galileo and his studies on motion, simply assumed that Santorio’s pulsilogium was a direct outcome of the former’s research into the properties of the pendulum. Unfortunately, there are many major problems with this hypothesis. The most important of which is that it takes no account of Santorio’s words and further assumes the primacy of Galileo’s over Santorio’s studies that is historically groundless.

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    $\begingroup$ Very interesting. I still fail to understand what the "added value" of measuring a heart beat in comparison to using a single pendulum actually was. How could that add some additional precision to the measurements? To me it sounds that it just added some errors. I had no idea that there is so little known about such important event in the history of physics. $\endgroup$
    – pisoir
    Dec 18, 2021 at 21:55
  • $\begingroup$ @pisoir By "added value" I assume that we mean "increased accuracy". The first article linked describes how Sanctorius wrote defending the accuracy of the pendulum when compared to other instruments in use at the time, so the "added value" is really in the pendulum itself. Note that the first figure (type A1) shows a pendulum independent of the measuring ruler so you're right that a pulsilogon is essentially the pendulum. The remaining four examples, where the pendulum is connected to the "ruler", would both ensure that the same pendulum and ruler where used and improve portability. $\endgroup$
    – nwr
    Dec 18, 2021 at 23:11
  • $\begingroup$ I think it would help to understand how measurements were actually taken. If I understand the article correctly, the physician changed the length of the pendulum until it was synchronised with the heartbeat, and then the length of the pendulum was the measurement value. So it was totally different from today when use a clock and count the heartbeats in a minute. The pulsilogium returned a relative value that didn't have a direct meaning but could be used to compare pulses in different patients or at different times. $\endgroup$
    – Stephan Matthiesen
    Dec 20, 2021 at 9:14
  • $\begingroup$ @StephanMatthiesen As far as I understand, a patient's measured pulse rate would be compared over a period of some days. Comparable differences would indicated "good" heart health while significantly different measurements may indicate "poor" heart health. Of course there would also be a range of pulse rates that was considered healthy, so a measurement outside of that range may have been thought to indicate poor heart health regardless of comparisons. $\endgroup$
    – nwr
    Dec 20, 2021 at 18:52

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