Che cosa è la macchina di Galton?
Il suo inventore, Sir Francis
Galton lo descrive così:
"It is a frame glazed in front, leaving a depth of about a quarter of an inch
behind the glass. Strips are placed in the upper part to act as a funnel. Below
the outlet of the funnel stand a succession of rows of pins stuck squarely into
the backboard, and below these again are a series of vertical compartments.
A charge of small shot is inclosed. When the frame is held topsy-turvy, all the
shot runs to the upper end; then, when it is turned back into its working position,
the desired action commences. Lateral strips, shown in the diagram, have the
effect of directing all the shot that had collected at the upper end of the frame
to run into the wide mouth of the funnel.
The shot passes through the funnel and issuing from its narrow end, scampers
deviously down through the pins in a curious and interesting way; each of them
darting a step to the right or left, as the case may be, every time it strikes
a pin. The pins are disposed in a quincunx fashion*, so that
every descending shot strikes against a pin in each successive row. The cascade
issuing from the
funnel broadens as it descends, and, at length, every shot finds itself caught
in a compartment immediately after freeing itself from the last row of pins."
Galton, p. 63
Quincunx: la parola deriva dal latino "quinque", cinque.
La disposizione a quinconce si realizza spostando ogni fila
di chiodi rispetto alla precedente in modo che i chiodi cadano
nel punto di mezzo
di quelli della
fila sovrastante. In tre file di chiodi si ha in pratica la
della faccia n. 5 di un dado. Da qui l'altro nome del macchinario: quinconce.