"steam-engine" "drove a ball weighing one talent over a distance
of six stadia." In a manuscript now in the library of the
Institut de France, Da Vinci describes this engine minutely. The
action of this machine was due to the sudden conversion of small
quantities of water into steam ("smoke," as he called it) by
coming suddenly in contact with a heated surface in a proper
receptacle, the rapidly formed steam acting as a propulsive force
after the manner of an explosive. It is really a steam-gun,
rather than a steam-engine, and it is not unlikely that the study
of the action of gunpowder may have suggested it to Leonardo.

It is believed that Leonardo is the true discoverer of the
camera-obscura, although the Neapolitan philosopher, Giambattista
Porta, who was not born until some twenty years after the death
of Leonardo, is usually credited with first describing this
device. There is little doubt, however, that Da Vinci understood
the principle of this mechanism, for he describes how such a
camera can be made by cutting a small, round hole through the
shutter of a darkened room, the reversed image of objects outside
being shown on the opposite wall.

Like other philosophers in all ages, he had observed a great
number of facts which he was unable to explain correctly. But
such accumulations of scientific observations are always
interesting, as showing how many centuries of observation
frequently precede correct explanation. He observed many facts
about sounds, among others that blows struck upon a bell produced
sympathetic sounds in a bell of the same kind; and that striking
the string of a lute produced vibration in corresponding strings
of lutes strung to the same pitch. He knew, also, that sounds