Leonardo's
"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
could be heard at a distance at sea by listening at one end of a tube,
the other end of which was placed in the water; and that the same
expedient worked successfully on land, the end of the tube being placed
against the ground.

The knowledge of this great number of unexplained facts is often
interpreted by the admirers of Da Vinci, as showing an almost occult
insight into science many centuries in advance of his time. Such
interpretations, however, are illusive. The observation, for example,
that a tube placed against the ground enables one to hear movements on
the earth at a distance, is not in itself evidence of anything more than
acute scientific observation, as a similar method is in use among almost
every