ore especially with John Collins, Dr Wallis,
and Henry Oldenburgh, the then Secretary of the Society. He was author
of a long list of scientific essays, two of which only are responsible
for his fame, viz., Theorice Medicaearum Planetarum, published in
Florence, and the better known posthumous De Motu Animalium. The first
of these two is an astronomical study in which Borelli gives evidence of
an instinctive knowledge of gravitation, though no definite expression
is given of this. The second work, De Motu Animalium, deals with the
mechanical action of the limbs of birds and animals and with a theory of
the action of the internal organs. A section of the first part of
this work, called De Volatu, is a study of bird flight; it is quite
independent of Da Vinci's earlier work, which had been forgotten and
remained unnoticed until near on the beginning of practical flight.

Marey, in his work, La Machine Animale, credits Borelli with the first
correct idea of the mechanism of flight. He says: 'Therefore we must be
allowed to render to the genius of Borelli the justice which is due
to him, and only claim for ourselves the merit of having furnished the
experimental demonstration of a truth already suspected.' In fact, all
subsequent studies on this subject concur in making Borelli the first
investigator who illustrated the purely mechanical theory of the action
of a bird's wings.

Borelli's study is divided into a series of propositions in which he
traces the principles of flight, and the mechanical actions of the wings
of birds. The most interesting of these are the propositions in which he
sets forth the method in which birds move their wings during flight and
the manner in which the air offers resistance to the stroke of the wing.
With regard to the first of these two points he says: 'When birds in
repose rest on the earth their wings are folded up close against their
flanks, but when wishing to start on their flight they first bend their
legs and leap into the air. Whereupon the joints of their wings are
straightened out to form a straight line at right angles to the lateral
surface of the breast, so that the two wings, outstretched, are placed,
as it were, like the arms of a cross to the body of the bird. Next,
since the wings with their feathers attached form almost a plane
surface, they are raised slightly above the horizontal, and with a
most quick impulse beat down in a direction almost perpendicular to the
wing-plan