every animal the hottest part is that which is around the
blood and veins; it is in a manner an internal fountain of fire, which
we compare to the network of a creel, being woven all of fire and
extended through the centre of the body, while the outer parts are
composed of air. Now we must admit that heat naturally proceeds outward
to its own place and to its kindred element; and as there are two exits
for the heat, the one out through the body, and the other through the
mouth and nostrils, when it moves towards the one, it drives round the
air at the other, and that which is driven round falls into the fire
and becomes warm, and that which goes forth is cooled. But when the heat
changes its place, and the particles at the other exit grow warmer, the
hotter air inclining in that direction and carried towards its native
element, fire, pushes round the air at the other; and this being
affected in the same way and communicating the same impulse, a circular
motion swaying to and fro is produced by the double process, which we
call inspiration and expiration.

The phenomena of medical cupping-glasses and of the swallowing of drink
and of the projection of bodies, whether discharged in the air or bowled
along the ground, are to be investigated on a similar principle;
and swift and slow sounds, which appear to be high and low, and are
sometimes discordant on account of their inequality, and then again
harmonical on account of the equality of the motion which they excite in
us. For when the motions of the antecedent swifter sounds begin to pause
and the two are equalized, the slower sounds overtake the swifter and
then propel them. When they overtake them they do not intrude a new
and discordant motion, but introduce the beginnings of a slower, which
answers to the swifter as it dies away, thus producing a single mixed
expression out of high and low, whence arises a pleasure which even the
unwise feel, and which to the wise becomes a higher sort of delight,
being an imitation of divine harmony in mortal motions. Moreover, as to
the flowing of water, the fall of the thunderbolt, and the marvels that
are observed about the attraction of amber and the Heraclean stones,--in
none of these cases is there any attraction; but he who investigates
rightly, will find that such wonderful phenomena are attributable to the
combination of certain conditions--the non-existence of a vacuum, the
fact that objects push one another round, and that