tes in producing disease, I shall endeavour to
lay down such rules for the preservation of health, as are the
result of reasoning on these subjects, and are also confirmed by
experience.

[1] [FIGURE] Suppose AC to be a lever, held in equilibrio by the
force B and weight W, then the whole momentum exerted at B must be
equal to that at W, but the forces will be different. For B x AC = W
x AB, and if AC = 10AB, then a force equal to ten times the weight to
be raised must be exerted by the muscle. Hence we see, that in the
actions of muscles there is a loss of power, from their insertions
being nearer the fulcrum than the weight. For example, suppose the
deltoid muscle to act and raise a weight of 55 lb.: the weight of the
arm is 5 lb., and the distance of its insertion is only 1/3 of the
arms length, hence the force exerted must be (55 + 5) x 3 = 180 lb.

[FIGURE] But by this contrivance we gain a greater extent of motion,
and also a greater velocity, and both with less contraction. Let A be
the centre of motion, or articulation; B the insertion of a muscle,
and AC the length of the lever or bone; then, by a contraction only
equal to B_b_, C is carried through C_c_, which is to B_b_ as AC to AB.
It is obvious also, that the velocity is greater, since C moves to _c_
in the same time as B to _b_.

A loss of power is likewise occasioned by the obliquity of the
muscular action, and the oblique direction of the fibres.

For, in this case, there is a compound of two forces, and a
consequent loss of power: for the forces are proportioned to the two
sides of a parallelogram, but the effects produced are proportioned
only to the diagonal.

LECTURE II.
RESPIRATION.

In the last lecture I took a short view of the human body, as a
moving machine, regulated by the will. We shall now proceed to
examine some of its functions more particularly.

I need not tell any of my audience, how necessary air is to the
living body; for every person knows that we cannot live when excluded
from this fluid; but, before we can understand the manner in which it
acts on the body, we must become acquainted with some of its
properties.

That the air is a fluid, consisting of such particles as have little
or no cohesion, and which slide easily among each other, and yield to
the slightest force, is evident from the ease with which animals
breathe it, and move through it. Indeed from its being transparent,
and therefore invisible, as well as from