conspicuous as were our efforts to tell what he was driving at. He was
describing an ordinary windlass hoist used at the shaft of a mine. He
said "There is a windlass at de top of de shaft around which is coiled a
rope, on de two ends of which is fastened two er--er--_pans_, one of
which is a _bucket_ and de oder a _platform_." I mention this because I
shall ask you to attribute my shortcomings in this lecture, not so much
to my lack of familiarity with my native tongue, as to--well, because I
was not educated at Cornell University.

We all know what free air is. You who are privileged to live upon these
beautiful hills overlooking Ithaca and the lake, doubtless know more
about free air than we do who are choked in the dusty confines of New
York City. Compressed air is simply air under pressure. That pressure
may be an active one, as in the case of the piston of an air compressor;
or passive, as with the walls of a receiver or transmission pipe. It is
usual to define compressed air as air increased in density by pressure,
but we know that we may produce compressed air by heat alone. A simple
illustration of this is the pressure which will blow a cork from an
empty bottle when that bottle has been placed near the fire. Here we
have pressure, or compressed air, in the bottle produced by heat alone.

Having defined compressed air, we must next define heat; for in dealing
with compressed air, we are brought face to face with the complex laws
of Thermodynamics. We cannot produce compressed air without also
producing heat, and we cannot use compressed air as a power without
producing cold. Based on the material theory of heat, we would say that
when we take a certain volume of free air and compress it into a smaller
space, we get an increase in temperature because we have the heat of one
volume occupying less space, but no one at this date accepts the
material theory of heat. Your distinguished director, Professor