d to produce a more perfect product
without increasing cost or investment in proportion. His reasoning led
him to the conclusion that this result could only be obtained through
the use of a kiln of comparatively great length, and his investigations
and experiments enabled him to decide upon a length of one hundred and
fifty feet, but with an increase in diameter of only six inches to a
foot over that of the sixty-foot kiln.

The principal considerations that influenced Edison in making this
radical innovation may be briefly stated as follows:

First. The ability to maintain in the kiln a load from five to seven
times greater than ordinarily employed, thereby tending to a more
economical output.

Second. The combustion of a vastly increased bulk of pulverized coal
and a greatly enlarged combustion zone, extending about forty feet
longitudinally into the kiln--thus providing an area within which
the material might be maintained in a clinkering temperature for a
sufficiently long period to insure its being thoroughly clinkered from
periphery to centre.

Third. By reason of such a greatly extended length of the zone of
oncoming material (and consequently much greater bulk), the gases and
other products of combustion would be cooled sufficiently between the
combustion zone and the stack so as to leave the kiln at a comparatively
low temperature. Besides, the oncoming material would thus be gradually
raised in temperature instead of being heated abruptly, as in the
shorter kilns.

Fourth. The material having thus been greatly raised in temperature
before reaching the combustion zone would have parted with substantially
all its carbon dioxide, and therefore would not introduce into the
combustion zone sufficient of that gas to disturb the perfect character
of the combustion.

Fifth. On account of the great weight of the heavy load in a long kiln,
there would result the formation of a continuous plastic coating on that
portion of the inner surface of the kiln where temperatures are
highest. This would effectively protect the fire-brick lining from the
destructive effects of the heat.

Such, in brief, were the essential principles upon which Edison based
his conception and invention of the long kiln, which has since become so
well known in the cement business.

Many other considerations of a minor and mechanical nature, but which
were important factors in his solution of this difficult problem, are
worthy of study by