uctive
power of the storm is confined to the limit of the conical cloud. All
movements for personal safety must extend entirely beyond the
circumference established by the rotary motion. The primary cause of
these tornadoes is probably due to a low barometric condition of the
atmosphere accompanied by a high temperature, and spreading over an
area of very irregular shape. An area of high barometer, accompanied
by a low temperature, encroaches upon the former, and then comes the
mighty effort to equalize these two different conditions of the
atmosphere and restore the equilibrium, which is the constant effort
of nature. The more diverse these two conditions are, the greater will
be the struggle of the giants in the contest. Of course the electrical
condition of the atmosphere existing at the time may form a very
important factor in the tornado which may follow. What was the
character and condition of the atmosphere on the memorable first of
July last, when the storm-cloud which spread desolation over a narrow
belt of not more than two hundred yards at most, swept across the
western half of Chester county, Penn'a? The middle part of the day was
hot and oppressive; the thermometer stood at about 92 and the
barometer about 29.6. The atmosphere seemed very close, and the
inhaling of air in the lungs was attended with rather more difficulty
than usual. I remarked to a friend that there was a peculiar condition
of the atmosphere, and yet who could have foretold the terrible
results of that afternoon? The oldest inhabitant had never heard of a
tornado in this section of country, and yet one was at hand. To give a
faithful and accurate description of the movements and conduct of this
storm-cloud, made it necessary for me to pass over the route of the
moving mass and observe critically its results, and also to inquire of
those living along and near its track what was its appearance, what
was the direction of its sweep through the forest trees, how far the
_debris_ were carried, what amount of life was destroyed, what was the
width of its track, and how the rotary motion of the cloud seemed to
affect the buildings and obstacles that vainly attempted to resist its
march in a direction a little south of east.

The first point of interest in connection with the tornado was about
one mile south-east of the Gap station, on the Penn'a Rail Road, and
about two miles west of the boundary line between Chester and
Lancaster counties. From