The storm-cloud of a tornado is nearly always funnel-shaped, the small end of the funnel extending downward. It looks like an immense balloon, and revolves on its axis with fearful rapidity. The air beyond the limits of this cloud is also in rapid-motion, but merely partakes of the character of a very high wind and is not particularly destructive. The death-dealing and destructive 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 this point the storm-cloud proceeded for about two miles in a direction south 70 degrees east, or about 20 degrees south of a line running due east. About three-fourths of a mile east of Sadsbury Meeting House a slight change of direction occurred in the movement of the cloud, and it took a direction a little to the north, running south 75 degrees east. It proceeded, with slight local variations, for about eight miles in a direct line, and cutting a track about 200 yards wide, until it reached the property of William Hamill, in East Fallowfield township, near Newlin’s Mill, and here it widened to about 300 yards, destroying the fences, crops, etc, on his property. At this point a slight change of direction occurred towards the south, bringing it into contact with the timber tracts of E. Phipps and Thos. Shields, in which a terrible destruction of forest trees occurred. It now veered to the north about ten degrees, and passed through the southern half of the village of Ercildoun in a line south 83 degrees east, or in a direction nearly due east, and continued thence until it arose in the air about half-a-mile east of Ercildoun, and proceeded, at a higher elevation, for about seven miles, relieving the farms and property of the intervening country from destruction, As it approached Broad Run, about a mile west of Marshallton, it descended sufficiently long to unroof and almost destroy the barns and out-buildings of two properties, owned respectively by Richard Baily and Joseph Marshall, of West Bradford township. Here it came to an end in its mad and reckless career. The two opposing currents of air had no doubt now become thoroughly blended and partook of the character of a high wind, fully relieved of its devastating properties. The storm-cloud was dissolved, or had permanently taken a higher elevation over a still greater amount of territory. The whole route of the tornado, as measured by its effects, was about 22 miles. The width of its track was from 100 to 300 yards, averaging generally about 200 yards. The following points also have been pretty clearly established by the use of the compass, and also by careful observation along the route of the storm:

First. That the general direction of the storm-cloud during the first half of its journey, to a point near Newlin’s Mill, one mile west of Ercildoun, was south 75 deg. east, but at that point it changed and its course afterwards was south 83 deg. east, or nearly due east.

Second. That the destruction of property was generally greater as the cloud moved across a valley.

Third. That the velo¬city of the moving mass varied at different periods from 5 to 15 miles an hour, but twelve miles an hour would be considered a fair average.

Fourth. That the trees along the southern side of the track of desolation were generally thrown with their tops towards the north, or at right angles to the direction of the progressive motion of the cloud, while those on the northern side were thrown in the opposite direction.

Fifth. That in some instances houses and buildings near the centre of the track were but slightly injured these cases, however, were rare.

Sixth. That from local and other causes, the lower part of the conical cloud frequently moved out of a straight course, while the upper or larger part of the cone kept in a line very nearly direct.

Seventh. That as soon as the cloud was formed, a roaring sound commenced, which continued without interruption during its entire course. This sound was not unlike continuous thunder.

Eighth. That the movement of the storm-cloud was unaccompanied with much rain or hail, though one or the other fell at some distance north or south of the track, the sun frequently shining at the time. To explain some of these phenomena, even with the aid of science, is difficult. The storm-cloud itself was an entirely exceptional phenomenon in this latitude. Such an event had never occurred before in eastern Pennsylvania, and we are without the benefit of previous observation and experience. The great destruction of property in crossing valleys has excited marked attention. The cloud undoubtedly required an immense amount of air to feed it as it went along. Persons near its track say that they breathed with the greatest difficulty. The surrounding air must have been very rare; in fact, a partial exhaustion must have resulted from the absorption of air by the moving mass. In crossing a valley at right angles, or nearly so, the dense air up and down on either side, would be at hand to furnish it with the necessary material, thus increasing its power and devastation; this is one explanation. Another theory, which is probably the correct one, could safely be advanced upon plausible grounds. Supposing electricity to be the primal cause of the cloud itself, in passing across deep and irregular valleys with rugged surface, more electricity would be developed, and greater power would be infused into the revolving cone as it moved forward. When passing over a smooth, level plateau, it would excite less of the electrical fluid, and would hence be disarmed of a portion of its ability to destroy buildings and fences.

A full description of the great tornado in Chester County, Pa, 1877

Continued in part 2, below