The large Circle D is said to have a difference of diameters of twenty-six feet, these being respectively 1189 feet and 1163 feet.
The Observatory Circle, which is the inclosure connected with the Octagon, was found to have been made with remarkable correctness. “The widest divergence between the line of the survey and the circumference of the true circle is four feet. It is therefore evident that the inclosure approaches in form very nearly an absolute circle.”
Professor Thomas also states with reference to the Observatory Circle, that the radius is almost an exact multiple of the surveyor’s chain.
The geometrical accuracy of the lines of embankments and of the inclosed areas in earthworks of such great dimensions, covering such large spaces of ground, is not the least strange fact concerning these works.
Archæologia Americana, Vol. I. The plan of the Marietta Inclosures is a reduction of a part of the survey made in 1837 by Mr. Charles Whittlesea, and published by the Smithsonian Institution in 1848.
Fort Du Quesne was built about the year 1752. It was situated near the spot where is now the town of Pittsburgh. In 1731 the Indians who then occupied the lands near Marietta formed an alliance with the French, and obtained their assistance in protecting them from the attacks of hostile tribes. These were probably the Iroquois, who at that period had made a treaty with the English, and were their allies during the wars against the French in Canada and this part of North America.
Archæologia Americana, Vol. I.