Assuming Stonehenge to be the prototype of our rude stone circles, it may be well to remember its general features, and particularly the dimensions of its plan. Its architecture consisted of an outer circle of ditch and earthen bank of an approximate diameter of three hundred feet, broken at the entrance from the north-east, where the banks are continued in that direction, and form an avenue of approach fifty feet in breadth. Within was a concentric circle one hundred feet in diameter, of upright stones supporting a continuous lintel. These stones are roughly squared, and the pillars now measure about fourteen feet above ground, whilst the lintels are about eleven and a half feet long. Ten feet within was a minor concentric circle of pillar stones, a few feet in height, arranged in pairs. Again, within were five huge trilithons arranged in the plan of a horse-shoe with a diameter of about fifty feet, and composed of stones similar in form to those of the outer peristyle, but varying in height to nearly twenty-five feet above the turf; one stone, for example, measuring, when exposed by excavation, twenty-nine feet eight inches in length. Finally, within the whole is the “altar-stone,” some sixteen feet by four, lying prone and within a broken, or horse-shoe shaped ellipse of a diameter of forty feet, composed of pillar stones about five or six feet high. Without the whole, and at a distance of two hundred and fifty feet from the centre, is a monolith, or “pointer,” sixteen feet high, known as the Friar’s Heel. It stands to the north-east and a fraction to the south of a line drawn from the altar-stone along the centre of the avenue. Another stone, now fallen, lies on the line just within the enclosure. From this very superficial description it will be noticed that there is a certain geometrical proportion to scale. The diameter of the outer bank is three times that of the peristyle, which, in turn, is twice that of the trilithons. The space betwen the peristyle and the outer bank equals the diameter of the former. The diameter of the outer circle of small pillar stones is twice that of the inner ellipse of pillar stones, and the distance of the Friar’s Heel from the peristyle is twice the diameter of the latter. Even admitting a wide margin for inaccuracy, the impression must remain that there is ground for the suspicion that some attempt at a decimal system prevailed in the general plan of this mysterious monument. These proportions are so obvious that it seems unlikely that they have escaped the attention of those who have studied the plan of Stonehenge. It was not, however, in relation to the great monument of the south that a possible system of geometrical mensuration suggested itself; but in the survey of our own hill-circles of Derbyshire, when it appealed so forcibly to observation that it prompted a reference to the prototype for possible confirmation.