Silbury Hill, which forms the third member of our group, is situated nearly due south from Avebury, at a distance of 1200 yards from the outside of the ring, of the former, to the foot of the hill, or, as nearly as may be, one Roman mile from centre to centre. Mr. Rickman[90] based an argument on the latter fact, as if it proved the post-Roman origin of the group; and like the many recurring instances of 100 feet and 100 yards, which run through all the megalithic remains, it may have some value, but, as a single instance, it can only be looked upon as a coincidence. The dimensions of the hill, as ascertained by the Rev. Mr. Smith, of Yatesbury,[91] are that it is 130 feet in height, 552 feet in diameter, and 1657 feet in circumference; that the flat top is 104 feet or 102 feet across,[92] according to the direction in which it is measured; this last being another Roman coincidence, as the top has no doubt both sunk and spread. The angle of the slope of the sides is 30 degrees to the horizon. In the year 1777 a shaft was sunk from the top of the mound to the base, by order of the then Duke of Northumberland and Colonel Drax, but no record has been preserved of what they found, or rather did not find, for had they made any discovery of the least importance, it certainly would have been communicated to some of the learned societies of the day. Subsequently, in 1849, a shaft was driven nearly horizontally from the southern face on the level of the original soil to the centre, where it met the Duke's shaft; and subsequently a circular gallery was carried round the centre, but in vain; nothing was found in these excavations that would show that the mound had ever been used for sepulchral purposes, or that threw any light whatever on its history or destination.[93]