This value of the azimuth, the mean of which is 49° 35′ 51″, is confirmed by the information, also supplied from the Ordnance Survey, that from the centre of the temple, the bearing to the N.E. of the principal bench mark on a hill, about 8 miles distant, the bench mark being very near a well-known ancient fortified British encampment named Silbury or Sidbury, is 49° 34′ 18″; and that the same line continued through Stonehenge, to the south-west, strikes another ancient fortification, namely, Grovely Castle, about 6 miles distant, and at practically the same azimuth, viz., 49° 35′ 51″. For the above reasons 49° 34′ 18″ has been adopted for the azimuth of the avenue. The summer solstice sunrise in 1901 was also watched for by Mr. Howard Payn on five successive mornings, viz., June 21 to 25, and was successfully observed on the last occasion. As soon as the Sun’s limb was sufficiently above the horizon for its bisection to be well measured, it was found to be 8′ 40″ northwards of the peak of the Friar’s Heel, which was used as the reference point; the altitude of the horizon being 35′ 48″. The azimuth of this peak from the point of observation had been previously ascertained to be 50° 39′ 5″, giving for that of the Sun when measured, 50° 30′ 25″; by calculation that of the Sun, with the limb 2′ above the horizon, should be 50° 30′ 54″. This observation was therefore completely in accordance with the results which had been obtained otherwise. The time which would elapse between geometrical sunrise, that is, with the upper limb tangential with the horizon, and that which is here supposed, would be about 17 seconds, and the difference of azimuth would be 3′ 15″. The remaining point was to find what value should be given to the Sun’s declination when it appeared showing itself 2′ above the horizon, the azimuth being 49° 34′ 18″. The data obtained for the determination of the required epoch were as follows:-- (1.) The elevation of the local horizon at the sunrise point seen by a man standing between the uprights of the great trilithon (a distance of about 8000 feet) is about 35′ 30″, and 2′ additional for Sun’s upper limb makes 37′ 30″. (2.) -Refraction + parallax, 27′ 20″. (3.) Sun’s semi-diameter, allowance being made for greater eccentricity than at present, 15′ 45″. (4.) Sun’s azimuth, 49° 34′ 18″, and N. latitude, 51° 10′ 42″. From the above data the Sun’s declination works out 23° 54′ 30″ N., and by Stockwell’s tables of the obliquity, which are based upon modern determinations of the elements of the solar system,[14] the date is found to be 1680 B.C. It is to be understood that on account of the slight uncertainty as to the original line of observation and the very slow rate of change in the obliquity of the ecliptic, the date thus derived may possibly be in error by 200 years more or less; this gives us a date of construction lying between say 1900 and 1500 B.C.