Page:Proceedings of the Royal Society of London Vol 69.djvu/150

142 for more than 1 300 feet from the centre of the temple, and particularly the line of the bottom of the ditch from which the earth was taken to form the bank, and which runs parallel to it. Measurements taken from this line assisted materially those taken from the crown of the bank itself. "With this help and by using the southern bank and ditch whenever it admitted of recognition, a fair estimate of the central line could be arrived at. To verify this, two pegs were placed at points 140 feet apart along the line near the commencement of the avenue, and four others at distances averaging 100 feet apart nearer the further recognisable extremity, and their directions were measured with the theodolite, independently by two observers, the reference point being Salisbury Spire, of which the exact bearing from the centre of the temple had been kindly supplied by Colonel Johnston, R.E., the Director-General of the Ordnance Survey. The same was also measured locally by observations of the Sun and of Polaris, the mean of which differed by less than 20" from the Ordnance value. The resulting observations gave for the axis of the avenue nearest the commencement an azimuth of 49" 38' 48", and for that of the more distant part 49 32' 54". The mean of these two lines drawn from the central interval of the great trilithon, already referred to, passes between two of the sarsens of the exterior circle, which" have an opening of about 4 feet, within a few inches of their middle point, the deviation being northwards. This may be considered to prove the close coincidence of the original axis of the temple with the direction of the avenue.

This value of the azimuth, the mean of which is 49 35' 51", is con- firmed by the information, also supplied from the Ordnance Survey, that from the centre of the temple the bearing of the principal bench mark on the ancient fortified hill, about 8 miles distant, a well-known 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 C 35' 51". For the above reasons 49 34' 18" has been adopted for the azimuth of the avenue.

The present solstitial sunrise was also watched for on five successive mornings, viz., June 21 to 25, and was successfully observed on the latter 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 xeference point, the altitude of the horizon being 35' 48". The azimuth of this peak from the point of observation had been pre- viously ascertained to be 50 39' 5", giving for that of the Sun when measured, 50 30' 25", and by calculation that of the Sun with the limb 2' above the horizon should be 50 30' 54". This observation