The dolmens have, I am convinced, been in many cases not graves originally, but darkened observing places whence to observe along a sight-line; this would be best done by means of an _allée couverte_, the predecessor of the darkened naos at Stonehenge, shielded by its covered trilithons. In order to obtain some measurements to test the orientation theory in Britain, I found that Stonehenge is the ancient monument in this country which lends itself to accurate theodolite work better than any other. Mr. Spence’s excellent work on astronomical lines at Stenness, where the stones, till some years ago at all events, have been more respected than further south, suggested a beginning there, but the distance from London made it impossible. Avebury and Stanton Drew are well known to a great many archæologists; there are also other very wonderful stone circles near Keswick and in other parts of England; but unfortunately it is very much more difficult to get astronomical data from these ancient monuments than it is in the case of Stonehenge, one reason being that Stonehenge itself lies high, and the horizon round it in all directions is pretty nearly the same height, so that the important question of the heights of the hills along the sight-line--a matter which is fundamental from an astronomical point of view, although it has been neglected, so far as I can make out, by most who have made observations on these ancient monuments--is quite a simple one at Stonehenge. Hence it was much easier to determine a date there than by working at any of the other ancient remains to which I have referred. In orientation generally--such orientation as has been dealt with by Mr. Penrose and myself in Egypt and in Greece--the question frequently was a change in direction in the axis of a temple, or the laying down of the axis of a temple, by means of observations of stars. Unfortunately for us as archæologists, not as astronomers, the changes of position of the stars, owing to certain causes, chiefly the precessional movement, are very considerable; so that if a temple pointed to a star in one year, in two or three hundred years it would no longer point to the same star, but to another. These star observations were requisite in order to warn the priests about an hour before sunrise so that they might prepare for the morning sacrifice which always took place at the first appearance of the sun. Hence the morning star to be visible in the dawn must be a bright one, and the further north or south of the sun’s rising place it rose, the more easily it would be seen. Some stars so chosen rose not far from the north point of the horizon. The alignments with small azimuths referred to in the British circles (p. 36) I believe to be connected with the Egyptian and Greek practice.