Doubtless such stones and barrows would be rendered obvious in the dark by a light placed on or near them. Cups which could hold oil or grease are known in connection with such stones, and a light thus fed would suffice in the open if there were no wind; but in windy weather a cromlech or some similar shelter must have been provided for it. Now if these standing stones or barrows were ever erected and still remain, accurate plans--not the slovenly plans with which Ferguson and too many others have provided us, giving us either no indication of the north or any other point, or else a rough compass bearing without taking the trouble to state the variation at the time and place--will help us. I have already pointed out that much time has been lost in the investigation of our stone circles, for the reason that in many cases the exact relations of the monuments to the chief points of the horizon, and therefore to the place of sunrise at different times of the year, have not been considered; and when they were, the observations were made only with reference to the magnetic north, which is different at different places, and besides is always varying; few indeed have tried to get at the real astronomical conditions of the problem. The first, I think, was Mr. Jonathan Otley, who in 1849 showed the “orientation” of the Keswick circle “according to the solar meridian,” giving true solar bearings throughout the year. In my opinion the most accurate plans conceivable, in the absence of a long and minute local inquiry, are the 25-inch maps of the Ordnance Survey, on which, I have it on the authority of Colonel Johnston the distinguished Director, each stone may be taken to be shown with a limit of error of 6 feet. With a large circular protractor azimuths can be read to one minute of arc, and in critical cases the true azimuth of the side lines, which are not necessarily meridians as latitudes are not marked, can be found on inquiry at the Ordnance Office, Southampton. Having then true azimuths, the next question concerns the declinations of the stars which may have been observed. The work of Stockwell in America, Danckworth in Germany,[19] and Dr. W. J. S. Lockyer in England, has provided us with tables of the changing declinations of stars throughout past time, or enough of it for our purpose. An accurate determination on the 25-inch map of either the _azimuth_ (angular distance from the N. or S. points) or _amplitude_ (angular distance from the E. or W. points) of the stone or barrow as seen from the centre of the stone circle will enable us to determine the declination of the star at the time when it was observed. I give a diagram which enables this determination to be made with the greatest ease for any monuments between Land’s End and John o’ Groats, whether the direction is recorded by amplitude or azimuth; the declination is read at the side from the value of either indicated, say, by a dot, at the proper latitude.