This, of course, only gives us a first approximation. The angular height of the point on the horizon to which the alignment or sight-line is directed by the stone or barrow from the centre of the circle must be most accurately determined, otherwise the declinations may be one or two degrees out. In the absence of measurements it is convenient to assume, in the first instance, that the horizon is half a degree high, as with this elevation refraction is compensated, as the following table will show: Elevation of actual Bessel’s horizon. refraction. Combined effect. 0°0′0″ 34′54″ -34′54″ 0°10′ 32′49″ -22′49″ 20′ 30′52″ -10′52″ 30′ 29′3·5″ +0′56·5″ 40′ 27′22·7″ +12′37·3″ 50′ 25′49·8″ +24′10·2″ 1°0′ 24′24·6″ +35′35·4″ In the absence of theodolite observations the actual elevation of the horizon can be roughly found by a study of the contour lines on the 1-inch map. The following heights will agree with the previous assumption of hills ¹⁄₂° high: Distance 1 mile Height = 46 feet „ 2 miles „ = 92 „ „ 4 „ „ = 184 „ „ 8 „ „ = 368 „ „ 10 „ „ = 460 „ [Illustration: FIG. 33.--Diagram for finding declination from given amplitudes or azimuths in British latitudes. ~Curves represent (from top) Lat. 49°, 51°, 53°, 55°, 57° and 59°.~] I also give other diagrams showing the changing declinations of the brightest stars, those which would naturally be observed, between the years 150 A.D. and 2150 B.C. These have been plotted from the calculations of the authorities I have named. Fig. 34 deals with the Northern stars. The stars are numbered as follows:-- Number. Name of star. 1 β Ursae Minoris. 2 α Ursae Minoris (Polaris). 3 α Draconis. 4 α Ursae Majoris (Dubhe). 5 γ Ursae Majoris. 6 η Ursae Majoris (Benetnasch). 7 γ Draconis. 8 β Cassiopeiae. 9 α Cassiopeiae. 10 α Persei. 11 α Aurigae (Capella). 12 α Cygni. 13 α Lyrae (Vega). 14 α Coronae. 15 α Geminorum (Castor). 16 β Geminorum (Pollux). 17 α Boötes (Arcturus). 18 β Leonis. 19 α Leonis (Regulus). 20 α Andromedae. 21 η Tauri (Alcyone). 22 α Tauri (Aldebaran). 23 α Canis Minoris (Procyon). 24 α Aquilae. 25 α Orionis (Betelgeuse). 26 α Virginis (Spica). On Fig. 35, dealing with the Southern stars, the names are given along the curves. * * * * * Now supposing that we have our plans; that we have determined the azimuth of the sight lines; and have found the declination of the star observed; we may find more than one star occupying that declination at various dates.