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13 We cannot rely on celestial observations, made within ten or twelve degrees of the horizon, on account of the density of the atmosphere being subject there to irregular variation. The quantity of refraction, at the same distance from the zenith, varies nearly as the height of the barometer, the tem- perature being constant. Every rise in temperature, equal to a degree Fahrenheit, diminishes refraction by about the -oth part. The refraction of the atmosphere is not affected by its hygrometric state. Dr Bradley found by calculation, the amount of refraction corresponding to each altitude. prev next |