In the second edition Wright also incorporated various improvements, including proposals for determining the magnitude of the Earth and reckoning common linear measurements as a proportion of a degree on the Earth's surface "that they might not depend on the uncertain length of a barley-corn"; a correction of errors arising from the eccentricity of the eye when making observations using the cross-staff; amendments in tables of declinations and the positions of the sun and the stars, which were based on observations he had made together with Christopher Heydon using a 6 feet quadrant; and a large table of the variation of the compass as observed in different parts of the world, to show that it is not caused by any magnetic pole.