  Terrestrial fire is presented as the weak variant of sidereal fire; it joins with oily substances and sulphur, for all of which Bacon introduces the Sulphur Quaternion. These quaternions comprise antithetical qualities: air and ether versus fire and sidereal fire. The struggle between these qualities is determined by the distance from the earth as the absolute center of the world system. Air and ether become progressively weaker as the terrestrial and sidereal fire grow stronger. The quaternion theory functions in Bacon's thought as a constructive element for constituting his own theory of planetary movement and a general theory of physics. This theory differs from all other contemporary approaches, even though Bacon states that “many theories of the heavens may be supposed which agree well enough with the phenomena and yet differ with each other” (Bacon IV [1901], 104). The diurnal motion of the world system (9th sphere) is driven by sympathy; it carries the heavens westward around the earth. The sidereal fire is powerful and, accordingly, sidereal motion is swift (the stars complete their revolution in 24 hours). Since the sidereal fire becomes weaker if it burns nearer to the earth, the lower planets move more slowly and unevenly than the higher ones (in this way Bacon, like Alpetragius, accounts for irregular planetary movement without reference to Ptolemy's epicycle theory). He applies his theory of consensual motion to physics generally (e.g., wind and tides) and thus comes into conflict with Gilbert's doctrine of the interstellar vacuum and Galileo's theory of the tides (for Bacon, the cycle of tides depends on the diurnal motion of the heavens but, for Galileo, on the earth's motion).