sulphur, electrical by friction, 48, 53, 56, 59.
[Greek: sundrome], *vj.
[Greek: sunentelecheia], 68.
Sussex, iron ore in, 22.
sympathy and antipathy, 65, 68, 112.

Tacitus, Cornelius, 25.
Taisner, or Taisnier, Joannes, 5, 107.
Tariassiona or Tarazona, 23.
terrella.
definition of, *vj, 12, 13.
poles and axis of, 13, 72, 81, 144.
divided into two parts, 72.
magnetick vigour, diagram of, 74, 75.
how small pieces of iron behave toward, 75, 76.
orbe of virtue of, 76, 77, 104.
"geography" of, 78.
aequinoctial circle of, 79, 144.
parallels of, 80, 211.
magnetick horizon of, 80.
proportion of the forces in, 81, 82.
experiment with iron sphere, 85.
small iron sphere and rod, 94, 102.
centre of magnetick virtue in, 95.
irregular terrella to exhibit variation, 155, 157.
to illustrate the dip of the needle, 190, 192.
analogy of, with the earth, 41, 78, 119, 211.
testing loadstones, methods of, 108.
Thales of Miletus, 11, 61, 68, 208, 210.
_theamedes_, the, 18.
Thebitius, or Thebit ben Korrah, 117, 236.
Themistius, 71.
Theophrastus, 1, 9, 11.
Thomas Aquinas, 3, 64.
tides, the cause of, 86.
Tycho Brahe, 174, 229.

Variation of the compass, 7, 46, 79, 116, 151-163, 166, 167, 180.
variation at the Azores, 4, 154, 156, 167.
versorium, magnetick, definition of, *vj.
use of, 13, 115, 147.
versorium, non-magnetick, use of, 48, 49, 50.
verticity, 28, 115, 119-147.
acquired, 67, 68, 84, 85, 104, 123, 125, 129, 138, 139, 141, 142, 211.
in iron plates touched by loadstone, 84.
in iron sphere, 85.
how, in iron, 123, 139, 212.
in bracket in tower of St. Augustine's Church, Rimini, 141.
similar at ends of rod touched in middle, 84, 129.
by percussion, 139.
through interposed matter, 67.
not in bodies other than magnetick, 142.
aequator separates two kinds of, 79.
possessed by the earth, as a "Cause," 117.
change of, through change of mass, 72.
definition of, *vj.
described, 119, 120, 121.
destroyed by heat, 66, 93, 124.
earth produces it in loadstone and iron, 42, 140, 211, 212.
excited through greater distances in iron than in air, 104.
exists in all shapes of loadstone, 76.
helps the earth to keep its orbit, 224.
inhaerent in wrought iron, 31, 115.
as a magnetick motion, 46.
mutation of, 120, 137.