Intellectual progress dependent on economic progress, i. 218
Intercalary month in the Celtic calendar of Gaul, ix. 342 sqq.
—— periods, customs and superstitions attaching to, ix. 328 sq.;
deemed unlucky, ix. 339 sqq.
—— periods of five days, ix. 339 sqq., 407 n. 1
Intercalation introduced to correct the vague Egyptian year, vi. 26, 27, 28, ix. 340 sq.;
in the ancient Mexican calendar, vi. 28 n. 3, ix. 339 sq.;
in Greek calendar, vii. 81, 83;
rudimentary, to equate lunar and solar years, ix. 325 sqq.