Pl. II. SEASONS.—Summer.
This line of magnetic, or heat activity, consequently varies with the earth's movements. On the 20th of June the flood of summer heat overspreads the northern portions of the earth; the sun thence apparently turns southward, and with its departure the relations of the line of heat activity change. The city of New York, which on the 20th of June is found nearest the centre of the solar current (Plate II. b), is, on the 21st of December, located at its greatest distance from the line of magnetic or heat intensity (Plate III. b), where the heat-producing forces are in operation in but low degree.
Pl. III. SEASONS.—Winter.