Sun-heat, therefore, like sunlight and gravity, is a veritable production, yet it is not due to the process of combustion. It is not dependent for its creation upon the destruction of fabulous quantities of substantial materials. _The rather does it originate in, and is it disseminated through the vast energies of spheres retro-acting upon spheres throughout the whole universe of matter._ FOOTNOTES: [6] Appendix, p. 99. [7] Appendix, p. 100. [8] Appendix, p. 100. [9] Appendix, p. 101. [10] Appendix, p. 102. [C] In the motions of the spheres through space, unlike all other forms of motion, there is no element of resistance. This form of motion is therefore incapable of developing _vis viva_. CHAPTER VI. THE SEASONS. _Why their varying Temperature?_ The usual explanation of these phenomena, _i. e._, the influence of direct and oblique sun-rays, has ever seemed insufficient and unsatisfactory; especially in view of the _fact_ that the heat comes not from the sun by continuity after the manner of progression as from a heated body. A philosophy more exact and consistent may be found in the development of the theory already advanced, and which is illustrated in the following plates. _The maximum of heat at the surface of the earth bears a very constant and intimate relation to the line of greatest diameters of the sun and earth._--Pl. II. a. Through this line the heat-producing functions of these great spheres are in operation in the highest degree. [Illustration: 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. [Illustration: Pl. III. SEASONS.--WINTER.] CHAPTER VII. GRAVITY. _Its Essential Nature, and its Source._ Gravity is not a separable entity, not a power _per se_. It is but a production, and an operation, of the same retro-action between sun-core, and earth-core. This retro-action gives rise to a stupendous magnetic circuit, as described, in which both sun and earth become the embodiments of magnetic force, or, in other words, great magnets.[11] The power thus developed is exercised in preserving the relative positions of the two bodies, and, on the part of the earth, as we know, in drawing unto itself all objects within its influence.