PLATE III. SEASON S. Fig. 1. reprefents, according to the Lecture, p. 134, twelve globes placed round the fun; the axis of each perpendicular to the plane of its orbit: of consequence, if we admit their daily rotation, that rotation must be coincident with the plane of their orbit. Every part of each globe, therefore, will be exactly half a day (i. e. half a rotation) in light, and the other half in darkness; nor does the smallest difference occur between any two globes, but as well 1, as 7, 4, as 10, 6, as 12, enjoys perpetual equinox. If one globe were placed fucceffively in the stations 2, 3, 4, &c. it would have no variation of Seafons, or other viciffitude than day and night, as is clear by the figure. Fig. 2. In the former figure we fuppofed what is not true; in this figure we attend folely to fact; for instead of being upright, the axis of the earth, N S, appears inclined throughout the whole of these figures; in which, preferving the idea of twelve globes, we obferve, that the oppofites are ftrongly contra-diftinguished. Except No. 1 and 7, which differ only, in that to one it is noon-day, to the other midnight. No. 1. N the North Pole, S the South Pole; 1, 2, the arctic circle; 3, 4, the tropic of Cancer; EQ, the equa tor, tor, or line; 5, 6, the tropic of Capricorn; 7, 8, the antarctic circle. [The fimilar lines on the other numbers correfpond to those on this figure.] The fun appears to be vertical in the equator, making equal night and day throughout this globe; as appears by inspection. We suppose a rotation from 1 to 2, 3 to 4, E to Q, 5 to 6, 7 to 8. No. 2. In this globe the North Pole lofes its diurnal share of light, and the South Pole gains it; because the verticity of the fun is not in the equator, but fouthward of it, being now at the dotted line, nearer the tropic of Capricorn. No. The fun's vertical station yet nearer the tropic. 3. No. 4. In this globe the North Pole is totally deprived of the folar beams: the daily rotation of the globe bringing no part of the arctic circle into his light, while the South Pole enjoys his continued influences. No. 5. The folar vertical ftation nearer the equator. No. 6. The fun ftill nearer the equator. No. 7. The fun in the equator (equinox). No. 8. The fun above the equator, advancing northward. The fame progrefs more apparent. No. 9. No. 10. The fun in the tropic of Cancer: the North Pole illuminated during the whole rotation: the South Pole constantly in darkness. The fame circumstances attend all places in each hemifphere, according to their distances from the pole. No. 11. The fun declining toward the equator. Obferve Obferve the oppofitions of thefe globes; No. 1. and 7. are alike, except that to one it is day, to the other night. No. 2. The fun below the line. No. 8. The fun above it. No. 4. The North Pole dark, confequently winter there and fummer in the fouth. No. 10. The North Pole light, confequently fummer there and winter in the fouth. The fame would be the feafons, &c. of a fingle globe, progreffively in the fituations 1, 2, 3, &c. as appears by the figure. Fig. 3. The plan of the folar verticity, progreffively advanc ing and receding on the furface of the globe. 1. The fun at in the equator; 2. 3. the fun advanced northward; 4. the folftice; 5. 6. the fun returning fouth; 7. the fun again on the line, going fouth; 8. 9. 10. the fouthern folftice; 11. 12. the fun returning to the line 1. r. The letters below the figure indicate the months of the year. Though it has been faid, the inclination of the earth's axis is conftantly the fame, yet this excludes not fome fmall variation; the attractive power of the fun and moon flightly dif turbing it at the folftice, the poles nodding, as it were, a few feconds, and returning to their places in about 18 years. There is alfo a motion which decreafes gradually the obliquity of the ecliptic, which formerly rather exceeded 23: 30'; but now is lefs nearly 2'. The equinoxes alfo do not happen in any fucceeding year precifely to a moment as the year before: but have a retrograde motion of about 50 yearly, arifing from the attraction of the celeftial luminaries. This motion completes a revolution in about 25,000 years. To fuch difturbances, though infenfible to its inhabitants, are subject the firmeft principles of the terreftrial motions! VII. H h PLATE PLATE IV. LENGTH OF DAY AND NIGHT. THESE three figures fhew more correctly (being larger) the principal effects arifing from the changing station of the folar verticity, to any particular place, as LONDON (L). The daily rotation of the earth supposed 1 to 2, 3 to 4, &c. The references are alike in all the figures. N the North Pole, S the South Pole, 1, 2, the arctic circle, 3, 4, the tropic of Cancer, E Q, the Equator, 5, 6, the tropic of Capricorn. Fig. 1. The fun in its higheft northern latitude (the tropic of Cancer); the North Pole wholly enlightened. LONDON (L) by much the greater part of the twenty-four hours enjoying the light of the fun; and when by the rotation of the globe brought to the fhadowed part of it, rather dipping into, than remaining in it. The femi-diurnal arc being 3a, 8 hours 16 minutes; the femi-nocturnal arc a 4, 3 hours 44 minutes. Fig. 2. The Sun in the equator; the femi-diurnal arc E C, equal to the femi-nocturnal arc CQ; confequently the equinox. Fig. 3. the reverse of fig. 1. The fun highly south, L enjoys but fhort day-light, being quickly brought again into the shadow the day is now the length of the night (7 hours 28 minutes) in fig. 1. and the night the length of the day (16 hours 32 minutes). Whenever the fun comes in the daily rotation to the line N S, it is fix o'clock; which in fig. 1, is long before the shadow begins; in fig. 2. is exactly at the beginning of the fhadow; in fig. 3. is immerfed confiderably in the shadow. PLATE |