« ПретходнаНастави »
fercher. The understanding of this single proposition, in all that is oredful to comprehend the phenomena of the seasons, and the various lengths of day and night.
284. The Earth, in its diurnal rotation, enn ries every place round in a circle which is equin distant from the equator; and all places which are exactly the same distance from the equatur, are carried round in the same circle.
Distance to the forth or south of the equator is called latitude ; and of course, if the Sun shines vertically at 10 degrees north of the equator, all places having 10 degrees of north latitude will pass exactly under the Sun on that day.
985. As the Sun, when vertical 10 degrees north of the equator, shines 10 degrees beyond the north pole, and 10 degrees shori of the south pole, it is evident that during ibat rotation of The Earth, no place within 10 degrees of the north pole can turu out of the sun-shine, so that to them it will be all day; and that no place within 10 degrees of the south pole, cum turn into the sun-sline, so that to then it will be all night.
280. When the Sun is vertical over the equator, he will then, of course, shine exactly as there as ench pole, and the boundary of day and night will cut all the circles made by the diure nal rotation of every place, into two equal parts: so that the day part of the circle being and to the night part, the days and nights will then, be equal all over the world.
207. The Sun is vertical over the equator on
the 21st of March; and the Earth desrends in its annual orbit for 9 days, till the 21st of June, wben the Sun is vernical over all places 235 degrees north of the equator; so thai, dure ing the on days, the Sun gradually gains the 294 degrees, and has been successively vertical over all countries within that distance of the equator.
288. During the same time, he has succe sively shone, by similar gradual advances, as many degrees beyond the north pole, and afforded to those countries perpetual day; and, of course, an ivereased length of day, to all places in the northern hemisphere, in proportion to their proximity to the pole, in consequence of his shining over the larger part of their diurnal circles.
289. The opposite phenomena will, necessarily, take place in the southern hemisphere of the Earth. The Sun's rays will fall short of the south pole as many degrees as he advances above the equator, and the country round that pole will be involved in darkness; and the nights will increase in length, in the same proportion as the days increase in the northern bemisphere.
Ons.--All this will be made evident, by hanging any round body somewhat below the level of aire or candles It will be seen, that the light shines over one pole, and does not reach the other. If then, the ball be turned round, it will be observad, that the circles performed by any parts of the surface are unequally divided by the light, and that it will be constant day near the north pole, and that all the phenomena will be revented in the other, or lower hemisphere.
290. When the earth has so ascended in its orbit, as to render the sun vertical 23 degrees north of the equator, it then begins to descend again after the same rate; and in 91 days, vir. the 21st of September, the Sun becomes vertical again over the equator.
The earth, however, continues to ascend in its orbit by successive gradations; till, on the 21st of December, the Sun becomes vertical at 23} degrees south of the equator, rendering it constant day at the south pole, and reversing all the phenomena which were experienced, when he was vertical 235 degrees to the north.
291. The earth then gradually descends in its orbit till the 21st of March, when the Sun being vertical over the equator, and inclining towards neither of the poles, the days, of course, are equal in each hemisphere; and spring is beginning in the northeru hemisphere, and autumn in the southern.
Look Nacure through, 'tis revolution all,
202. The heat of summer is produced by two, auscs; first, by the greator intensity of the
Sun's rays, owing to their falling with increased perpendicularity and density; and secondly, by the excess of the length of the days over that of the nights ; so that the heat imbibed in the day is not wholly parted with during the night.
Obs. 1. The effect of obliquity, in regard to rays, will be evident, if a board be held perpendicularly be fore a fire. It will then receive a body of rays equal to its breadth. But if it be placed obliquely, at an angle of 45 degrees, then only balf the rays will fall on its surface, and the other balf will paso over it. So it is with the sur face of the eartla in surimer and in winter.
2. The Ipcreased heat in the polar regions is amazing. ly great, owing to six months' continued day, which melto, or nearly, the ice and snow, produced during the six months night. Hence, in northern countries, where the day lasts 18 and 20 hours, or wbere the Sun is aboro the horizon for any number of days togther, the heat of the summer lo equal to that of any part of the world.
293. The air which we breathe, und in which we live, just as fishes live in water, surrounds the whole earth, to the height, as is supposed, of 40 miles. It is of a blue colour, aud therefore gives that colour to the beavens. The passage of the Sun's rays through it, is supposed to genérate part of their heat, as well as their action on the ground and on various bodies.
294. The solar beat is supposed, indeed, to be excited, chiefly, by its action on terrestrial bodies; it being found, that in the hottest climates, snow and ice lic unthawed all the year in the tops of mountains; and that the heat is, in a considerable degree, in proportion to the height of the situation, or to the warmth imbibed by the surroupding atmosphere from the objects in contact.
203. As the rays of light are turned out of their course, or refracted half o degree when they enter the atmosphere, the Sun's rays, when they reach the earth, represent him a degree higher than he is; and, as he is a de gree in diameter, he appears just above the horixon, when, in reality, his upper edge is just touching it.
So it is at his setting; and with his appear rance and disappearance in the polar regions; and also, with the Moon, and all the heavenly bodies.
206. The denominations of various parts of the onrth are derived from the phenomena pro duced by the Sun in the Earth's descent and as cont in its orbit. All these are given beneath, and they should not be forgotten. North Pole, 90 degrees from the Equator.
Frozen mona. Bouth Pole, 90 degrees from the Equator. 207. The space between the tropics, twice