Charles. At the poles, the matter is, I suppose, still different. Tutor. There one half of the ecliptic never sets, and the other half never rises; consequently the sun continues one half year above the horizon, and the other half below it. The full moon being always opposite to the sun can never be seen to the inhabitants of the poles, while the sun is above the hori son. But all the time that the sun is below the horizon, the full moons never set. Consequently to them the full moon is never visible in the summer; and in their winter they have her always before and after the full, shining for 14 of our days and nights without intermission. And when the sun is depressed the lowest under the horizon, then the moon ascends with her highest altitude. James. This indeed exhibits in a high degree the attention of Providence to all his creatures. But if I understand you, the inhabitants of the poles have in their winter a fortnight's light and darkness by turns. Tutor. This would be the case for the whole six months that the sun is below the horizon; if there were no refraction ;* and no subsitute for the light of the moon. But by the atmosphere's refracting the sun's rays, he becomes visible a fortnight sooner, and continues a fortnight longer in sight than he would otherwise do were there no such property belonging to the atmosphere. And in those parts of the winter, when it would be absolutely dark in the absence of the moon, the brilliancy of the Aurora Borealis, is probably so great as to afford a very comfortable degree of light. Mr. Hearne in his travels near the polar circle, has this remark in his journal; " December 24. The days were so short, that the sun only took a circuit of a few points of the compass above the horizon, and did not at its greatest altitude rise half way up the trees. The brilliancy of the Aurora Borealis, however, and of the stars, even without the assistance of the moon, made amends for this deficiency, * The subject of refraction will be very particularly explained when we come to Optios. for it was frequently so light all night, that I could see to read a small print.” These advantages are poetically described by our Thomson: By dancing meteors then, that ceaseless shake WINTER, 1. 859. sufficient observations have not been made to ascertain whether he has a diurnal motion on his axis. James. Would you then conclude that he has such motion? Tutor. I think we ought; because it is known to exist in all those planets upon which observations of sufficient extent have been made, and therefore we may surely infer, without much chance of error, that it belongs also to Mercury, and the Herschel, the former from its vicinity to the sun, and the latter from its great distance from that body, having at present eluded the investigation of the most indefatigable astronomers. Charles. At what distance is Mercury from the sun ? Tutor. He revolves round that body at about thirty-seven millions of miles distance, in eighty-eight days nearly; and therefore you can now tell me how many miles he travels in an hour. James. I can; for supposing his orbit circular, I must multiply the 37 millions y 6,* which will give 222 millions of miles for the length of his orbit; this I shall divide by 88, the number of days he takes in performing his journey, and the quotient resulting from this, must be divided by 24, for the number of hours in a day; and by these operations, I find that Mercury travels at the rate of more than 105,000 miles in an hour. Charles. How large is Mercury? Tutor. He is the smallest of all the planets. His diameter is something more than 3200 miles in length. James. His situation being so much nearer to the sun than ours, he must enjoy a considerably greater share of its heat and light. Tutor. So much so, as would indeed infallibly burn every thing belonging to the earth to atoms, were she similarly situated. The heat of the sun at Mercury, must be seven times greater than our summer heat: * See p. 222. |