from the equator, towards the arctic circle, they would have their days longer than their nights, whilst those on the south side of the equator would have their nights longer than their days; so that in this case, there would be continual summer on the north side of the equator, and continual winter on the south side of it. But as the globe turns round its axis, move your hand slowly forwards, so as to carry it from H towards E, and the boundary of light and darkness will then approach towards the north pole, and recede from the south pole; the northern places will go through less and less of the light, and the southern places through more and more of it; which shows how the northern days decrease in length, and the southern days increase, whilst the globe proceeds from H to E. When the globe is at E, it is at a mean situation between the lowest and highest parts of its orbit; the candle is directly over the equator; the boundary of light and darkness just reaches to both the poles; and all places on the globe go equally through the light and dark hemispheres; which shows that the days and nights are then equal on every part of the earth, the poles only excepted; and there, it is evident, the sun is setting to the north pole, whilst he is rising to the south pole. Continue moving the globe forward, and as it goes through the quarter A, the north pole will recede farther into the dark hemisphere, and the south pole advance more into the light, as the globe comes nearer to; and when the centre of it is N at F, the candle will be directly over the tropic of Capricorn; so that the days are then at the shortest, and nights at the longest in every part of that hemisphere, from the equator to the arctic circle; and the reverse in the southern hemisphere, from the equator to the antarctic circle; within which circles, it is dark to the north frigid zone, and light to the south. Continue both motions as before, and as the globe moves through the quarter B, the north pole advances towards the light, and the south pole recedes towards the dark; the days lengthen in the northern hemisphere, and shorten in the southern; and when the globe comes to G, the candle will be again over the equator, as it was at E, and the days and nights will be equal as before; so that the north pole will be just coming into the light, and the south pole going out of it. From this experiment, we see the reason why the days lengthen and shorten from the equator to the polar circles every year; why there is sometimes no day or night for many revolutions of the earth, within the polar circles; and why the days and nights are equally long all the year round at the equator, which is always equally cut by the terminator, or the circle which is the boundary of light and darkness. All this beautiful variety is occasioned by the inclination of the earth's axis to the plane of its orbit. - "Some say he bid his angels turn askance The poles of earth twice ten degrees and more MILTON. The earth's orbit being elliptical, and the sun constantly keeping in one and the same focus, which is about one million three hundred and seventyseven thousand miles from the centre, the earth will, therefore, be two million seven hundred and fifty-four thousand miles nearer to the sun, at one time of the year than at another; and as the sun appears constantly larger, or under a greater angle in winter than in summer, it is evident that the earth must be nearer to the sun in the former season than in the latter. But here this question will naturally arise; Why have not we the hottest weather when we are nearest to the sun? The earth is above two millions of miles nearer to the sun in December than it is in June, and yet in June it is the middle of summer, and in December the depth of winter; this seems a paradox. In answer to this apparent contradiction, it may be observed, that the excentricity of the earth's orbit, or one million three hundred and seventyseven thousand miles, bears no greater proportion to her mean distance from the sun, than seventeen does to one thousand, and therefore can occasion but little difference in the heat and cold of different seasons. But the principal cause of this difference is, that the sun's rays, in winter, fall so obliquely upon us, and have so large a portion of the atmosphere to pass through, that they come with less force, and spread over a larger space than they do in summer, or when the sun is at a greater height above the horizon. In the winter long nights, we have also a greater degree of cold than can be com [LET. XI. pensated for by the return of heat in the short days; and on both these accounts the cold will be much increased. Whereas in summer, the sun's rays descend more perpendicularly upon us, and therefore fall with a greater force, and in a greater quantity, upon any particular place, than when they come more obliquely. The sun is also much longer above the horizon than in winter, and, consequently, a greater degree of heat will be imparted by day, than can fly off by night; so that the heat, on all these accounts, will continue to increase. LETTER XII. OF THE NATURAL AND ARTIFICIAL DIVISIONS OF TIME. THE more we extend our views, the more we are perplexed and embarrassed. Things which once appeared the most familiar to our understandings, are now hid under an impenetrable veil, and become totally mysterious and inexplicable. Education is a new birth to man; but, with all the advantages that art and nature can bestow, he is still a limited and confined being. Numberless are the questions that may be put to the most profound philosopher, which, if he be ingenuous, he must confess his entire inability to resolve. What, for instance, is time, space, matter, or motion? Every one, who speaks of these things, imagines himself to be clearly understood, even by the most illiterate; and yet if you require an explanation of the question, no one is able to give a rational answer to it. If nobody asks me, said St. Augustine, what time is, I know; but if any body asks me, I do not know. Another philosopher being desired to explain the nature of motion, got up and walked. I cannot define it, said he, but I'll show you the thing itself. But of all the definitions that ever were given of motion, that of Aristotle is the most curious; who tells us that it is "the act of a being in power, as far as it is in power;" which, as Mr. Locke observes, is such a jargon, as would puzzle |