Part 24 (1/2)

_Emily._ You say, that the superior heat of the equatorial parts of the earth, arises from the rays falling perpendicularly on those regions, whilst they fall obliquely on these more northern regions; now I do not understand why perpendicular rays should afford more heat than oblique rays.

_Caroline._ You need only hold your hand perpendicularly over the candle, and then hold it sideways obliquely, to be sensible of the difference.

_Emily._ I do not doubt the fact, but I wish to have it explained.

_Mrs. B._ You are quite right; if Caroline had not been satisfied with ascertaining the fact, without understanding it, she would not have brought forward the candle as an ill.u.s.tration; the reason why you feel so much more heat if you hold your hand perpendicularly over the candle, than if you hold it sideways, is because a stream of heated vapour constantly ascends from the candle, or any other burning body, which being lighter than the air of the room, does not spread laterally but rises perpendicularly, and this led you to suppose that the rays were hotter in the latter direction. Had you reflected, you would have discovered that rays issuing from the candle sideways, are no less perpendicular to your hand when held opposite to them, than the rays which ascend when your hand is held over them.

The reason why the sun's rays afford less heat when in an oblique direction, than when perpendicular, is because fewer of them fall upon an equal portion of the earth; this will be understood better by referring to plate 10. fig. 1, which represents two equal portions of the sun's rays, s.h.i.+ning upon different parts of the earth. Here it is evident, that the same quant.i.ty of rays fall on the s.p.a.ce A B, as fall on the s.p.a.ce B C; and as A B is less than B C, the heat and light will be much stronger in the former than in the latter; A B, you see, represents the equatorial regions, where the sun s.h.i.+nes perpendicularly; and B C, the temperate and frozen climates, where his rays fall more obliquely.

_Emily._ This accounts not only for the greater heat of the equatorial regions, but for the greater heat of our summers, as the sun s.h.i.+nes less obliquely in summer than in winter.

_Mrs. B._ This you will see exemplified in figure 2, in which the earth is represented, as it is situated on the 21st of June, and England receives less oblique, and consequently a greater number of rays, than at any other season; and figure 3, shows the situation of England on the 21st of December, when the rays of the sun fall most obliquely upon her.

But there is also another reason why oblique rays give less heat, than perpendicular rays; which is, that they have a greater portion of the atmosphere to traverse; and though it is true, that the atmosphere is itself a transparent body, freely admitting the pa.s.sage of the sun's rays, yet it is always loaded more or less with dense and foggy vapour, which the rays of the sun cannot easily penetrate; therefore, the greater the quant.i.ty of atmosphere the sun's rays have to pa.s.s through in their way to the earth, the less heat they will retain when they reach it. This will be better understood, by referring to fig. 4. The dotted line round the earth, describes the extent of the atmosphere, and the lines which proceed from the sun to the earth, the pa.s.sage of two equal portions of the sun's rays, to the equatorial and polar regions; the latter you see, from its greater obliquity, pa.s.ses through a greater extent of atmosphere.

_Caroline._ And this, no doubt, is the reason why the sun, in the morning and in the evening, gives so much less heat, than at mid-day.

_Mrs. B._ The diminution of heat, morning and evening, is certainly owing to the greater obliquity of the sun's rays; and they are also affected by the other, both the cause, which I have just explained to you; the difficulty of pa.s.sing through a foggy atmosphere is perhaps more particularly applicable to them, as mist and vapours are prevalent about the time of sunrise and sunset. But the diminished obliquity of the sun's rays, is not the sole cause of the heat of summer; the length of the days greatly conduces to it; for the longer the sun is above the horizon, the more heat he will communicate to the earth.

_Caroline._ Both the longest days, and the most perpendicular rays, are on the 21st of June; and yet the greatest heat prevails in July and August.

_Mrs. B._ Those parts of the earth which are once heated, retain the heat for some length of time, and the additional heat they receive, occasions an elevation of temperature, although the days begin to shorten, and the sun's rays to fall more obliquely. For the same reason, we have generally more heat at three o'clock in the afternoon, than at twelve, when the sun is on the meridian.

_Emily._ And pray, have the other planets the same vicissitudes of seasons, as the earth?

_Mrs. B._ Some of them more, some less, according as their axes deviate more or less from the perpendicular, to the plane of their orbits. The axis of Jupiter, is nearly perpendicular to the plane of his...o...b..t; the axes of Mars, and of Saturn, are each, inclined at angles of about sixty degrees; whilst the axis of Venus is believed to be elevated only fifteen or twenty degrees above her orbit; the vicissitudes of her seasons must therefore be considerably greater than ours. For further particulars respecting the planets, I shall refer you to Bonnycastle's Introduction to Astronomy.

I have but one more observation to make to you, relative to the earth's motion; which is, that although we have but 365 days and nights in the year, she performs 366 complete revolutions on her axis, during that time.

_Caroline._ How is that possible? for every complete revolution must bring the same place back to the sun. It is now just twelve o'clock, the sun is, therefore, on our meridian; in twenty-four hours will it not have returned to our meridian again, and will not the earth have made a complete rotation on its axis?

_Mrs. B._ If the earth had no progressive motion in its...o...b..t whilst it revolves on its axis, this would be the case; but as it advances almost a degree westward in its...o...b..t, in the same time that it completes a revolution eastward on its axis, it must revolve nearly one degree more in order to bring the same meridian back to the sun.

_Caroline._ Oh, yes! it will require as much more of a second revolution to bring the same meridian back to the sun, as is equal to the s.p.a.ce the earth has advanced in her orbit; that is, nearly a degree; this difference is, however, very little.

_Mrs. B._ These small daily portions of rotation, are each equal to the three hundred and sixty-fifth part of a circle, which at the end of the year amounts to one complete rotation.

_Emily._ That is extremely curious. If the earth then, had no other than its diurnal motion, we should have 366 days in the year.

_Mrs. B._ We should have 366 days in the same period of time that we now have 365; but if we did not revolve round the sun, we should have no natural means of computing years.

You will be surprised to hear, that if time is calculated by the stars instead of the sun, the irregularity which we have just noticed does not occur, and that one complete rotation of the earth on its axis, brings the same meridian back to any fixed star.

_Emily._ That seems quite unaccountable; for the earth advances in her orbit with regard to the fixed stars, the same as with regard to the sun.

_Mrs. B._ True, but then the distance of the fixed stars is so immense, that our solar system is in comparison to it but a spot, and the whole extent of the earth's...o...b..t but a point; therefore, whether the earth remain stationary, or whether it revolved in its...o...b..t during its rotation on its axis, no sensible difference would be produced with regard to the fixed stars. One complete revolution brings the same meridian back to the same fixed star; hence the fixed stars appear to go round the earth in a shorter time than the sun by three minutes fifty-six seconds of time.

_Caroline._ These three minutes fifty-six seconds is the time which the earth takes to perform the additional three hundred and sixty-fifth part of the circle, in order to bring the same meridian back to the sun.

_Mrs. B._ Precisely. Hence the stars gain every day three minutes fifty-six seconds on the sun, which makes them rise that portion of time earlier every day.