_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, shining 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 shines 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 shines 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.

When time is calculated by the stars it is called sidereal time; when by the sun, solar, or apparent time.

_Caroline._ Then a sidereal day is three minutes fifty-six seconds shorter, than a solar day of twenty-four hours.

_Mrs. B._ I must also explain to you what is meant by a sidereal year.

The common year, called the solar or tropical year, containing 365 days, five hours, forty-eight minutes and fifty-two seconds, is measured from the time the sun sets out from one of the equinoxes, or solstices, till it returns to the same again; but this year is completed, before the earth has finished one entire revolution in its...o...b..t.

_Emily._ I thought that the earth performed one complete revolution in its...o...b..t, every year; what is the reason of this variation?

_Mrs. B._ It is owing to the spheroidal figure of the earth. The elevation about the equator produces much the same effect as if a similar ma.s.s of matter, collected in the form of a moon, revolved round the equator. When this moon acted on the earth, in conjunction with, or in opposition to the sun, variations in the earth's motion would be occasioned, and these variations produce what is called the precession of the equinoxes.

[Ill.u.s.tration: PLATE XI.]

_Emily._ What does that mean? I thought the equinoctial points, were fixed points in the heavens, in which the equator cuts the ecliptic.

_Mrs. B._ These points are not quite fixed, but have an apparently retrograde motion, among the signs of the zodiac; that is to say, instead of being at every revolution in the same place, they move backwards. Thus if the vernal equinox is at A, (fig. 1. plate XI.) the autumnal one, will be at B, instead of C, and the following vernal equinox, at D, instead of at A, as would be the case if the equinoxes were stationary, at opposite points of the earth's...o...b..t.

_Caroline._ So that when the earth moves from one equinox to the other, though it takes half a year to perform the journey, it has not travelled through half its...o...b..t.

_Mrs. B._ And, consequently, when it returns again to the first equinox, it has not completed the whole of its...o...b..t. In order to ascertain when the earth has performed an entire revolution in its...o...b..t, we must observe when the sun returns in conjunction with any fixed star; and this is called a sidereal year. Supposing a fixed star situated at E, (fig. 1. plate XI.) the sun would not appear in conjunction with it, till the earth had returned to A, when it would have completed its...o...b..t.

_Emily._ And how much longer is the sidereal, than the solar year?

_Mrs. B._ Only twenty minutes; so that the variation of the equinoctial points is very inconsiderable. I have given them a greater extent in the figure, in order to render them sensible.

In regard to time, I must further add, that the earth's diurnal motion on an inclined axis, together with its annual revolution in an elliptic orbit, occasions so much complication in its motion, as to produce many irregularities; therefore the true time cannot be measured by the apparent place of the sun. A perfectly correct clock, would in some parts of the year be before the sun, and in other parts after it. There are but four periods in which the sun and a perfect clock would agree, which is the 15th of April, the 16th of June, the 23d of August, and the 24th of December.

_Emily._ And is there any considerable difference between solar time, and true time?

_Mrs. B._ The greatest difference amounts to between fifteen and sixteen minutes. Tables of equation are constructed for the purpose of pointing out, and correcting these differences between solar time and equal or mean time, which is the denomination given by astronomers, to true time.

Questions

1. (Pg. 92) What does the line A B, (fig. 2 plate 8.) represent, and what are its extremities called?

2. (Pg. 92) What is meant by the equator, and how is it situated?

3. (Pg. 92) There are two hemispheres; how are they named and distinguished?

4. (Pg. 92) What are the circles near the poles called?

5. (Pg. 92) What do the lines I K, and L M, represent?

6. (Pg. 92) What circle is in part represented by the line L K?

7. (Pg. 92) Against what mistake must you guard respecting this line?