Part 2

§ 19. It remains for us to speak of that class of bodies known by the name of _Comets_, (see Lesson I, § 6). Of these an unknown number belongs to our Solar System.—(Some philosophers have estimated their number to be about Twentyone; others think it must amount to several hundred). They move round the Sun in exceedingly long ovals, having their transparent tails always turned away from that luminary. What is most remarkable about them is the astonishing degree of heat to which they are exposed on account of passing so near the Sun, and the astonishing velocity with which they travel.

The Comet which appeared in the year 1680, is supposed to sustain a heat nearly Two Thousand times greater than that of red hot iron, and to move at the rate of several Hundred Thousand miles an hour!!

RECAPITULATION OF LESSON III.

QUESTIONS.

[§ 14.] Do all Planets need the same time to complete a whole revolution round the Sun? Why not?

If the pupils are old enough to understand the use of Dividers, it will perhaps be well for the teacher to let them draw the Solar System on a piece of paper.—If not, he ought to let them explain Plate IV, or an orrery, if one be at hand.

[§ 15.] What is the time called, which our Earth needs for a complete revolution round the Sun? How many days are there in a year? How many hours are there in a day?

What is the revolution of the Earth round the Sun, the means of?

[§ 16.] What time does Mercury require for a complete revolution round the Sun? What time does Venus require for the same purpose? What time does Ceres, Vesta, Juno and Pallas need? What, Jupiter, Saturn, and Herschel?

[§ 17.] What motion does the Moon make whilst the Earth is travelling round the Sun? How many days does the Moon need for a complete revolution round the Earth? Does the Moon during its revolution round the Earth always exhibit the same shape? What changes then does she gradually undergo? What do you call the successive changes in the Moon’s appearance? What do you call the time during which the Moon is not seen? What, that, during which she exhibits her full shape? What, the different periods of her waxing and waning? When does the First Quarter take place? When the last?

[The teacher might now require the explanation of Plate V; the elder pupils may draw the Diagram.]

[§ 18.] What does frequently occur during the Moon’s motion round the Earth? What is the consequence of the Moon’s position in a direct line between the Sun and ourselves? What is such a diminution of light in consequence of the Moon being placed between us and the Sun, called? What takes place when the Earth is placed between the Sun and the Moon? What is such an obscuration of the Moon, in consequence of the Earth’s position between her and the Sun, called?

[The younger pupils ought now to explain Plate VIII; the elder pupils ought to draw the Diagram on a slate or paper.]

[§ 19.] Is the number of Comets belonging to our Solar System precisely ascertained? How many are there _supposed_ to belong to our System? In what manner do they move round the Sun? What remarkable property do they possess?

FOOTNOTES:

[2] If the teacher has an orrery at hand, if it be even of the most simple construction, he may exhibit it now. But it is the author’s belief that a considerable portion of the pupil’s interest is lost, if he be acquainted with it, at the beginning of the study; or, as it is the custom in some schools, if an orrery is hung up among the charts and maps of the school room. The pupil ought not to see the orrery, until he knows that it is but a _faint illustration_ of the infinite grandeur of the heavens. Nothing detracts so much from our estimation of things as a too familiar acquaintance with them, before we know their real value.

[3] The exact numbers are given in Table II, at the end of the book.

LESSON IV.

ROTATION OF THE SUN, THE EARTH, AND THE REST OF THE PLANETS ON THEIR AXES.—DAY AND NIGHT.—INCLINATION OF THE EARTH’S AXIS.—SEASONS.

§ 20. Besides the progressive motion of the Planets, of which we have spoken in the last Lesson, they have yet a peculiar motion like a wheel turning on its Axle-tree. This motion is called the _rotation of the Planets on their Axes_; because each of them seems to move round a straight line passing through its centre; like a ball turning round a piece of wire run through the middle of it.—Moreover it is customary to call such a straight line imagined to be drawn through the centre of a Planet—_the Axis of that Planet_.

The double motion of the Planets,—progressive and rotary,—is perhaps one of the most difficult things for young pupils properly to understand, without some popular illustration. If the teacher, therefore, has no orrery to show this motion to his pupils, he may compare it to a screw which is turned round whilst it suffers at the same time a progressive motion; or perhaps with more propriety to a spinning-top, which is continually turning on its Axis, while at the same time it describes large circles.[4]

§ 21. If you have well understood what has just been said, you will be able to comprehend, that _our Earth, while it is performing its great journey round the Sun in Three Hundred and Sixtyfive days, is, at the same time, every Twentyfour hours turning on its Axis_. This _rotary_ motion of our Earth on its Axis is the cause of the successive changes of day and night; that portion of the Earth which is turned toward the Sun having always day, when the other, which is turned away from him, has night.

This is again a wise dispensation of God’s providence. For if the Earth would always keep the same relative position to the Sun, then that portion of it, which would then be turned toward the Sun, would have continual day, whilst the other, which would then be turned away from him, would be enveloped in perpetual darkness. But as it is now arranged by the Earth’s rotation on its Axis, most every portion of its surface must at least _once_ every Twentyfour hours be turned toward the Sun and receive from him light and heat. Without this, one great half of our Earth would have a perpetual winter, destructive to plants and animals, while an everlasting summer would scorch the other half and render it equally unfit for the support of man.

The following Diagram, Plate IX, may serve to give you an idea of the Earth’s rotation round its Axis, and the alternate succession of Day and Night, resulting from it. When the Earth is situated as represented in the Diagram, then that portion of it, which is marked A, will have Day, because it is turned toward the Sun; and the portion marked B, will have Night. But in the course of the next Twelve hours the order will be reversed. The portion which is marked B, will be turned toward the Sun and have day, whilst the portion A will be turned from him, and have night.

§ 22. The rotation of the Earth on its Axis is also the cause of the _Rising_ and _Setting of the Sun_. For no portion of our Earth is at once turned toward or from the Sun; but moves toward or from it by degrees (as you may see by slowly turning a ball near the flame of a candle). This gradual motion of each portion of the Earth’s surface toward or from the Sun, makes the Sun himself appear to us as rising and going down; while, in fact, we ourselves are turning towards, or receding from him. This is a kind of deception similar to that which you experience when slowly gliding down a river; when the objects on shore have the appearance of receding from you, while in fact, it is you, yourself, who are travelling away from them.

The gradual rise and setting of the Sun is another excellent provision of nature. Were we from the darkness of night at _once_ exposed to the luminous rays of the Sun, it would dazzle our eyes and render them unfit to distinguish a single object.—It is only by a gradual transition from darkness to light that we are able to accustom our eyes to the brilliancy of noon.

§ 23. There is another peculiarity in the situation of our Earth with regard to the Sun, which you have not yet learned. The Earth’s Axis is not even (parallel) with that of the Sun; but is somewhat inclined towards it, as represented in the last Diagram. To this is owing the _Change of the Seasons_. For on account of the inclination of the Earth’s Axis, the Sun’s rays fall, sometimes nearly perpendicular upon us, while at other times they are striking us more obliquely[5]. This is the principal cause of those changes of temperature which we are in the habit of distinguishing by the names, _Spring_, _Summer_, _Autumn_ and _Winter_.

In winter the Sun’s rays strike us most obliquely; it is therefore the coldest season of the year. In summer they are most perpendicular;—Summer therefore is the hottest season. Spring and autumn are standing in the middle between these two. From spring till mid-summer the Sun’s rays are striking us more and more perpendicularly; from mid-summer till winter more and more obliquely.

A similar change of temperature is felt every day from Sun-rise (when the Sun-beams are most oblique) till noon, (when they are most perpendicular); and from noon again towards evening or Sun-set, when they are again oblique.

§ 24. It has been mentioned (Lesson IV, § 20), that each Planet in our Solar System is regularly turning on its Axis. But all of them do not perform this rotation with equal _velocity_. The Planets which are farther from the Sun are turning quicker than those which are near him. Jupiter, for instance, turns on its Axis twice as fast as our Earth. The nights in Jupiter, therefore, do not last half as long as ours.

This is, in some degree, necessary. For in proportion as a planet is further from the Sun, it receives less light and heat, which deficiency is, in part, made up by a more frequent exposition to his rays.

§ 25. The _Sun_ himself is also known to turn regularly on his Axis, and to complete one whole rotation in about Twentysix of our days. This we have been able to perceive from the spots which have been discovered on its surface, and which gradually move toward and disappear on one side, when in a short time after they appear again on the other.

§ 26. The Moon, and the Satellites of the other Planets have no rotary motion; but have always the same side turned towards their Planets. Thus the moon keeps constantly the same side turned towards the Earth; but her monthly motion round the Earth (Lesson III, § 17) is equal to a rotation on her axis; because by this means every part of her is, at least, once every Twentyseven days turned toward the Sun; as you may see from the Moon’s phases, represented on plate VII. A day in the Moon, therefore, is equal to Twentyseven of our days; because the Moon moves in Twentyseven of our days round the Earth, which is equal to turning once on her axis.

RECAPITULATION OF LESSON IV.

QUESTIONS.

[§ 20.] Is the progressive motion of the Planets round the Sun the only one which they are performing? What other motion have they besides this? What is this motion called? Why is it called so? What do you call a straight line imagined to be drawn through the centre of a Planet?

To what may the double motion of the Planets’ progression and rotary be compared?

[§ 21.] What time does the Earth need to turn once on its axis? How many times does the Earth turn on its axis during its whole journey round the Sun?[6] What is this rotary motion of our Earth the cause of? Which portion of the Earth has day? Which night?

What would be the case, if the Earth was always to keep the same relative position with regard to the Sun? What is the advantage derived from the rotation of the Earth on its Axis. Would the Earth be habitable without it or not?

[The pupil ought now to explain plate IX.]

[§ 22.] What is the rotation of the Earth on its axis further the cause of? What does the gradual motion of each portion of our Earth towards the Sun produce?

What would be the case, if from the darkness of night we were at once exposed to the luminous rays of the Sun? Could our eyes endure the brightness of noon without a _gradual_ transition from darkness to light?

[§ 23.] What other peculiarity is there in the situation of the Earth with regard to the Sun? What is this the cause of? Do the rays of the Sun strike us at all times equally perpendicular or obliquely? What changes of temperature are thereby created?

How do the rays of the Sun strike us in winter? What season, therefore, is it? How do the rays of the Sun strike us in summer? What season, therefore, is summer with regard to temperature? How are the rays of the Sun striking us from spring till mid-summer? How, from mid-summer till winter? What similar change of temperature do we experience every day from Sun-rise till noon, and from noon till evening or Sun-set?

[§ 24.] Do all Planets turn on their axes with equal velocities? Which Planets turn quicker, those which are nearer or those which are further from the Sun? Give an instance. How long are Jupiter’s nights in comparison to our’s?

Why is this, in some degree, necessary?

[§ 25.] Is the Sun himself also known to turn on its axis like the Planets? How many days does he need for one complete rotation? By what means have we been able to observe this motion?

[§ 26.] Have the Moon and the Satellites of the other Planets also a _rotary_ motion? In what position does the Moon remain with regard to the Earth? But what is her monthly revolution round the Earth equal to? Why? What is a day in the Moon equal to? Why?

FOOTNOTES:

[4] The propriety of this comparison becomes still more evident, when we reflect that the Axis of the top is generally somewhat inclined to the plane.

[5] If the terms _perpendicular_ and _oblique_, should not be perfectly understood by the pupils, it will be easy for the teacher to explain their meaning.

[6] If the Earth requires 365 days to travel round the Sun; and each day has twentyfour hours; then the Earth will, during the whole of this time, turn Three Hundred and Sixtyfive times on its axis.

LESSON V.

OF THE APPEARANCE AND PECULIARITIES OF THE MOON AND SOME OF THE PLANETS WHEN VIEWED THROUGH A TELESCOPE.

§ 27. Next to the Sun there is no heavenly body so interesting to us as the _Moon_. When viewed through a good Telescope[7] she has nearly the same appearance as in Figure I, Plate X. The bright parts are supposed to be lofty mountains and tracts of land; (which is evident also from the shadow which they cast) and the dark spots are supposed to be valleys and caverns. Many of the mountains of the Moon are higher than the largest mountains on the Earth. Some of them are volcanos, and their eruptions have been distinctly observed by many distinguished Philosophers. Some of the caverns are ascertained to have a depth of many miles and a width of almost Three miles.—No water has as yet been discovered in the Moon. Hence if she is inhabited, as we have reason to believe, her inhabitants must be very differently constructed from ourselves.

§ 28. Among the Planets _Venus_ is by far the most beautiful in appearance. She is known also by the name of the _Morning_ and _Evening Star_. Her light is so bright that she is often seen at _Noon_. When viewed through a good telescope she exhibits phases similar to those of the Moon (Lesson III, § 17), which proves her spherical form (Lesson I, § 3). The mountains in Venus have been calculated to be at least Six times as high as those on our Earth. Her Atmosphere is only half as dense as ours.

§ 29. Mars appears in many respects similar to our Earth. His light is red and changeable; his surface exhibits black changeable spots (see Figure II, Plate X). Some philosophers pretend to have noticed a region of ice on his poles. His atmosphere is twice as dense as ours.

§ 30. Jupiter, viewed through a telescope, exhibits a surface covered with stripes. These are supposed to be clouds. A representation of them is given in Fig. III, Plate X. His light is very white and subject to but little variation. His atmosphere is nearly Twentyseven times denser (thicker) than ours.

§ 31. Very remarkable, as we have already observed, is the Planet Saturn, on account of its luminous ring. Viewed through a telescope it has the appearance, represented in Figure IV. It is highly probable that to the inhabitants of that Planet, this ring has an entirely different appearance from what it has to us. It appears to be a solid opaque mass, and is probably inhabited like the Planet, which it constantly accompanies on its journey round the Sun. Saturn’s atmosphere is nearly Ninety times denser than that of our Earth. No mountains have as yet been discovered on its surface.

* * * * *

Herschel is too remote, for us to know much about its surface. Its atmosphere is supposed to be Three Hundred and Sixty-one times thicker than ours.

Mercury, being the nearest Planet to the Sun, has a very bright light; but is only seen early before Sun-rise, and immediately after Sun-set. It exhibits Phases like the Moon.

RECAPITULATION OF LESSON V.

QUESTIONS.

[§ 27.] What do you know about the Moon’s surface? What, about its mountains, volcanos and caverns? Have any great waters been discovered in it?

[§ 28.] What do you know about the Planet Venus? By what other name is she known? What does she exhibit, when viewed through a telescope? What do you know about her mountains? What, about her atmosphere?

[§ 29.] What do you know about the Planet Mars? What, about his light, surface and atmosphere?

[§ 30.] What surface does Jupiter exhibit when viewed through a telescope? Of what color is the light of Jupiter? What do you know about his atmosphere?

[§ 31.] What do you know about the Planet Saturn? What does the ring of Saturn appear to be? What do you know about the atmosphere of this Planet?

FOOTNOTES:

[7] This is an instrument through which we can see things much clearer and larger than we could with the naked eye.

LESSON VI.

DESCRIPTION OF THE EARTH’S SURFACE.—DIVISION OF IT INTO ZONES.—TORRID, TEMPERATE AND ARCTIC ZONES.—THE FIVE GREAT CONTINENTS.—RELATIVE DIMENSIONS OF AMERICA, EUROPE, ASIA, AFRICA AND AUSTRALIA.

§ 32. Having learned the relation of our Earth to the Sun and the Planets, it will be well to acquaint ourselves with the principal objects on its surfaces.—What the _Interior_ of our Earth consists of, we have as yet no idea, because we have not yet been able to penetrate deeper than a few hundred feet; and this is, in proportion to the Earth’s Diameter, little more than nothing. But as regards her surface, we know that it consists partly of land and partly of water. Little more than one fourth of the Earth’s surface is covered with land; all the rest is water.—The great lands are called _Continents_; the great waters are called _Oceans_. Smaller portions of land surrounded on all sides by water, are called _Islands_. Smaller bodies of water surrounded by land are called _Lakes_.

§ 33. The land on our Earth is divided into Five Continents: America, Europe, Asia, Africa and Australia.—These, however, are not all of the same extent. Europe is the smallest of them. America and Asia are the greatest.

The adjoining Plate No. XI, will give you an idea of the proportion of land and water on our Earth, and of the relative extent of the Five Continents.

Fig. I represents the surface of the Earth divided into land and water. Were all land on our Earth put together in a circle, and the water placed round it, then the land would only fill the inner circle, the water occupying the surrounding ring, a space nearly four times as large as the circle. Fig. II, III, IV, V and VI represent the comparative surfaces of America, Europe, Asia, Africa and Australia. Fig. VII represents the comparative surface of the Moon.

Upon close inspection of these figures you will perceive;

1. That the extent of America and Asia are nearly equal; but that each of these Continents is several times larger than either Europe or Australia.

2. That the next greatest portion of our Globe is _Africa_, which is more than three times larger than Europe.

3. That Europe is the smallest Continent of our globe.

4. That the whole surface of the Moon would not be more than enough to cover either America or Asia.

§ 34. The Earth’s surface is not throughout equally illumined or heated by the Sun; because the Sun’s rays strike some portions of the Earth more perpendicular than others. Our Earth, therefore, is divided into _Climates_ or _Zones_, which you will understand better from Plate XII.

You will see from it that the Sun’s rays are perpendicular to the central part of the Earth’s surface; but that toward the extremities of the Earth’s Diameter these rays strike us more and more _obliquely_. The greatest heat, therefore, must be felt by the people living between the two circles EF and IK. The circle GH, which is exposed to the perpendicular rays, is termed the _Equator_; and the two circles EF and IK, _which are at equal distance from the Equator, are called Tropic Circles_. The whole surface included by these two circles is called the _torrid Zone_. The space between either of the circles CD and EF, or IK and LM, is called a _temperate Zone_; because the Sun’s rays striking these portions neither perpendicular nor very obliquely, their inhabitants suffer neither great heat nor cold. In one of these Zones are situated the United States of America and the greater portion of Europe. Beyond them, toward the extremities of the Diameter AB, are the two _icy_ or _arctic_ zones. The Sun’s rays strike them very obliquely; which is the cause of their being almost continually covered with ice or snow.

The two circles, CD and LM, are called _Polar_ circles; and the two extremities, A and B, of the Earth’s Diameter, situated in those regions, are called _the Poles_. A is called the _North-Pole_ and B the _South-Pole_ of the Earth.

§ 35. The different zones of which we have just spoken, are not equal to one another. Plate XIII, will show their relative extent.

Fig. I represents the surface of the Earth divided proportionally into its three zones: the _torrid_, the _temperate_ and the _arctic_. The inner circle represents both the arctic zones; the yellow ring b, which surrounds it, represents the united extent of the two temperate zones; and the outmost red ring, the whole of the two _torrid zones_.

Fig. II represents separately the whole torrid zone;—Fig. III one of the temperate zones;—Fig. IV one of the arctic zones; and Fig. V the whole extent of land on our globe.

The next Plate, No. XIV, represents the comparative surfaces of these zones, drawn separately in form of squares; and the last figure on that Plate, shows the extent of the five continents, compared to one of the temperate zones.