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much warmer in the middle of the day, than it is either in the morning or evening. The cause, of course, is the greater directness of the sun's rays when on the meridian.

QUESTIONS FOR EXAMINATION ON CHAPTER III.

Pages 38-43.-The arguments in proof of the earth's motion round the sun? 2. The illustrations? 3. By what argument is it proved that a body projected into pure space will continue in motion for ever, in a straight line, and with uniform velocity? 4. Can you state the argument? 5. Can you explain by a diagram the causes of the earth's annual motion? 6. Can you go through the demonstration in this and the preceding page? 7. By what combination is the circular motion of the earth and the other planets produced? 8. What other names are given to the forces of projection and attraction? 9. The meaning of the terms tangential and centripetal? 10. If the earth at its creation had been projected towards or too near the sun, what must have happened? 11. If too remote from the sun, the consequences? 12. What is an ellipse? 13. Why is the orbit of the earth elliptical? 14. The meaning of the term Aphelion? 15. Perihelion? 16. Can you go through the preceding demonstration? 17. In what part of her orbit is the earth when the centripetal force is greatest? 18. In what part, when it is least? 19. How is it that the tangential force prevails over it in the former case, and yields to it in the latter? 20. Does the orbit of the earth differ much from a circle ? 21. Why has it been given so elliptical in the diagram? 22. The difference in length between the longer and shorter axis or diameter of the earth's orbit? 23. Why is this difference almost nothing? 24. Is the earth as near the sun in winter as it is in summer? 25. How do you explain this? 26. How do you illustrate this by the polar summer? 27. The warmest time of the day? 28. Why? 29. The warmest time of the year? 30. The coldest time of the night and year? 31. Why is the sun when risingon the meridian-and setting-at the same distance from us?

a Strictly speaking, the greatest warmth is not in the middle of the day, but in two hours or so after, because the heat continues to accumulate for some time after the sun has reached the meridian; just as midsummer is not the hottest part of the year, but two months or so after. And for a similar reason, the night is colder towards morning than it is at midnight; and with regard to midwinter, we need only quote the old proverb, "As the day lengthens the cold strengthens."

b A contributing cause is, that the more direct the sun's rays are, the less of the atmosphere they have to travel through in reaching the earth.

d

CHAPTER IV.

MAGNITUDE AND MEASUREMENT OF THE EARTH.

HAVING explained the figure and motions of the earth we have now to show how its MAGNITUDE has been determined. As the earth is a spherical body, its magnitude will depend upon the length of its DIAMETER and CIRCUMFERENCE. But how can the length of either be ascertained? We cannot follow a straight line through the centre of the earth, from side to side, to ascertain its length; nor can we even travel round the surface of the earth in a circle' to measure its circumference. Nor is it necessary to attempt either. For, as the circumference of the earth, like every other circle, is conceived to be divided into 360 equal parts, or degrees, it is evident, that if we can ascertain the length of any one of these parts, we have only to multiply it by 360, to find the length of the entire circumference. And as the proportion between the diameter of a globe and its circumference is known to be nearly as one to three, it is plain that the circumference of the earth will give us the length of its diameter. For example, it has been ascertained by actual measurement that the length of a degree on the earth's surface is about 69 English miles, which, multiplied by 360, gives nearly 25,000 miles for the whole circumference; and as the diameter of a globe or circle is something less than one-third of the circumference, it follows that the diameter of the earth is about 8,000 miles in length.

But how is a degree on the earth's surface measured? The process is easily understood, but it requires a previous knowledge of the CIRCLES which, for the purpose of measur

a Diameter, from the Greek words dia, through, and metreo, to measure. A diameter measures a globe or circle through the centre, from any point in the circumference to the point opposite.

Circumference, a line or circle carried round the surface of the earth, so as to divide it into two equal parts or halves,

Of course in a great circle. See note, page 31.

d See page 35, for the origin of the division of the circle into 360 degrees; and it should also be noted that degrees are subdivided into sixty equal parts, called MINUTES; and that minutes are also subdivided into sixty equal parts, called SECONDS. The following marks are used to denote degrees, minutes, and seconds-"'". For example, 5°26′ 20′′ means five degrees, twenty-six minutes, and twenty seconds.

ing the earth's surface, and determining the position of places, astronomers have supposed to be drawn round both the CELESTIAL and TERRESTRIAL SPHERES.

H

B

M

In this figure, which represents on a plane surface one-half of the terrestrial sphere, CD is the one-half of the equator, which, as we mentioned before, is a circle supposed to be drawn round the middle of the earth, or at an equal distance from each pole. As the plane of the equator passes through the centre of the earth, it divides it into two equal parts, and is, consequently, A GREAT CIRCLE. The half of the globe above or north of the equator is called the NORTHERN HEMISPHERE, and the half below or south of the equator is called the SOUTHERN HEMISPHERE. The word hemisphere means half of a sphere or globe.

A and B are the POLES of the earth, or its extreme northern and southern points; and the lines or circles drawn from A to B are MERIDIANS.

The circles which are drawn parallel to the equator_are called PARALLELS. And in the northern hemisphere, I K

a

If you stand opposite to a globe, and at some distance from it, the half of the equator, as in the diagram, will appear to you to be a straight line, and not a semicircle. The half of a meridian presents a similar appearance when you stand opposite to it, as A B in the diagram

represents the tropic of Cancer, and E F, the Arctic circle; and in the southern hemisphere, L M is the tropic of Capricorn, and G H, the Antarctic circle. L K represents the one half of the ecliptic, but it refers to the heavens. See page 35, and note at bottom.

[Young persons may be led by familiar illustrations, such as the following, to form clear and correct ideas of LATITUDE and LONGITUDE. Put a pin up to the head in any part of an orange, which is equidistant from the top and bottom of it; and having attached a thread to it, carry it fairly round the middle of the orange till it comes to the pin again. Now attach the thread to the pin by giving it a turn or two round its head; and the circle formed in this way will represent the Equator, and its division of the earth into two equal parts or HEMISPHERES. The half of the orange above the circle formed by the thread represents the Northern, and the half which is below it, the Southern hemisphere.

In that part of the surface of the upper half of the orange, which is farthest from the circle formed by the thread, put a pin up to the head; and in the opposite, or lowest point of the surface of the under half of the orange, put in another pin in the same way, and the heads of these pins will represent the North and South Poles of the earth. Connect these two points, on both sides of the orange, by a thread drawn along its surface; and the circle formed in this way will represent a meridian, and its division of the earth into two equal parts or HEMISPHERES. And should this meridian pass through London, we call it the First Meridian; and the two hemispheres into which it divides the earth will be called, the one the Eastern, and the other the Western hemisphere. The Eastern hemisphere is to the east of the First Meridian, and the Western hemisphere to the west of it.

Now, LATITUDE is the measurement of the earth from the equator to the poles; and there is no more difficulty in conceiving how this is done than you would have in measuring the distance between any point of the circle of thread round the middle of the orange, and the head of the pin at the top or bottom of it. This, of course, you would do by drawing the shortest line along the surface of the orange, from the thread to the head of either of the pins; but this line will obviously be the fourth part of a circle, and it contains, as we know ninety degrees, for all circles, however they may differ in size from each other, contain the same number of degrees, namely, 360. A degree, or the 360th part of a small circle like this, is almost too ininute for measurement; but the degrees of a meridian, or great circle of the earth, are each sixty geographical, or nearly seventy English miles in length. And hence, we see that the distance of each of the poles of the earth, from the equator, is 90 degrees of a meridian circle or, in other words, about 6,000 miles.

And hence we see, also, that the distance of any place between the equator and the poles, must be less than 90 degrees; and if we wish to ascertain the exact distance from the equator, or, in other words, its latitude, we have only to draw a line through it from pole to pole. The circle formed by this line will be its meridian, and its latitude will evidently be the arc of its meridian intercepted between it and the equator. And if we follow the parallel of the place to any meridian that is graduated, we shall find the number of degrees which this arc contains; for as all meridians are of the same size, all equal or corresponding parts of them must contain the same number of degrees; and hence, we see that if one meridian be graduated, it will answer for all the rest. Such a meridian on the globe is called a UNIVERSAL MERIDIAN.

To make this perfectly clear, divide two quarters of a meridian into nine equal parts each, beginning at the equator; and at the end of the first part put the figure 10, because it contains ten degrees; at the second, 20; at the third, 30; at the fourth, 40; at the fifth, 50; and so on. Now, if from each of these parts circles be drawn round the globe parallel to the equator, it will be evident that all the places through which they pass are the same number of degrees distant from the equator, that is, 10, 20, 30, 40, or 50, as the case may be. And hence the parallels, though really circles of longitude, are called Parallels of Latitude. Without their assistance we could not tell the latitude of places upon maps, unless all the meridians were graduated.

In the same way, it should be shown that LONGITUDE is the measurement of the earth, from the First Meridian eastward or westward to the Anti-Meridian, or half round the globe, and that consequently, the longitude of a place is the arc of its parallel intercepted between it and the first meridian. This arc is not graduated, but the number of degrees which it contains will be found by following the meridian of the place to the Equator; for as the graduation of one meridian answers for all the rest, so the graduation of the equator answers for the graduation of all the circles that are parallel to it. But it should be kept in mind, that degrees of longitude, except on circles equidistant from the equator, are of different lengths, because they are the 360th parts of unequal circles, namely, the parallels and the equator. On the contrary, the degrees of latitude are all of the same length, because they are the 360th parts of equal circles, namely, the meridians.]

The LATITUDE of a place is its distance from the equator. If a place is north of the equator, it is said to be in NORTH LATITUDE, and if south, in soUTH LATITUDE. It is evident, that the entire northern hemisphere is in north latitude, and the entire southern hemisphere in south latitude.

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