17. What appeared during the eclipses of 1851 and 1852 ?

18. Where was that of 1860 visible?

19. When will others happen, and where?

20. What causes an eclipse of the Moon ?

21. Why does it not follow that she should be eclipsed when between the Sun and us?

22. Illustrate the way in which she is eclipsed.

23. At what part of the Earth's shadow must she be?

24. How long does the phenomenon last, and during what time is she hidden ?

25. On what does the duration of the phenomenon depend?

26. What do we perceive on looking at the Moon?

27. Explain the cause of this.

28. What phenomenon would the inhabitants of the Moon have ?

29. Why may not the Sun be totally eclipsed to them?

30. What effect is always seen from an eclipse of the Moon?

31. What may be noticed about the stars?

32. To what places are the eclipses visible?

CHAPTER XVIII.

THEORY OF THE TIDES.

1. The tides are alternate movements of the ocean, owing to the attraction of the Sun and Moon on the waters.

2. In the space of 24 hours 50 minutes a great wave is carried from the equator to the poles, and from the poles to the equator. This, in its passage over the earth, causes the water to rush up the rivers, and each place to which it comes has what is called high water.

3. When the wave has passed, the water gets lower and lower until it is low water.

4. The high tides take place when the water is going from the equator towards the poles, and low water when it returns. 5. The first of the Greeks who noticed the cause of the tides was Pytheas, who lived 300 years before Christ.

6. The Moon passes successively in front, as it were, of each point of the ocean, and, by virtue of the law of attraction, it draws the waters into a heap. This heap moves as already stated, and is called a tidal wave.

7. On account of the irregular motion of the Moon, she comes to the meridian about 50 minutes later every day; it is then high water at the place; and it follows that the time of high water is later every day by the same amount.

8. The tides are much higher when the Moon is in perihelion, or nearest the Earth.

9. At full or new moon the tides are greatest, for the Sun joins its attraction to that of the Moon; but at the time of her quadrature they are least.

10. The waters on that side of the Earth which is turned away from the Moon are less attracted than the centre of the Earth, or the Earth is drawn away from the water; consequently another tidal wave is formed in the opposite hemisphere. We have, therefore, two tidal waves in 24 hours 50 minutes.

11. The attractive force of the Sun on the Earth, although not above one third of that of the Moon, is sufficient to produce a flux and reflux; hence two sorts of tides may be distinguished, viz., solar and lunar.

12. The most elevated part of the tidal wave is not exactly under the Moon, but always a little to the east, though never more than 15 degrees.

13. This is because the waters do not immediately obey the attraction of the Moon, on account of the resistance they meet with in the forms of the land.

14. Newton has calculated that if there were tides in the Moon the attraction of the Earth would be sufficient to raise the waters to seven times the greatest height of our tidal

waves.

QUESTIONS ON CHAPTER XVIII.

1. What are tides? How are they caused?

2. How is high water caused?

3. What is the cause of low water?

4. When do high tides take place?

5. What ancient noticed their cause?

6. What is a tidal wave? How is it caused ?

7. Why is the time of high water 50 minutes later every day? 8. What increases the height of the tides ?

9. When are they greatest?

10. Why have we two tidal waves? In what time?
11. What influence has the Sun on the waters?
12. Where is the most elevated part of a tidal wave?
13. How is it retarded ?

14. What calculation has Newton made?

CHAPTER XIX.

CIRCLES OF THE EARTH.

1. A circle is a figure contained or bounded by one line called the circumference, and all lines drawn from its centre to the circumference are equal, and are called radii. One of these is called a radius.

2. The principal circles on the surface of the Earth are, the equator, meridians, and parallels of latitude.

3. Circles are divided into two classes, great and less circles.

4. A great circle is one whose plane (or imaginary flat surface, bounded by the circumference,) passes through the centre of the sphere on which it is drawn.

5. A less circle is one whose plane does not pass through the centre of the sphere.

6. A sphere is a solid body all the diameters of which are equal. If it be cut straight through in any direction the flat surface exposed will be a circle.

7. The earth is not a perfect sphere, for it is flattened at the poles, one diameter being longer than the other by 26 miles.

8. A meridian is a great circle, passing through both poles, and cutting the equator perpendicularly or at right angles.

9. The equator is a great circle passing round the Earth, at an equal distance from each pole.

10. Parallels of latitude are less circles, passing round the Earth, parallel to, or running in the same direction with, the equator.

11. The latitude of a place is its distance north or south of the equator, measured on a meridian, or the arc or part of a meridian, intercepted between the place and the equator.

12. The longitude of a place is its distance east or west of some particular meridian measured on the equator. The meridian from which it is measured is called the first meridian.

13. Englishmen measure their longitude from the meridian of Greenwich; Frenchmen from that of Paris.

14. Latitude and longitude are measured in degrees, marked thus (°).

15. A degree is equal to 60 geographical miles, or about 69 English or common miles. Each geographical mile is called a minute, marked thus ('), and minutes are again divided into 60 parts called seconds (").

16. Thus the latitude of Greenwich is 51° 28′ 50′′, and its longitude is 0°.

17. The difference of latitude between two places is the arc of a meridian intercepted between them.

18. The difference of longitude between two places is the arc of the equator intercepted between the points where the meridians cut that circle.

19. When the places are on different sides of the equator their difference of latitude is the sum of their latitudes, and when they are on different sides of the first meridian their difference of longitude is the sum of their longitudes.

20. The greatest latitude a place can have is 90°, or from the equator to one of the poles; the greatest longitude is 180°, or half round the Earth.

21. A circle is divided into 360°; therefore half a circle is 180°, and a quarter of it is 90°.

22. The Earth being nearly a sphere, it is evident that the equator must be greater than any circle drawn parallel to it.

23. Hence, if the longitude of a place were measured on a parallel of latitude passing through it, it would not be the same as if measured on the equator, but the length of a degree would be diminished the nearer we approached the poles: for this reason a degree at the equator is taken as the measure of longitude.

24. It has been already stated in Chapter IV. (§ 13) that the Earth moves round the Sun, with its axis pointing in the same direction; this causes the Sun to appear to describe in the heavens throughout the year a circle, which appears to

D

cut the equator at an angle of 2340; this circle is called the ecliptic.

25. On maps of the world this circle is represented by a curved line, which rises as much above the equator into the northern hemisphere, as it goes below into the southern, cutting the equator in two opposite points.

26. The surface of the Earth is divided into five zones or belts: viz., two frigid or cold, extending from each pole to the borders of the Arctic and Antarctic circles respectively; one torrid or hot, extending 231° on each side of the equator; and two remaining parts called temperate zones.

27. There are two particular parallels of latitude called the tropics of Cancer and Capricorn, the former passing through the most northern point and the latter through the most southern point of the ecliptic. These mark the boundaries of the torrid zone on each side of the equator.

28. On the 21st of March the Sun is over one of the points where the ecliptic cuts the equator, on the 21st of September he is above the other; on the 21st of June he is over the tropic of Cancer, and on the 21st of December over that of Capricorn.

29. If the Earth's axis were perpendicular to the plane of its orbit, the parts round the poles would receive little or no light from the Sun; if parallel, then the parts at the equator would be in a similar position, and one pole would receive no light.

1. What is a circle ?

QUESTIONS ON CHAPTER XIX.

2. Name the principal circles on the Earth.

3. Into what are they divided?

4. What is a great circle? 5. Define a less circle?

6. What is a sphere ?

7. Of what shape is the Earth?

8. What is a meridian ?

9. What is the equator?

10. Define parallels of latitude.

11. What is the latitude of a place ?

12. What is its longitude?

13. From what places do Englishmen and Frenchmen measure? 14. How are latitude and longitude measured?

15. How is a degree divided?