THE COMET OF 1843 was so brilliant that it was visible in full daylight. It was so near the sun at perihelion as "almost to graze his surface.”

ENCKE'S COMET has a period of only 3 years. A most interesting discovery has been made from observations upon its motion. The comet returns each time to its perihelion about 2 hours earlier than the calculations indicate. Hence, Prof. Encke has been led to conjecture that space is filled with a thin, ethereal medium capable of diminishing the centrifugal force, and thus contracting the orbit of a comet.

DONATI'S COMET (1858) was the subject of universal wonder. When first discovered, in June, it was 240,000,000 miles from the earth. In August, traces of a tail were noticed, which expanded in October to about 50,000,000 miles in length. This comet, though small, has never been exceeded in the brilliancy of the nucleus and the graceful curvature of the tail. It will return in about 2,000 years.

THE "GREAT COMET OF 1882" had, soon after passing its perihelion, a nucleus as bright as a star of the 1st magnitude, and a tail 60,000,000 miles long. The aphelion of its orbit is six times further than Neptune from the sun, and the comet's period is estimated at between eight and nine centuries.

V. ZODIACAL LIGHT.

Description.-If we watch the western horizon in March or April, just after sunset, we shall sometimes see the short twilight of that season illuminated by

a faint nebulous light, of a conical shape, flashing upward, often as high as the Pleiades. In September and October, at early dawn, the same appearance

can be detected near the eastern horizon. The light can be seen in this latitude only on the most favorable evenings, when the sky is clear and the moon absent. Even then, it will be frequently confounded with the Milky Way or auroral lights. At the base,

it is of a reddish hue, where it is so bright as often to efface the smaller stars. In tropical regions, the zodiacal light is perpetual, and shines with a brilliancy sufficient, says Humboldt, to cast a sensible glow on the opposite part of the heavens.

Origin.-The commonly-received opinion is, that it is caused by a faint, cloud-like ring, perhaps a meteoric zone, that surrounds the sun, and becomes visible to us only when the sun himself is hidden below the horizon. Others maintain that, since it has been seen in tropical regions in the east and the west simultaneously, it can be explained only on the theory of a "nebulous ring that surrounds the earth within the orbit of the moon."

PRACTICAL QUESTIONS.

1. Would the earth rise and set to a Lunarian?

2. Could there be a transit of Neptune?

3. Why does Mars's inner-moon rise in the west ?

4. In what part of the sky do you always look for the planets?

5. Show how it was impossible for the darkness that occurred at the time of the Crucifixion of Christ to have been caused by an eclipse of the

sun.

6. Is there any danger of a collision between the earth and a comet?

7. How are aërolites distinguished?

8. When do we see the old moon in the west after sunrise?

9. When do we see the moon high in the eastern sky in the afternoon before the sun sets?

10. When is a planet morning, and when evening, star?

11. Is the sun really hotter in summer than in winter?

12. Why is a planet invisible at conjunction?

13. Must an inferior planet always be in the same part of the sky as the sun? A superior planet?

14. Why, in summer, does the sun, at rising and at setting, shine on the north side of certain houses?

15. What effect does the volume of a planet have upon the force of gravity at its surface?

16. In what part of the heavens do we see the new moon? The old moon? The crescent moon?

17. What is the Golden Number in the almanac ?

18. Why do we have more lunar than solar eclipses ?

19. In what direction do the horns of the moon turn?

20. Is the "tidal-wave " an actual movement of the water?

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21. Why does the sun cross the line" in some years on March 21, and, in others, on March 22?

22. Do we ever see the sun where it really is?

23. At Edinburgh, Scotland, there are times when the sun rises at 31 o'clock A. M. and sets at 8 o'clock P. M., and the twilight lasts the entire night. When and why is this?

24. Which is the longest day of the year?

25. Is the moon nearer to us when it is at the horizon, or at the zenith? 26. How many solar eclipses would happen each year if the orbits of the sun and the moon were in the same plane?

27. Is there any heat in moonlight?

28. Can we see the moon during a total eclipse?

29. Which of the planets are repeating a portion of the earth's history?

30. How many times does the moon turn on its axis each year?

31. Can you explain the different signs used in the almanac ?
32. Show how the moon is a prophecy of the earth's future.
33. Does the sun really rise and set?

34. Are the bright portions of the moon mountains or plains?
35. Which of the heavenly bodies are self-luminous ?
36. Why is not a solar eclipse visible on the whole earth?
37. What is meant by the "mean distance" of a planet?
38. What keeps the earth in motion around the sun?
39. Do we ever see the sun after it sets?

40. When does the earth move the most rapidly in its orbit?
41. Have we conclusive evidence that any planet is inhabited?
42. When is the twilight the longest? The shortest? Why?
43. What is a moon?

44. To a person in the south temperate zone, where would the sun be

at noon?

45. Is it correct to say that the moon revolves about the earth, when we know that, according to the law of Physics, they must both revolve about their common center of gravity ?*

46. During a transit of Venus, do we see the body of the planet itself on the face of the sun?

47. How many real motions has the sun? How many apparent ones? 48. How many real motions has the earth?

49. Can an inferior planet have an elongation of 90° ?

50. How do we know the intensity of the sun's light on the surface of any of the planets?

51. Why is the Tropic of Cancer placed where it is?

52. What planets would float in water?

53. How must the moons of Jupiter appear during their transit across the disk of that planet?

54. "The shadow of the satellite precedes the satellite itself when Jupiter is passing from conjunction to opposition, but follows it between opposition and conjunction." Explain.

55. What facts point to the conclusion that Mars may, perhaps, have passed his planetary prime?

56. Why may we conceive that Saturn and Jupiter are yet in their planetary youth?

57. Show how, if the Nebular Hypothesis (p. 256) be accepted, the fashioning of a planet must require an enormous length of time.

58. Do we know the cause of gravitation?

* "Strictly speaking, the moon does not revolve around the earth, any more than the earth around the moon; but, by the principle of action and reaction, the center of each body moves around the common center of gravity of the two bodies. The earth being eighty times as heavy as the moon, this center is situated within the former, about three-quarters of the way from its center to its surface."- Newcomb's Astronomy, p. 91.