9. (Pg. 93) Describe what is intended by the plane of the earth's orbit. 10. (Pg. 93) Extending this plane to the fixed stars, what circle would it form, and among what particular stars would it be found? 11. (Pg. 93) What is fig. 1. plate 9, designed to represent? 12. (Pg. 93) The ecliptic does not properly belong to the earth, for what purpose then is it described on the terrestrial globe? 13. (Pg. 93) What does the obliquity of the ecliptic to the equator serve to show? 14. (Pg. 93) Within what limits do you find the torrid zone? 15. (Pg. 93) What two zones are there between the torrid, and the two frigid zones? 16. (Pg. 93) Where are the frigid zones situated? 17. (Pg. 93) What is meant by the term zone; and are the frigid zones properly so called? 18. (Pg. 93) How do meridian lines extend, and what is meant by the meridian of a place? 19. (Pg. 93) What is said of the meridian to which the sun is opposite, and where is it then midnight? 20. (Pg. 94) What hour is it then, at places exactly half way between these meridians? 21. (Pg. 94) How are greater and lesser circles distinguished? 22. (Pg. 94) What part of a circle is a degree, and how are these further divided? 23. (Pg. 94) What is the diameter, and what the circumference of a circle, and what proportion do they bear to each other? 24. (Pg. 94) What part of a circle is a meridian? 25. (Pg. 94) How many degrees are there between the equator and the poles? 26. (Pg. 94) Into what parts, besides degrees, is the ecliptic divided? 27. (Pg. 94) How are degrees of latitude measured, and to what number do they extend? 28. (Pg. 94) On what circles are degrees of longitude measured, and to what number do they extend? 29. (Pg. 94) What is a parallel of latitude? 30. (Pg. 95) Degrees of longitude vary in length; what is the cause of this? 31. (Pg. 95) What is the length of a degree of latitude, and why do not these vary? 32. (Pg. 95) What causes the equator to be somewhat larger than a great circle passing through the poles, and what effect has this on degrees of longitude measured on the equator? 33. (Pg. 95) What is the cause of this form being given to the earth? 34. (Pg. 96) What would have been a consequence of the centrifugal force, had the earth been a perfect sphere? 35. (Pg. 96) A body situated at the poles, is attracted more forcibly than if placed at the equator, what is the reason? 36. (Pg. 97) What effect would be produced upon the gravity of a body, were it placed beneath the surface of the earth, and what supposing it at its centre? 37. (Pg. 97) What two circumstances combine, to lessen the weight of a body on the equator? 38. (Pg. 97) Why could not this be proved by weighing a body at the poles, and at the equator? 39. (Pg. 98) What is a pendulum? 40. (Pg. 98) What causes it to vibrate? 41. (Pg. 98) Why are not its vibrations perpetual?