17. Q. What is the cause of tides? A. The attractive force of the moon and sun.
18. Q. What shores have the greatest tides? A. All eastern shores have far greater tides than western.
19. Q. What is the mean distance of the moon from the earth? A. Two hundred and forty thousand miles.
20. Q. What is the diameter of the moon in round numbers? A. Two thousand miles.
21. Q. What is the time of its revolution about the earth and of its axial revolution? A. Twenty-nine and one-half days.
22. Q. How clearly do the best telescopes we are now enabled to make reveal the moon? A. No more clearly than it would appear to the naked eye if it were 100 or 150 miles away.
23. Q. What is said about the moon presenting the same side to us? A. The moon always presents the same side to the earth.
24. Q. What is the difference of heat on the moon in the full blaze of its noon-day and midnight? A. No less than five hundred degrees.
25. Q. What is said as to the presence of air and water on the moon? A. There are no indications of air or water on the moon.
26. Q. What is said of the maps of the side of the moon toward us? A. They are far more perfect than those of the earth.