3. Q. What is inertia? A. If a body is at rest, inertia is that quality by which it will forever remain so, unless acted upon by some force from without; and if a body is in motion, it will continue on at the same speed, in a straight line, forever, unless it is quickened, retarded, or turned from its path by some other force. 4. Q. What is the result of the action of attraction and inertia upon two revolving bodies? A. They circle about each other as long as these forces endure. 5. Q. What would be the solution of the problem of a simple revolution of one world about another in a circular orbit? A. It would always be at the same distance from its center, and going with the same velocity. 6. Q. In the case of the moon, how many causes are there that interfere with such a simple orbit? A. Over sixty. 7. Q. What is heat? A. A mode of motion. 8. Q. Through what do all the light and heat of the sun that appear upon our earth come? A. Through space that is two hundred degrees below zero, and through utter darkness. 9. Q. At what velocity does light travel? A. One hundred and eighty-five thousand miles per second. 10. Q. What is the highest velocity we can give a rifle ball? A. Two thousand feet a second. 11. Q. How long does it take light to travel from the sun to the earth? A. About eight minutes. 12. Q. What is light? A. The result of undulations in ether. 13. Q. What are the different effects we call color? A. They are simply various velocities of vibration. 14. Q. How does sunlight melt ice? A. In the middle, bottom, and top at once. 15. Q. What is the effect of dark heat on ice? A. It only melts the surface. 16. Q. What can you say of the passage of the heat of the sun and the heat of a furnace or stove through glass? A. Nearly all the heat of the sun goes through glass without hindrance; only a small portion of the heat of a furnace or stove goes through the same substance. 17. Q. If our air were as pervious to the heat of the earth as it is to the heat of the sun, how cold would the temperature of the earth become every night? A. Two hundred degrees below zero. 18. Q. What is said of worlds so distant as to receive from the sun only a thousandth part of the heat we enjoy? A. They may have atmospheres that retain it all. 19. Q. What is probable as to the temperature of Mars? A. It is probable that Mars, that receives but one-quarter as much heat as the earth, has a temperature as high as ours. 20. Q. What two radically different kinds of telescopes are made? A. The refracting telescope, and the reflecting telescope. 21. Q. Why is the refracting telescope so called? A. Because it is dependent on the refraction of light through glass lenses. 22. Q. Why is the reflecting telescope so called? A. Because it acts by reflecting the light from a concave mirror. 23. Q. What is the loss of light in the use of each kind of telescope? A. In passing through glass lenses it is about two-tenths. By reflection it is often one-half.