Question I.—State the relations existing between the pressure, temperature, and density of a given gas. How is it proved that when a gas expands its temperature is diminished?
Answer.—Now the answer to the first part of this question is, that the square root of the pressure increases, the square root of the density decreases, and the absolute temperature remains about the same; but as to the last part of the question about a gas expanding when its temperature is diminished, I expect I am intended to say I don't believe a word of it, for a bladder in front of a fire expands, but its temperature is not at all diminished.
Question 2.—If you walk on a dry path between two walls a few feet apart, you hear a musical note or ``ring'' at each footstep. Whence comes this? <p 176>
Answer.—This is similar to phosphorescent paint. Once any sound gets between two parallel reflectors or walls, it bounds from one to the other and never stops for a long time. Hence it is persistent, and when you walk between the walls you hear the sounds made by those who walked there before you. By following a muffin man down the passage within a short time you can hear most distinctly a musical note, or, as it is more properly termed in the question, a ``ring'' at every (other) step.
Question 3.—What is the reason that the hammers which strike the strings of a pianoforte are made not to strike the middle of the strings? Why are the bass strings loaded with coils of wire?
Answer.—Because the tint of the clang would be bad. Because to jockey them heavily.
Question 4.—Explain how to determine the time of vibration of a given tuning- fork, and state what apparatus you would require for the purpose.
Answer.—For this determination I should require an accurate watch beating <p 177>seconds, and a sensitive ear. I mount the fork on a suitable stand, and then, as the second hand of my watch passes the figure 60 on the dial, I draw the bow neatly across one of its prongs. I wait. I listen intently. The throbbing air particles are receiving the pulsations; the beating prongs are giving up their original force; and slowly yet surely the sound dies away. Still I can hear it, but faintly and with close attention; and now only by pressing the bones of my head against its prongs. Finally the last trace disappears. I look at the time and leave the room, having determined the time of vibration of the common ``pitch'' fork. This process deteriorates the fork considerably, hence a different operation must be performed on a fork which is only lent.
Question 6.—What is the difference between a ``real'' and a ``virtual'' image? Give a drawing showing the formation of one of each kind.
Answer.—You see a real image every morning when you shave. You do not see virtual images at all. The only people who see virtual images are those people <p 178>who are not quite right, like Mrs. A. Virtual images are things which don't exist. I can't give you a reliable drawing of a virtual image, because I never saw one.