University of Virginia Library

1880.—ACOUSTICS, LIGHT AND HEAT PAPER.
Science and Art Department.

The following are specimens of answers given by candidates at recent examinations in Acoustics, Light and Heat, held in

connection with the Science and Art Department, South Kensington. The answers have not of course all been selected from the same paper, neither have they all been chosen for the same reason.

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?

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

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

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.

Question 8.—How would you disprove, experimentally, the assertion that white light passing through a piece of coloured glass acquires colour from the glass? What is it that really happens?

Answer.—To disprove the assertion (so repeatedly made) that «white light passing through a piece of coloured glass acquires colour from the glass,» I would ask the gentleman to observe that the glass has just as much colour after the light has gone through it as it had before. That is what would really happen.

Question 11.—Explain why, in order to cook food by boiling, at the top of a high mountain, you must employ a different method from that used at the sea level.

Answer.—It is easy to cook food at the sea level by boiling it, but once you get above the sea level the only plan is to fry it in its own fat. It is, in fact, impossible to boil water above the sea level by any

amount of heat. A different method, therefore, would have to be employed to boil food at the top of a high mountain, but what that method is has not yet been discovered. The future may reveal it to a daring experimentalist.

Question 12.—State what are the conditions favourable for the formation of dew. Describe an instrument for determining the dew point, and the method of using it.

Answer.—This is easily proved from question 1. A body of gas as it ascends expands, cools, and deposits moisture; so if you walk up a hill the body of gas inside you expands, gives its heat to you, and deposits its moisture in the form of dew or common sweat. Hence these are the favourable conditions; and moreover it explains why you get warm by ascending a hill, in opposition to the well-known law of the Conservation of Energy.

Question 13.—On freezing water in a glass tube, the tube sometimes breaks. Why is this? An iceberg floats with 1,000,000 tons of ice above the water line. About how many tons are below the water line?

Answer.—The water breaks the tube because of capallarity. The iceberg floats on the top because it is lighter, hence no tons are below the water line. Another reason is that an iceberg cannot exceed 1,000,000 tons in weight: hence if this much is above water, none is below. Ice is exceptional to all other bodies except bismuth. All other bodies have 1090 feet below the surface and 2 feet extra for every degree centigrade. If it were not for this, all fish would die, and the earth be held in an iron grip.

P.S.—When I say 1090 feet, I mean 1090 feet per second.

Question 14.—If you were to pour a pound of molten lead and a pound of molten iron, each at the temperature of its melting point, upon two blocks of ice, which would melt the most ice, and why?

Answer.—This question relates to diathermancy. Iron is said to be a diathermanous body (from dia, through, and thermo, I heat), meaning that it gets heated through and through, and accordingly contains a large quantity of real heat. Lead is said to be an athermanous body

(from a, privative, and thermo, I heat), meaning that it gets heated secretly or in a latent manner. Hence the answer to this question depends on which will get the best of it, the real heat of the iron or the latent heat of the lead. Probably the iron will smite furthest into the ice, as molten iron is white and glowing, while melted lead is dull.

Question 21.—A hollow indiarubber ball full of air is suspended on one arm of a balance and weighed in air. The whole is then covered by the receiver of an air pump. Explain what will happen as the air in the receiver is exhausted.

Answer.—The ball would expand and entirely fill the vessell, driving out all before it. The balance being of greater density than the rest would be the last to go, but in the end its inertia would be overcome and all would be expelled, and there would be a perfect vacuum. The ball would then burst, but you would not be aware of the fact on account of the loudness of a sound varying with the density of the place in which it is generated, and not on that in which it is heard.

Question 27.—Account for the delicate shades of colour sometimes seen on the inside of an oyster shell. State and explain the appearance presented when a beam of light falls upon a sheet of glass on which very fine equi-distant parallel lines have been scratched very close to one another.

Answer.—The delicate shades are due to putrefaction; the colours always show best when the oyster has been a bad one. Hence they are considered a defect and are called chromatic aberration.

The scratches on the glass will arrange themselves in rings round the light, as any one may see at night in a tram car.

Question 29.—Show how the hypothenuse face of a right-angled prism may be used as a reflector. What connection is there between the refractive index of a medium and the angle at which an emergent ray is totally reflected?

Answer.—Any face of any prism may be used as a reflector. The connexion between the refractive index of a medium and the angle at which an emergent ray does not emerge but is

totally reflected is remarkable and not generally known.

Question 32.—Why do the inhabitants of cold climates eat fat? How would you find experimentally the relative quantities of heat given off when equal weights of sulphur, phosphorus, and carbon are thoroughly burned?

Answer.—An inhabitant of cold climates (called Frigid Zoans) eats fat principally because he can't get no lean, also because he wants to rise is temperature. But if equal weights of sulphur phosphorus and carbon are burned in his neighbourhood he will give off eating quite so much. The relative quantities of eat given off will depend upon how much sulphur etc. is burnt and how near it is burned to him. If I knew these facts it would be an easy sum to find the answer.