' iliE WORLD OF SOUND.* IN The World of Sound Sir
William Bragg has written one of the most attractive, popular science manuals that have appeared for a very long time. It is delightfully written, quite short, and, though the matter is explained in clear, untechnical language, the reader does not feel that he is being condescended to.
Originally based on six Christmas lectures given at the Royal Institution, the book deals with sounds in the town, sounds in the country, sounds in music, the behaviour of sounds under water, and sounds in war. The last chapter will be particularly delightful to boys, with its simple explanations of submarine- detecting apparatus, sound-ranging stations, and so forth.
The author also envisages delightfully the musical policeman who determines the rate of the approaching car which is exceeding the speed limit by the drop in pitch of the sound made by the car as it passes him. When the fire-engine dashes by the sound of the bell which is being rung continuously is obviously lowered in pitch as the engine passes :—
"As the car or the fire-engine is coming towards an observer, the sound pulses, which it is pouring out at the rate of so many in each second, run ahead to the observer, whose sense of the pitch of the note depends on how many pulses reach him every second. This number is artificially increased by the movement of the source. Each pulse is given a little handicap on the pulse before it, so that it follows its predecessor at a smaller interval than it would have done if the source had been at rest. It may be helpful to consider one or two illustrations. Suppose that a man travelling home to London from Rome in a leisurely fashion, one or two hundred miles each day, were to write daily to a London friend ; the friend would, on an average, receive rather more than a letter a day."
Here, fortunately for our reason, he turns to the acoustics of halls and the Whispering Gallery of St. Paul's.
We cannot imagine a more charming present than Sir William Bragg's book for an intelligent boy or girl of about fifteen, or indeed for any properly curious person.
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