SCIENCE SIFTINGS

New Zealand Tablet, 25 March 1920, Page 46

SCIENCE SIFTINGS

(By "Vow.")

- . , ' Secrets of Sound. ;. Professor W. H. Bragg told a war-story in his lecture on the "World of Sound" at the Royal Institution iecently, that will -i be new to most people (writes a correspondent of the 'Manchester Guardian). His 'friend, Dr. Richard Paget, had a wonderful facility for recognising the number of vibrations per second in a note, especially when the note was heard under water, as in the case of that from the propeller of a submarine, and he had a no less wonderful way of using his gift. He would strip, said the Professor, and get his head under the water. Presently he would reappear humming a note, then he would tap his forehead with his finger and call out 256 or some such number. Now it is extraordinarily difficult to recognise the exact pitch of a note without the help of a tuning fork .or instrument to compare it with. Dr. Paget needed nothing of the kind. When he tapped his forehead it always resounded to one note which he had ascertained with great accuracy. By comparing the note he was humming with the one sounding inside his head he recognised its pitch, and his scientific knowledge enabled him to calculate at once the number of vibrations per second that had caused it. This is not a sense of "absolute pitch," but it is an amazing sense of relative pitch. The lecturer told many other stories and showed many experiments all bearing on ordinary sounds. He had a miniature whispering gallery behind his lecture table, and, using a whistle and a sensitive flame, he showed how the sound of the whistle did not run straight across the gallery, but was reflected from point to point around the walls. A screen set up anywhere close to the reflecting surface prevented the flame from responding to the whistle, whereas an obstacle put directly between the flame and the whistle had no effect. This led him to talk about the acoustics of various buildings. He spoke of the "superstition" that putting wires across a hall would prevent echoes. The best way to tost the acoustic properties of a hall, he said, was to stand in. the middle of it and clap one's hands. If the sound was sharp and ceased instantly, as it did in the theatre of the Royal Institution, the hall would be perfect from a speaker's standpoint. If the sound persisted for loss than two seconds it would be fairly good for speaking. For music a little longer might be allowed, but when the resonance lasted for 20 seconds, as it did in the octagonal hall of the Victoria and Albert Museum, both speaking and music would be impossible. The reason why the whistle of an engine drops in pitch as it passes through a station was explained. "When the whistle is approaching you," said Dr. Bragg, "the vibrations are being poured out towards you, and you are getting more than you ought, and as it is receding you are getting fewer. By the drop in the note it would be possible," he said, "to tell at what rate a motor-car is travelling. And a very intelligent policeman mightcatch you out in that way. If you saw an intense look in his face, you could quicken up a little bit and deceive him, for the note would not go down at all. But I am wondering what he would think if the note went up instead of down. He would probably come to the conclusion that, in spite of appearances, you were really going backwards." The singing kettle on the hob and the roaring flame up the chimney also gave up the mystery of their welcome sounds. The little bubbles of steam at the bottom of the kettle try to rise to the surface, and as they do so they get into colder parts of the water and so collapse. "A collapsing bubble makes a noise like a tiny hammer stroke." The roar of flame up the chimney was explained by holding a lighted gas ring inside a length of rainwater pipe, which gave off a deep and sonorous note as the air rushed up the heated tube. The lecturer concluded with a striking reference to the Colossus of the Plain. According to the fable, the figure on the right, as the sun struck it in the morning, used to sing. - In later times one of the kings of Egypt had the figure repaired, and it never sang again; but the descriptions showed that in a former age the image was the object of great pilgrimages, and that proconsuls and other famous men from all parts had visited the scene. The phenomenon, was explained by the lecturer as being due to the heat of the sun acting on the stone. "Thus," he concluded, "one stone sliding on another, owing to the heat of the morning sun, was a world wonder and the source of famous -pilgrimages."