The World of Sound – Sir William Bragg’s Royal Institution Lectures, 1919
By Hugh Dominic W Stiles, on 25 July 2018
Sir William Henry Bragg (1862–1942) was a Cumbrian physicist, who was awarded a Nobel Prize in 1915 along with his son Lawrence for their discovery of the new science of x-ray crystallography, which eventually led to Rosamund Franklin’s photographs of DNA. He was appointed a UCL Quain Professor of Physics in 1915, and around the same time was appointed to the Board of Invention and Research. The Admiralty eventually appointed Bragg to lead research at Aberdour into the use of hydrophones for detecting submarines. In 1919 the Royal Institution invited him to give their annual Christmas Lectures. He gave six lectures, published in 1920 as The World of Sound –
- What is Sound?
- Sound in Music
- Sounds in the Town
- Sounds of the Country
- Sounds of the Sea
- Sounds in War
All around us are material objects of many kinds, and it is quite difficult to move without shaking some of them more or less. If we walk about on the floor, it quivers a little under the fall of our feet; if we put down a cup on the table, we cannot avoid giving a small vibration to the table and the cup. If an animal walks in the forest, it must often shake the leaves or the twigs or the grass, and unless it walks softly with padded feet it shakes the ground. The motions may be very minute, far too small to see, but they are there nevertheless. (p.1)
In his first lecture, he repeated experiments demonstrated by John Tyndall in the RI ‘half a century ago’ (presumably 1865 or 1873). Bragg said most of Tyndall’s apparatus was still there. He demonstrated how sound could travel from a musical box in the basement up a long rod, and that when a tea tray was placed on the top of the rod, it transmitted the sound to everyone in the room ((p.4-6).
To illustrate how sound waves spread out, he used a ‘ripple tank’ which held a shallow trough about a yard square, witha plate-glass bottom, and an arc lamp under that. Light passed through the water to an angled mirror, that then reflected onto the walls (p.13-14).
In ‘Sounds of the Town,’ he demonstrated how Lord Rayleigh had explained and demonstrated how the Whispering Gallery in St. Paul’s Cathedral works. The sound is ‘continuously reflected by the wall without ever getting too far away from it,’ and then he repeated Rayleigh’s experiment (p.84-6).
In ‘Sounds of the Country,’ he describes how Charles Gahan told him that he was able to get a death-watch beetle to respond when he tapped with a pencil. The beetle raps its head on wood to signal to other beetles. He also explains the twisting and fluttering of a leaf – the poplar being particularly prone to this fluttering due to the leaf stemallowing the leaf to twist, and sometimes the natural period of vibration of a leaf means it flutters more than its neighbours (p.119). In ‘Sounds of the Sea’ we learn how fish have no cochlea but are able to respond to minute changes in pressure on pits in the skin of the head (p.136-7).
The last chapter describes the use of ‘Sound in War.’ Bragg had lost a son Robert, at Gallipoli. He discusses the use of the hydrophone, and the use of sound-ranging to find enemy guns or to locate mining operations.