Page 44 - January 2009

experience is this: that an echo returns in double the interval of time in which the primary sound reached the sound- reflecting object. For example, if the sound-reflecting object was distant 600 ft., the return of the echo would take place within the same interval of time in which the primary sound would have traversed 1200 ft. if it had not been reverberated.
And this fact has often been of great use to me in meas- uring the distance of places. For example, when I was stand- ing on the bank of the river Thames opposite the town of Woolwich, the echo of a monosyllabic sound has been rever- berated from the opposite houses in six half seconds from which I infer that the width of the River Thames at the at point, from the margin of one bank to the margin of the other
is 1712 English feet, or over a quarter of a mile. For, as 9.25 half-seconds are to 5280 feet (a mile), so are six half-seconds to 3423.8 feet – the half of which is 1711.9 feet.
Finally, in this way the height of thunderclouds and the distance of the thunder and lightning itself may be easily ascertained. [The idea of determining the distance to a thun- derstorm by counting seconds from the lightning flash to the report of thunder is not original with Derham. This idea was expressed by members of the Accademie del Cimento in Florence prior to Derham’s paper. See the English translation by Waller of these writings (reference given in the first Table above).]
Finis.
The portrait of William Derham is printed with the kind permission of The Royal Society ©. The picture of James C. Welling is printed with the kind permission of the Library of Congress and is from the Brady-Hardy Collection. The picture of Thomas B. Gabrielson is printed with the kind permission of the Annotator.
The Formula 1 in Room Acoustics
• Efficient import of 3D models • Speed of calculations
• Reliability of results
• Transmission between rooms • Quality of auralisation
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De Motu Soni 43