Sound : a course of eight lectures delivered at the Royal Institution of Great Britain / by John Tyndall.

  • John Tyndall
Date:
1869
Catalogue details

Licence: Public Domain Mark

Credit: Sound : a course of eight lectures delivered at the Royal Institution of Great Britain / by John Tyndall. Source: Wellcome Collection.

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    that of a stratum of mercury about 30 inches high. At the summit of Mont Blanc the barometric column is not much more than half this height; and, consequently, the elasticity of the air upon the summit of the mountain is not much more than half what it is at the sea-level. If we could augment the elasticity of air, without at the same time augmenting its density, we should aug- ment the velocity of sound. Or, if allowing the elasticity to remain constant we could diminish the density, we should augment the velocity of sound. Now, air in a closed vessel, where it cannot expand, has its elasticity augmented by heat, while its density remains unchanged. Through such heated air sound travels more rapidly than through cold air. Again, air free to expand has its den- sity lessened by warming, its elasticity remaining the same, and through such air sound travels more rapidly than through cold air. This is the case with our atmosphere when heated by the sun. It expands and becomes lighter, bulk for bulk, while its pressure, or in other words its elas- ticity, remains the same. And now you see the reason of the phrase that the velocity of sound in air6 at the freezing temperature9 is 1,090 feet a second. At all lower tem- peratures the velocity is less than this, and at all higher temperatures it is greater. The late M. Wertheim has determined the velocity of sound in air of different tem- peratures, and here are some of his results :— Temperature of air. Velocity of sound. O^0 centigrade . . 1089 feet 2*] 0 . 1091 „ 8*5 . 1109 12-0 . 1113 26*6 . 1140 At a temperature of half a degree above the freezing point of water the velocity is 1,089 feet a second; at a temperature of 26*6 degrees, it is 1.140 feet a second, or a
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