"How can that be?" inquired Jack.
"Why, very easily. Sound increases or diminishes in intensity according as the wind carries it on or retards it."
"What, then, is sound, that the wind can blow it about, most learned brother?"
"It is a result of the compression of the air, that from its elasticity extends and expands, and which causes a sort of trembling or undulation, similar to that which is observed in water when a stone is thrown into it."
"And you may add," said Becker, "that bodies striking the air excite sonorous vibrations in this fluid; thus it rings under the lash that strikes it with violence, and whistles under the rapid impulsion of a switch: it likewise becomes sonorous when it strikes itself with force against any solid body, as the wind when it blows against the cordage of ships, houses, trees, and generally every object with which it comes in contact."
"I can understand," replied Jack, "how this sonorous effect is produced on the particles of air in immediate contact with the object struck; but how this sound is propagated, I do not see."
"Very likely; but still it travels from particle to particle, in a circle, at the rate of three hundred and forty yards in a second."
"Three hundred and forty yards in a second!" said Willis, who was beginning by degrees to recover his self-possession. "Well, that is what I should call going a-head."
"And by what sort of compasses has this speed been measured, Master Ernest?"
"The first accurate measurement, Master Jack, was made at Paris in 1738. There are there two tolerably elevated points, namely, Montmartre and Montlhéry—the distance between these, in a direct line, is 14,636 toises. Cannons were fired during the night, and the engineers on one of the elevations observed that an interval of eighty-six seconds and a half elapsed between the flash and the report of a cannon fired on the other."