of air
rotating round a translated axis with a diameter of one hundred or one
hundred and fifty miles, the axis moving in a curvilinear track with a
progressive advance of twenty or twenty-five miles an hour, and the
motions being in opposite directions in opposite hemispheres of the
globe.

The equatorial calms and trade-winds accounted for on physical
principles, it was admitted that the winds of high latitudes,
proverbially uncertain as they are, depend in like manner on physical
causes.

With these palpable movements there are others of a less obvious kind.
Through the air, and by reason of motions in it, sounds are transmitted
to us.

[Sidenote: Of sounds; their velocity.] The Alexandrian mathematicians
made sound a favourite study. Modern acoustics arose from the
recognition that there is nothing issuing from the sounding body, but
that its parts are vibrating and affecting the medium between it and the
ear. Not only by the air-pump, but also by observations in the rare
atmosphere of the upper regions, it was shown that the intensity of
sound depends upon the density. On the top of a mountain the report of a
pistol is no louder than that of a cracker in the valley. As to the
gradual propagation of sounds, it was impossible to observe fire-arms
discharged at a distance without noticing that the flash appears longer
before the report in proportion as the distance is greater. The
Florentine academicians attempted a determination of the velocity, and
found it to be 1148 feet in a second. More accurate and recent
experiments made it 1089.42 feet at the freezing-point of water; but the
velocity, though independent of the density, increases with the
temperature at the rate of 1.14 foot for each degree. For other media
the rate is different; for water, about 4687 feet in a second, and in
cast iron about 10-1/2 times greater than in air. All sounds,
irrespective of their note or intensity, move at the same velocity, the
medium itself being motionless in the mass. No sound can pass through a
vacuum. The sudden aerial condensation attending the propagation of a
sound gives rise to a momentary evolution of heat, which increases the
elasticity of the air, and hence the velocity is higher than 916 feet in
a second, otherwise the theoretical rate.

[Sidenote: Acoustic phenomena.] Turning from soniferous media to
sounding bodies, it was shown that the difference between acute and
grave sounds depends on the frequency of vi