748 Marvels of the Universe 
AIR CURRENTS AND WAVES 
BY F. W. HENKEL, B.A., F.R.A.S. 
Tue air, as a fluid, is almost constantly in motion, the state of rest being a rare exception and one 
only found existing over limited areas and for a short time. Our earth is everywhere covered with 
this atmosphere of usually invisible gas, whose height is not less than fifty miles, in which “ we live 
and move and have our being,’’ which presses on every square inch of its surface with a force equal 
to the weight of 14.7 lbs. The unequal heating by the sun of different regions of our planet on 
account of their varying presentment towards the central body is the primary cause of the great 
atmospheric movements or currents. Every disturbance communicated to the air, however small, 
gives rise to greater or less move- 
ments of the latter. The vibrations 
of a sounding body give rise to the 
states of alternate compression and 
rarefaction in the air, whereby the 
sound waves are propagated, which, 
falling upon the tympanum of the 
ear, set the latter in motion, and thus 
ultimately the brain receives the im- 
pression to which the name of “‘sound”’ 
is given. 
The greater heating of the equa- 
torial regions and the deficient supply 
of the polar ones give rise to the two 
main currents of our atmosphere— 
the equatorial warm current north- 
wards and southwards from _ the 
equator towards either pole and the 
polar cold currents from the latter 
towards the equator. Air _ being 
heated becomes lighter and ascends, 
THE SUBSTANCE OF AIR. its place being taken by colder air 
Here the propeller is making 1,300 revolutions a minute. Two from surrounding regions. But this 
vortices will be noticed in the air, caused by the action of the two F 2 2 : : 
blades; these are spiral and extend into space, only a vertical section 1s not all. The earth 1S all the while 
of each spirall appearing in the photograph. turning round on its axis from west 
to east (the diurnal motion of rotation), and this motion is quicker near the equator than in higher 
latitudes. The air moving polewards, since it is coming from a region of quicker rotation towards 
one of slower motion, is deviated towards the east, and conversely a current coming from the 
region of the poles will lag behind in the opposite direction. Poisson and Herrel developed this 
theory, and gave a general explanation of the great atmospheric currents. Air-currents coming from 
the equator northwards are deflected, as stated above, and thus in our hemisphere the south wind 
becomes a south-west one, and the polar current flowing equatorwards becomes a north-eastern wind. 
The latter is known as the north-east trade wind, and blows with great persistency over the North 
Atlantic and North Pacific Oceans. Similarly in the southern hemisphere we have the south-east 
trade winds. In England, as is well known, the south-west (rain-bearing) wind is more common 
than any other, and the late James Glaisher, for a long time in charge of the Meteorological Depart- 
ment at Greenwich Observatory, estimated that on the average this wind blows for over two thousand 
seven hundred hours per annum. 
