ed in
below to fill the vacated space. What if he took the equatorial
regions or great tracts of arid desert for the heated room? The air
over them, subjected by the heat to constant rarefaction, must
rise, must overflow above, and must force the colder air from the
surrounding regions in below. Two sheets of air will thus set in
vertically on both sides, rise, and again separate above. Here was an
explanation of the great, steady, uninterrupted aerial currents which,
at the rate of from fifteen to eighteen miles per hour, sweep the
surface of the Atlantic and Pacific oceans. The candle, no doubt, was
wasted, but the secret of the trade-winds was discovered.

The idea was correct as far as it went. It did not go very far, it is
true. It had not taken into account the earth's rotation, whose force,
according to Herschel, "gives at least one-half of their average
momentum to all the winds which occur over the whole world;" nor the
infinite variation in the movements of the atmosphere which we call
winds, caused by the change in the sun's motion, by the differing
amounts of vapor held in them, by the physical configuration of the
earth below, by the vicinity of the sea or arid deserts, and by the
passage of storms or electric currents.

The science of meteorology, especially as regards wind, is as yet
searching for general principles, which can only be deduced from
countless facts. We do not now, like Saint Paul, talk of the wind
Euroclydon as of a special agent of God, but describe it by stating
that it is an aerial ascending current over the Mediterranean,
produced by the heated sands of Africa and Arabia. We can even measure
its heat at 200 deg. Fahrenheit, and its velocity at fifty-four miles per
hour. But it attacks us just as unexpectedly as it did the apostle,
and brings disease and death to Naples or Palermo to-day just as
surely as it did to Cambyses. The popular verdict on the matter
would no doubt be that when meteorologists can not only describe the
sirocco, but give warning of its coming, their science will justify
its claim to consideration. The common sense of mankind always demands
as a royalty from every science daily practical benefits to the mass
of men and women. It is not enough for meteorologists to have proved
that the atmosphere varies in weight, in temperature or velocity of
motion according to fixed rules, or to be able to explain why no rain
falls on a certain portion of the coast of Portugal,