CLIMATE
CHANGE OF SOLAR RAYS INTO LIGHT, HEAT, AND OTHER FORMS OF ENERGY
AS THEY ARE ABSORBED BY OUR ATMOSPHERE OR AS THEY ENCOUNTER THE
EARTH—TEMPERATURES OF WATER, EARTH, AND AIR—HOW SANITARY HOMES
MAY BE CHEAPLY CONSTRUCTED BELOW GROUND, COOL IN SUMMER AND WARM
IN WINTER
=Difference between Climate and Weather.= One may speak of the
weather of to-day or of some time that is past, but not of the
climate of to-day, or of any day, month, or year that is gone: for
the climate of a place is determined by a study of its weather
records for a long period of years. Climate changes so slowly that
we speak of the movement as a mutation rather than as a change. The
time that has elapsed since the discovery of the barometer and the
thermometer—about two and a half centuries—is so short as to show
little if any change in climate, while the weather changes from day
to day.
=The Sun Our Only Source of Appreciable Heat.= Each one of the stars
visible to the eye and many of the millions that are not visible, are
suns accompanied by planets. Their conditions are similar to those
of our sun, except that most of them are larger than our sun, some
a million times larger. But their distance is so great that they
exercise little or no influence in the heating of the earth. Light
travels at about the rate of 186,400 miles per second, and yet these
stars are so distant that if the nearest one had been created at the
time of the signing of the Declaration of Independence we still would
be in ignorance of its existence, for its first rays of light would
not reach us for many years yet to come; and light from some of the
remote suns that we call stars requires thousands of years to come.
It is apparent therefore that we depend exclusively upon our own
luminary for the heat that warms our atmosphere and gives life to the
surface of the earth.
[Illustration: FIG. 21.—Equinoxes, March 21 and September 22. Axis
perpendicular to Sun’s rays. Day and night everywhere equal.]
=Different Temperatures with the Same Quantity of Solar Heat.= On
the same day of each year at the same place practically the same
amount of heat falls upon and into the earth’s atmosphere from the
sun, but rarely does the same temperature and weather occur, and
often there is wide variation in the weather of the same day of two
different years. The first of July may be cold enough to wear an
overcoat at midday, or the first of January may be so temperate as
to permit the donning of summer habiliments, while, according to the
amount of heat received from the sun, there would have occurred the
usual seasonal conditions on the days named had there been no other
influence than the direct action of the sun’s heat. The cause of
these seeming inconsistencies is due to the motions of the atmosphere
in a stratum only five to seven miles in depth, air cooling by
expansion as it ascends in cyclonic whirls and heating as it descends
in anti-cyclonic movements. Condensation, in the form of cloud or
rain or snow, also introduces complications, usually producing a
cooling effect in summer and a warming in winter. In other words:
interference in the uniform and gradual change in temperature, of
the lower stratum of air in which we live, from the heat of summer
to the cold of winter, and then the reverse process, is due entirely
to the heating and the cooling of the lower air by its upward and
downward motions.
[Illustration: FIG. 22.—Summer Solstice, June 21. North Pole leans
towards Sun’s rays.]
[Illustration: FIG. 23.—Winter Solstice, December 21. North Pole is
dark now instead of light, as at Summer Solstice. Pole leans in same
direction but Earth being on opposite side of its orbit rays come
from opposite direction. Refer to Figure 24.]
If the earth’s axis were vertical to the plane of its orbit all
places on its surface always would have days of twelve hours each
and the nights would be of the same length; sunshine would just
touch both poles (Figure 21) throughout the entire course of the
earth around the sun and there would be no seasons. One would need
to change one’s location on the earth in order to get a change of
weather, which would be monotonous and quite different from the
active conditions of the atmosphere that we now enjoy. The whole
conditions of life would be altered for the worse. You have seen a
top tilt over to one side as it spun on the floor. In the same way
the earth spins on its axis as it pursues its course around the sun
without changing the direction towards which its axis points, as
shown by Figure 24.
[Illustration: FIG. 24.—Note that direction of axis does not change
as Earth moves around Sun. This causes variation in area of surface
illuminated. If axis were perpendicular to plane of orbit there would
be no seasons.]
[Illustration: FIG. 25.—As angle of incidence decreases from 90° to
10° the heat received on upper end of blocks is spread over greater
area at bottom, and its temperature diminished. (Abbe.)]
The intensity of the sun’s rays at sunrise and at sunset is less than
at midday because the quantity of heat received at the outer limits
of the atmosphere on a given area, as for instance at the area of
the upper ends of the blocks in Figure 25, passes through a deeper
stratum of air the lower the angle of incidence, and because it is
distributed over a larger area when it reaches the surface of the
earth.
As the heat of day increases from morning until midday and then
decreases, so does the heat of the year increase from midwinter
to midsummer and then decrease, and for the same reason: change
in obliquity of the sun’s rays, to which must be added change in
distance from the central luminary. Figure 26 shows that the sun
reaches its greatest midday altitude on June 21st and its least on
December 21st.
[Illustration: FIG. 26.—Observer at center of picture at latitude
45°. Showing altitude attained by the Sun at midday and length of its
track above the horizon at the Summer and Winter Solstices and at the
two Equinoxes.]
=Solar Rays Absorbed by the Atmosphere.= The atmosphere of the earth
absorbs about seventy-six per cent. of the solar rays that pass
through it. About one half is absorbed by a cloudless atmosphere,
and nearly all is absorbed or reflected away by a cloudy air. On the
average about fifty-two per cent. of the earth’s surface is obscured
by clouds all the time, which reduces the total amount of heat that
reaches the earth to but twenty-four per cent. But in regions like
the high plateau of the Rocky Mountains, where there is little
cloudiness or moisture in the air, fully fifty per cent. reach the
earth. At the equator, when the sun is in the zenith at noon, the
rays strike the earth perpendicularly and reach the earth through
the shortest air distance possible; but for latitudes far north or
south of the equator, the rays are more oblique and must pass through
an ever-increasing thickness of air as the latitude increases.
Consequently the heat that reaches the earth at high latitudes
decreases, not only on account of the greater obliquity of the sun’s
rays, but also because of the longer path of atmosphere traversed,
which causes a further loss by absorption.
=The Lag of Earth Temperatures.= The solar rays reach their greatest
intensity on June 21st, in the Northern Hemisphere, when the sun
attains the farthest point north, and the obliquity of its rays is
the least, but the highest temperature of the air for the year does
not occur on the average for a month or six weeks later, due to the
capacity of the earth and air to absorb heat; and the maximum for the
earth does not occur until still later. The sun is the farthest south
on December 21st, but the minimum air temperature of the year, on the
average, does not occur until a month later, and at a later period in
the earth. At Munich, Bavaria, at a depth of four feet, the minimum
annual temperature occurs on the 2d of March, and the maximum on the
24th of August. For each increase of four feet in depth the time
of occurrence of either maximum or minimum temperature is retarded
twenty-one days, the minimum not occurring until the 23d of May at
a depth of 20.2°, and the maximum being retarded until the 17th of
November.
=Annual Range in Air Temperature.= The difference in temperature
between winter and summer increases from the equator northward and
from all oceans toward the interior of continents, and is greater
in the middle latitudes on the eastern side of large bodies of land
than on their western side. Yakutsk, Siberia, has experienced 80°
below zero in January and 102° above in July, making a range of 182°.
Dawson, Canada, has a record of 68° below for winter and 94° above
for summer, making a range of 162°. In marked contrast with these
large differences, shown in the northern interior of continents, is
the annual range at Samoa, from a maximum of 92° to a minimum of 62°,
a range for the year of only 30° for this island of the Pacific,
located near the equator.
=Reversal of the Seasons in the Two Hemispheres.= The summer is
shorter in the Southern Hemisphere than in the Northern and the
winter is longer, but the Southern Hemisphere is nearer to the sun in
the summer and farther away in winter, conditions that tend to add to
the extremes of both seasons. Because of the slowness of the earth
in passing through one half of its orbit, the northern summer lasts
ninety-three days, while that of the Southern Hemisphere lasts but
eighty-nine days. The result is that during like seasons and during
the whole year the two hemispheres receive exactly the same quantity
of heat.
=Only Water Vapor Protects the Earth from Death by Freezing.=
In Chapter IV you are told that the earth is surrounded by four
atmospheres that conduct themselves each quite independently of the
others, and that water vapor (aqueous vapor) is one of them. Water
vapor plays the most important part in absorbing incoming rays and
in absorbing and reflecting back outgoing heat rays from the earth.
Without the vaporous atmosphere the sun’s rays would be but slightly
absorbed as they entered and radiation from the earth would readily
escape through the atmosphere to outer space. No matter how fiercely
the sun might shine, life on the earth would be entirely destroyed by
cold.
When water vapor, clouds, or dust motes intercept certain portions
of the sun’s rays, they change them from vibrations in ether to
the motions of molecules, and the motions of these molecules are
expressed in a rise in temperature in the vapor, cloud, or dust.
Earth radiations of heat, having longer and slower wave lengths
than those that come from the sun, are more readily absorbed by the
atmosphere.
One of the principal functions of the atmosphere is to protect the
earth from the intense cold of outer space, which must be near or at
absolute zero—459° below the zero mark.
=Why Should Not Mountain Peaks Be Warm? They Are Nearer the Sun.= The
absorption by the atmosphere of both solar and terrestrial radiation
is greater in the lower levels of the air, where water vapor, cloud,
and dust are the densest, while the transmission of both incoming and
outgoing radiation is more rapid through the pure air aloft. Thus we
account for the coolness of all mountain peaks, and the perpetual
freezing temperatures of some, even though they be located in the
tropics, and though their tops occupy positions several miles nearer
the sun than the bases from which they rise.
=How the Earth Cools at Night.= Radiation from the earth goes on
day and night, winter and summer. During daylight the gain of heat
is greater than the loss, while at night the reverse is true. After
sunset both the earth and the air continue to cool by radiation
unchecked by the incoming heat of the daytime. The earth loses heat,
even under a clear sky, more freely than the air, with the result
that the surface of the ground and of vegetation may fall to a
temperature ten to fifteen degrees lower than that of the air at a
few hundred feet elevation. This condition is called “temperature
inversion.” The greater difference will occur when there is little
wind to mix the air. On a clear night the radiation outward will be
rapid; then, if the wind be light, there may occur an increase in
temperature up to a height of two hundred to four hundred feet, and
then a fall, reaching the surface temperature at about two thousand
feet elevation, unless the ground be wet, or the location be adjacent
to a considerable body of water.
=A Cloud Covering Cools by Day and Warms by Night.= One of the
principal functions of clouds is to conserve the heat of the sun. A
covering of cloud, fog, or dense haze may not only screen off the
heat of day, but greatly retard the lowering of temperature at night
by reflecting and radiating back to the ground much of the heat that
it has lost.
=The Temperature of Oceans, Lakes, and Rivers.= The same quantity
of heat falling upon different kinds of matter produces different
temperatures, depending on the capacity (specific heat) of each kind
of matter to absorb or hold heat; this is notably apparent when
the matter is land, water, or air; for the same quantity of heat
will raise the temperature of a water surface only about one fourth
as much as it will a land surface. Water rejects by reflection a
considerable amount of the solar rays that fall upon it, while land
reflects but a small part; and of that which is received upon the top
layer of water much is rendered latent in the process of evaporation
and does not impart warmth to the water. Solar rays also penetrate
water to a considerable depth and are quite uniformly absorbed by the
whole stratum penetrated. These conditions cause large water surfaces
and the air immediately over them to have a much lower temperature
during the day and a much higher temperature during the night; and
also lower temperatures during summer and higher temperatures during
winter, than occur over a land surface of the same latitude.
=Fresh Water and Salt Water Have Different Freezing Temperatures.= In
the ratio of 93.5 to 100 the specific heat of sea water is less than
that of fresh water. Sea water is a better conductor of heat, so that
it penetrates to a greater depth in salt water in the same period of
time than it does in fresh water. Sea water regularly contracts with
falling temperature until its greatest density occurs at four degrees
below freezing, when it becomes solid ice and expands in the process
of freezing; otherwise it would not float.
=A Wonderful Phenomenon.= In this respect a most wonderful and
unexplainable phenomenon occurs with regard to fresh water. Not only
sea water but practically all other forms of matter—liquid, solid,
and gaseous—expand with increasing heat and contract with decreasing
heat, except fresh water between 39° and 32°, which actually expands
with falling temperature. It seems as though the Creator had gone
over His work and made revisions and corrections here and there, for
unless the law with regard to the contraction of liquids with falling
temperatures had been reversed for fresh water between 39° and 32°
our rivulets, streams, lakes, and rivers would freeze from the bottom
upward and the life of inland water be wholly or partly destroyed.
Even more calamitous would be the floods of springtime, for melting
snows and falling rains would spread over and erode the cultivated
fields of the husbandman instead of being carried away by the open
channels of streams, as is largely done now.
=The Freezing of Fresh and of Salt Bodies of Water.= The freezing of
water does not take place upon the surface of water only, as many
suppose. Congelation takes place about millions of minute atoms of
matter carried by the water in suspension. Water expands in the
process of freezing and each particle of ice, no matter in what part
of the body of water it is formed, immediately rises to the surface
because of the gain in its buoyance as it changes from the liquid to
the solid form.
When the surface of water cools by radiation to a cooler air it
gains in specific gravity and sinks and warmer water comes up to
take its place and in turn be cooled and sink; thus a circulation is
established which continues in fresh water until every part of the
body of water has fallen to 39° and in salt water to 28°. At these
temperatures the two waters reach their maximum density. With the
further cooling of salt water particles of ice form and rise to the
top, as already described. With the cooling of fresh water below 39°
the law that holds good for all higher temperatures is reversed and
expansion of volume begins, which continues until 32° is reached.
Therefore, fresh water of any temperature between 39° and 32° may
float upon water that is considerably warmer; in fact, it has less
specific gravity at 32° than at 46°. At 32° that which was a liquid
becomes a solid and still further suddenly expands its volume.
=The Cold of Ocean Bottoms.= Few have any idea of the enormous volume
of cold water that lies upon the surface of the earth, three fourths
of which is covered with oceans whose depths average two miles and
in many places are five miles. Below one mile in depth these oceans
are always at about the freezing point of salt water, which is 28°,
except in the tropics, where it is but little warmer, varying between
34° and 36°.
=How Temperatures of Inclosed Seas Differ from Those of Oceans.= We
will take the Red Sea as an example. It is 180 miles wide and extends
in a nearly north and south direction for 1450 miles, about one half
of it lying within the tropics. Evaporation takes place at a rapid
rate, but only the surface water of the Indian Ocean on the south
is able to enter to take the place of that which is lost, for a bar
or sill at the entrance, extending from the bottom to within twelve
hundred feet of the surface, separates the deep water of the sea from
that of the outside ocean. Its surface temperatures vary about as
the Indian Ocean, being 85° in summer and 70° in winter. Both bodies
of water decrease in temperature at about the same rate down to the
level of the sill, where the temperature remains constant the year
through at 70°. Here a marked difference occurs, for the sea, which
has a depth of 7200 feet, maintains the same temperature of 70° all
the way down to the bottom; while the ocean continues to decrease
in temperature down to a depth of about six thousand feet, where a
temperature of 34° to 36° prevails throughout the year. A similar
condition exists with relation to the Mediterranean and the Atlantic
Ocean. At the top of the sill, which is 1140 feet below the surface,
the temperature of both bodies is 55°, and this degree of heat is
maintained all the way down to the bottom of the Mediterranean,
while in the Atlantic Ocean, at the same depth as the bottom of the
Mediterranean, the temperature is only 35°.
=How the Temperature of Water Changes with Latitude, Season, and
Depth.= It is impossible to name a given temperature as prevailing
over bodies of water at all places on the same parallel of latitude,
because ocean currents soon move water heated in one latitude to a
higher or a lower position. At the equator the surface temperature is
between 82° and 84°; it changes less than one degree between day and
night, and not over five degrees between winter and summer; and below
twenty-four hundred feet there is no difference between the seasons,
the daily variation ceasing at less than a hundred feet. Below six
thousand feet the temperature is always near the freezing point of
fresh water.
In the middle latitudes the surface variation is from 50° in winter
to 68° in summer.
At latitude 70° N. the surface temperature has but a small daily
variation, and a yearly range of from 35° for winter to 45° for
summer; at a depth of twenty-four hundred feet it remains steady at
32°.
From this level there is a gradual decrease to a depth of six
thousand feet, where a constant temperature of 28° exists, and below
this there is no change. The temperature of Lake Superior decreases
down to a depth of two hundred forty feet, where a temperature of 39°
continues throughout the year, as it does downward for the remainder
of the distance to the bottom, which has an average depth of nine
hundred feet.
=Direction of Wind Affects Shore Temperature of Water.= Onshore winds
skim off the warm surface water and drive it shoreward, where it
banks up, and, pressing downward, causes the colder water beneath to
flow back seaward. In like manner, offshore winds blow off the top
water near the shore and send it out to sea, and colder water rises
to take its place.
=Great Heat of the Earth’s Interior.= We are ignorant of the
conditions of matter under the heating effect of the enormous
pressure that exists near the center of the earth, but it is probable
that pressure prevents it from changing from a solid to a liquid or a
gaseous form. The surface of the solid earth rises to a much higher
temperature as the solar rays fall upon it than does a water surface,
or the air immediately above, because it is a poor reflector, a poor
conductor, and a poor radiator, and when dry does not get any cooling
effect from evaporation. Solar heat ceases to be apparent at a depth
that varies with the latitude and the conditions of the soil with
regard to moisture and specific heat, but everywhere at less than
fifty feet.
At the poles and for some distance away the earth is covered with ice
or snow the entire year and is frozen to a considerable depth. In
the interior of Siberia and some parts of Alaska only a thin stratum
of soil thaws out under the heat of summer. Beginning at about fifty
feet, there is an increase of temperature downward, but it is not
the same for all places, varying from a degree for forty feet to a
degree for one hundred feet. Taking the average of the increase with
depth, water would boil at ninety-five hundred feet and the hardest
rock be molten at thirty miles. At a depth of 3490 feet near Berlin,
the temperature was found to be 116°, while it was only 108° at the
same depth at Wheeling, West Virginia, and in both places there is no
change from day to night or from winter to summer.
=Soil Usually Warmer Than Air Next Above.= In summer, June to August,
the bare, dry, top soil is warmer than the air ten feet above during
all hours of the day and night, at times the difference being as much
as forty degrees at midday. During winter, December to February, it
is slightly cooler, except between 9 A.M. and 3 P.M. when the excess
is seldom more than ten degrees. Evaporation from a wet soil lowers
its temperature below that of the air immediately above through the
rendering latent of a large quantity of heat. A melting snow surface
also is below the temperature of the air because of the heat employed
in changing the snow to the liquid form.
=Let Mother Earth Cool and Refresh You During the Heat of Summer.=
How little the average man realizes the possibilities for improving
his condition that lie close at hand. He does not know, or he is
indifferent to the fact, that only three feet from the surface of
the ground it is as cool at midday as at midnight, and that there is
no diurnal variation in temperature below that depth, and no annual
variation below a depth of from thirty to forty feet. If one were
to set down the temperature of each day, add the numbers at the end
of the year, and divide the sum by 365 the quotient would equal the
temperature always found at that place at a depth of about thirty
feet. The temperature of a deep-flowing spring is always about the
mean annual air temperature of the place. Here is health-giving
coolness for summer and warmth for winter of which one takes little
heed and derives practically no profit.
Remarkable, is it not? And these beneficent conditions are universal
and available for all, except to those crowded into congested centers
of population. The temperature is 54° in the Mammoth Cave in Kentucky
and shows no change from day to day and from winter to summer.
During the extreme heat of summer and the cold of winter many could
profitably, healthfully, and pleasantly live below ground. During
such periods the cellar of the house, which should be deep and
spacious, even extending beyond the dimensions of the edifice above,
if a continuous supply of pure air could be forced through it, or
natural ventilation accomplished by the plan outlined below, should
be the lounging, resting, and sleeping place of the occupants of the
household. It is not impossible or extremely difficult to change the
stagnant, moist, germ-laden, ill-smelling air of the average cellar,
in which it is positively dangerous to spend much time, into active,
pure, and delightfully healthful air,—air in which the worn and
weary worker from the heat of the farmer’s field, or the artisan and
the clerk from the debilitating temperatures of the factory and the
office could recuperate from the toil of the day, and from which they
would go forth each morning invigorated for another day’s efficient
service, instead of dragging weary limbs from hot, sleepless beds,
each morning less in energy than the day before. As is shown in
other parts of this book, the researches of Huntington have proven
conclusively that man is at his lowest physical and mental points of
efficiency, and more subject to the contraction of disease through
weakness, in midsummer and midwinter, and that the hotter the summer
and the colder the winter the less is his energy and the lower is his
power of resistance.
The whole problem is one of ventilation. While this is simple, it
must be scientifically done. The ideal location for a living cellar
is a hillside. It is easy to install ventilators in the roof of a
cellar no matter where located, but these are of no avail whatever
if there is not adequate air drainage at the bottom of the cellar.
From the cellar in a hillside a conduit can lead from the bottom
of the inclosure and have its opening at a lower level, thereby
accomplishing drainage and circulation, which are all-important in
the creating of a sanitary condition of air under the cool earth. For
each thousand cubic feet of cellar space there should project from
the roof, to a height of at least six feet above ground, a separate
ventilator shaft of at least one square foot cross-section dimension.
A like ventilating capacity should be provided from the bottom
outward to a lower level, but here two or more shafts may be combined
in one, so the proper capacity is secured. During the day the draft
will be upward through this system. But at night, except when the
wind is brisk, the direction of movement of the air is reversed, and
the cool air of the minimum temperature of night or early morning,
because of its greater density, drops down into the cellar. The
drainage shaft should be provided with a damper, which should be
closed in the early morning, about daybreak, entrapping the cold air
of night. The lower opening should be covered with wire netting, to
exclude small animals, and the whole construction be of concrete,
rendering it imperishable and rat-proof.
=Inexpensive but Efficient Cold Storage.= Such a sanitary cellar
as described above provides an excellent storage for fruits and
vegetables, comparing favorably with the much more expensive
artificial refrigeration. By an intelligent manipulation of the
damper in the lower shaft, cool storage may be provided for fruit and
other produce in the early fall, and protection secured against the
extreme cold of winter.
=Why Does Air Cool with Ascent and Heat with Descent?= If a mass of
air be elevated 183 feet it will be found to have lost one degree in
temperature, because there is less air above to exert pressure upon
it and it therefore expands to greater volume, and in the process
of expansion work is performed which employs heat and renders it
latent. One minute, one hour, or a thousand years thereafter, if this
same air be lowered to its former elevation, it will be compressed
into its previous dimensions and the heat energy that formerly was
employed to expand it will be restored to the sensible condition.
This ratio of 183 feet to one degree does not hold for any extended
movement, because, as soon as the dew point of the air is reached,
condensation in the form of cloud or rain occurs and the heat of
condensation is released; that is to say, the same quantity of heat
employed to create the water vapor at some previous time and thereby
rendered latent is now become sensible and partly makes up for the
loss by expansion as the air ascends. The average is therefore about
three hundred feet for one degree.
=Height of Freezing Cold in the Free Air.= The frost level remains
constant, winter and summer, over the equator at about eighteen
thousand feet. Elsewhere this level rises and falls with the seasons,
the amplitude of the movement increasing with latitude and being
greater over land than over water on the same parallel.
=Daily Range of Temperature in the Free Air.= The difference between
the temperature of day and that of night decreases with altitude
in the free air and ceases at about eight thousand feet. It is
greatest during clear weather and least in cloudy weather. Narrow
valleys may show a greater daily range than hilltops. When the sky
is clear, radiation from the hillsides may heat the air in a valley
to almost furnace heat at midday, while at night the air, coming in
contact with cool vegetation higher up, chills and, gaining in weight
by contraction, flows down and collects in the valley, making the
bottom of the valley warmer during day and colder during night than
the air above. Often moisture-laden winds precipitate much of their
water vapor as the air cools by expansion in passing over a mountain
range. These winds carry a comparatively dry air over to the leeward
side of the mountain, where the daily range of temperature will be
much greater than on the windward side at the same elevation. San
Francisco, where the prevailing winds come from the ocean, has a less
range than New York, where the predominating winds are from the land;
but New York is influenced by its proximity to the ocean, for its
range is much less than at Denver, in the interior of the continent.
The range is less on the east side of Lake Michigan than on the west
side, as it is with relation to all similar bodies of water.
=Man Soon Adjusts Himself to Changes in Altitude.= In Colonial days
it was noted that horses coming down from the mountains in North
Carolina ran swifter in the races the first day or two after changing
to a lower level. In going to a higher altitude an increase in the
number of red corpuscles in the blood enables it to absorb oxygen
more readily, and thus compensate for the loss in the density of the
air. Because of this gain in the chemical activities of the life
current, one feels a marked increase in strength on coming to a
lower level, but the gain lasts for only a short time before there
is a readjustment to former conditions. Persons with weak hearts may
not be able to live at an altitude of four thousand feet, and most
people experience inconvenience, at least, on first reaching ten
thousand feet; but nature is accommodating, and a number of large
cities prosper at altitudes of from one to two miles.