â– Loc. cit.
CHAPTER VII.
HEAT.
LAWS OF HEAT — THE ZONES — HOW THE TEMPERATURE OF THE EARTH'S
SURFACE IS MAINTAINED — DIFFERENT CONDITIONS OF HEAT —
ANOMALIES OF EXTREME HEAT IN THE EASTERN STATES, AXD ITS
DANGERS — RELATIONS OF HEAT TO ALTITUDE.
Heat it the fundamental power of climate. The atmosphere is the
life of the world, but if the atmosphere were not the carrier of heat there
would be no life.
It is heat which sets the air in motion, governs the course of the
winds, moves the currents of the ocean, supplies the atmosphere with
moisture, distributes the rains, and distils the dews. Hence a knowl-
edge of the laws which govern heat are of primary importance in the
study of climate.
Experiments show that nearly all bodies are subject to expansion, or
increase in volume by the application of heat, and recover their original
size when reduced to the initial temperature. Thus if the temperature
of an iron bar be raised 5°, and thereby increased in length one inch, by
elevating the temperature to 10° it will have- increased two inches. Or
if a ball of iron be made of such a size as to pass through an aperture
at a temperate warmth, it will not fall through when heated. Liquids
are in like manner amenable to the law of expansion. If a glass tube be
inserted through a cork into a flask tilled with water, and heat applied,
the water will rise in the tube, and the higher as it becomes warmer. It
is in accordance with the same law that the thermometer was con-
structed, and operates by the expansion of mercury, or, rather, by
dilating in equal amounts for equal increments of heat.
But water is an extraordinary exception to the law of expansion by
heat. It contracts till cooled down to the temperature of 40° F., after
which it expands. It is in consequence of this peculiarity that ice
always floats. This effect of heat on water is illustrated by another, and
a very curious consequence. When a pool or a well of melted ice hap-
pens to be formed on the upper surface of a mass of ice, as not infre-
quently happens in the glaciers of -cold regions, the well quickly increases
in depth until it entirely penetrates the ice to the earth beneath. Sup-
posing the water at the surface when the pool or well first forms to be
HEAT.
about or but little above the freezing point of 82 , as it, becomes heated
by the air and Bun'srays, instead of being thereby expanded (as it would
ii ii oonformed to the law of expansion), and rendered specifically
lighter and detainedal the Burface, as it, more nearly approaches the
temperature of 40 it becomes heavier, and therefore sinks down to the
bottom; but there, by melting some of the ice, and consequently becom-
ing cooled, it is again made, lighter, and, rising again to the surface,
gives place to another descending portion, which has, from exposure,
acquired the same properties, and so on; the circulation and digging
cease only when the water has bored its way quite through the ice to the
surface of the ground.
The manner in which heat is communicated through liquids, is
by convection. When applied to the lowest part of the mass, for ex-
ample, to the bottom of a tube containing water, the particles heated
expand, and becoming lighter, ascend, while the colder particles from
above descend and supply their place. The process may be clearly shown
by throwing into the tube, while the water is boiling, some insoluble
powder whose specific gravity is the same as that of the water, when a
series of upward and downward currents will be exhibited, which will
continue as long as the boiling.
Gases become heated in the same way as liquids, though the currents
nave more of a wavy or cloudy form, unless, as is seldom the case, the
air is perfectly still, when smoke is seen to ascend vertically, and fogs
and vapors rise in the same manner.
Gases and vapors expand by heat; and air which is a gaseous com-
pound, conforms to the law.
The results of scientific experiments show that the ratio of expansion
of a volume of air in passing from the freezing to the boiling point of
water is £{j |; ,f ; that is to say, a quantity of air equal in volume to 1,000
cubic inches at 32° F. will expand to 13G5.5 cubic inches at 212°, or
by an increase of 180° of heat. Hence, as it has been proved that the
expansion is the same for each additional degree of heat, the expansion
will be equal to 0.00203G, or j^V.tt ^ or eacn degree : —
Four hundred and ninety-one cubic feet of air at a temperature of
32 will become 492 at 33°
493 at 34°
490 at 31°
489 at 30°
488 at 29°, etc.
Solid bodies lose or acquire heat by conduction. The particles nearest
the source acquire heat and transmit it to those in contact with them,
and these to the next, and so on till the whole mass has attained the heat
of the surrounding medium. If the temperature of this medium, sup-
pose the air, be reduced, the surface of the heated body radiates forth
its heat from the interior till equilibrium is restored.
56 HEAT.
According to the greater or less rapidity with which heat is diffused
throughout a substance, so it is said to be a good or a bad conductor of
heat. Metals are good conductors; gases, liquids, and earthy matters,
scarcely conduct at all. The loss of heat by radiation greatly depends
upon the nature of the surface: smooth or polished surfaces radiate much
more slowly than rough surfaces.
Glass and wood are bad conductors of heat. For illustration, a piece
of wood may be held in the hand and burned until within a short dis-
tance from the flame, or a glass rod held by the fingers within an inch
from the flame of a blow pipe, without inconvenience, whereas the end
of an iron wire, which is a good conductor, held in the same flame and
heated to redness, will be heated and rendered intolerable for a foot or
more from the flame.
Air is a bad conductor, and hence the utility of double windows in
cold climates, which include a layer of air between that prevents the
radiation of heat from the rooms into the colder atmosphere without.
Ice is a bad conductor, and snow a still worse. A surface of ice pre-
vents the cooling of water underneath ; and snow prevents the ground
from freezing, and protects the delicate roots of plants from the effects,
of severe cold.
The barks of trees are bad conductors, and exhibit a structure of
porous material, with pores filled with air ; which finds its imitation in
the cover of the boilers of steam engines with wood and felt. And for
the same reason, ice is wrapped in flannel because the flannel is a bad
conductor, and prevents the external heat from reaching it.
The crust of the earth is almost wholly composed of bad conductors,
of heat. Hence, although the interior of the earth is well known to be
of a much higher temperature than the superficial layers, no heat is com-
municated to the surface from the interior. And for the same reason,
the sun's rays, upon which the surface of the earth wholly depends for
its warmth, never penetrate to any considerable depth — the temperature
of mines and wells, even a few feet deep, continue about the same for
summer and winter.
Liquids are also bad conductors. If a tube be filled with water, and
held aslant over the flame of a spirit-lamp, the water may be made to
boil from the top, while a piece of ice may be retained at the bottom.
Although the earth is a warm body, which at the beginning was
probably incandescent, it has so cooled down at the surface by the lapse
of ages as to retain scarcely a trace of its original temperature. Never-
theless, we know that the temperature of the earth increases as we de-
scend into the interior at the rate of about 1° to every 112 feet, and
that the internal heat must be very great underneath volcanoes. Nine
thousand feet of depth, or about one mile and seven-tenths gives the
temperature of boiling water ; at the depth of thirty miles, at the same
ratio, the heat is sufficiently intense to melt all the rocks and metals
i hat. 57
contained in the earth's crust, and to account for the torrents <>f molten
ticry lava belched from the craters of raging volcanoes. It is to this in-
ternal heal of the earth that hot springs, and the warm rater 01 deep
artesian wells are due. Bnt all the heat available for the purpose ot
organic life is unquestionably due to the influence of the the
sun.
Though the sun pours its life-giving rays in a uniform and uninter-
rupted stream upon the earth's surface, the spherical form of the earth
and its movements of daily rotation on its axis and annual revolution
around the sun, establish permanent differences of temperature, in every
latitude between the poles and the equator, and periodical difference
upon the seasons, and upon the diurnal rotation for day and night.
The division of climate into zones is based upon the permanent differ-
ences in the power of the sun's rays on the earth's surface.
Two imaginary parallel lines to the equator upon the surface of the
globe, at the distance of 23° 28' in each hemisphere, designate two cir-
cles between which the sun passes across the zenith at certain epochs of
the year : these are the tropics. That of the northern hemisphere is
known as the Tropic of Cancer, because during the summer solstice the
sun passes at its zenith and is in the zodiacal sign of Cancer. That of
the southern hemisphere is known as the Tropic of Capricorn, because
the sun passes at its zenith during the winter solstice in the zodiacal
sign of Capricorn. The space included between these two circles com-
prises that portion of the earth's surface over which the sun rises to its
greatest and most vertical altitude, and sheds forth its rays with the
greatest degree of intensity, is termed the torrid zone.
Two other circles, distant respectively 6G° 32' from the equator, or
23° 28' from the poles, in each hemisphere, mark the lines below which,
the sun may remain for several days together, and above which it attains
its least altitudes: these are the polar circles, which include the frigid
zones. During one-half of the year, the sun rises above the polar circles
to the height of 23° 28', and during the other half descends below them
to the same amount.
The temperate zone is that portion of the earth's surface which is be-
tween the torrid and frigid zones.
The areas in square miles of the respective zones is: —
North tropical zone, . . 39,109,628 ) Warm region,
South " " . . . 39,109,628) 78,219,256.
North polar " . . . 8,229,748 / Cold region,
South " " . . . 8.229,748) 16,459,946,
North temperate zones, . . 51,110,763 ) Temperate region,
South " " 51,110,763)" 102,221,526,
It thus appears that the regions most favorable to mankind, the tem-
perate regions, are far the most extensive; and next, the warm regions;
58 HEAT.
while the frigid zones, unsuited for human progress, extend over a com-
paratively inconsiderable portion of the earth's surface.
In the regions of, and near the equator, in both hemispheres, the
various causes which influence the action of the sun's heat vary but
little throughout the year. The day has about the same length all the
year round; the meridian height of the sun undergoes but little varia-
tion, and the four seasons differ very little in regard to temperature, the
one from the other. For an entirely different cause, in the regions
where the length of the day varies very much in the course of the year,
or where the meridian height of the sun at one solstice is very different
from that of the other, the seasons are very dissimilar both to the north
and south of the equator.
The angle at which the solar rays reach the surface of the earth are
the chief cause of the succession of climates from the equator to the poles,
and if the earth's surface were perfectly regular in shape and consistence,
instead of being divided into land and water, forests and arid plains,
table-lands, mountains and valleys, etc., the diminution of temperature
would be progressive and regular. These irregularities of the surface
give rise to the various differences of climates in the same latitudes, and,
under the influence of heat, tend to maintain a perpetual circulation of
the atmosphere, and the equalization of temperature. Thus the trade-
winds, which establish a double current between the equator and the
poles, temper the cold of the high latitudes over which they pass, and the
heat of the tropical regions where they arise, heating the former and
cooling the latter.
Gases and vapors, as before shown, possess the property of absorbing
heat rays, and consequently the atmosphere absorbs a portion of the
rays transmitted by the sun. But the power of absorption by different
gaseous substances greatly differs. Professor Tyndall, after many ela-
borated experiments on the absorptive power of different gases, concludes
that on an average day the water present in the air absorbs about sixty
times as much heat as the air itself; and Professor P. M. Garibaldi has
shown that the absorptive power of heat, under a barometric pressure of
39.92, by different gases, stands in the proportion of
Atmospheric air (oxygen and nitrogen) 1
Carbonic acid 92
Ammonia 546
Vapor of water 7,937
Hence it appears that the actual amount of heat of the air derived
from the sun's rays in their transit, compared with that which is radiated
from the earth's surface, is very small.
The lower portions of the air are heated by radiation from the earth's
surface, communicated by convection, and to this source may be traced
the greatest portion of the heat it exhibits. The greater portion of the
in: AT.
Bun's rays which are received by the earth are deprived of their power
of being reflected back again into space by t he interposing moisi are, and
its greal capacity for heat.
But the amount of specific or absolute heat which different sub-
stances contain greatly differs, although their temperature, as indicated
by the thermometer, may appear to be the same. If, for example, two
glass tubes, in every way alike, one containing mercury ami the other
water, be subjected to the same degree of heat by plunging them into a
vessel of hot water, the mercury will attain the degree of the surround-
ing medium in half the time the water will take; and on removing
tubes and allowing them to cool, the mercury will take only half the
time to recover the initial temperature as the water: this effect ari
from the difference in the absorbing power of the two substances, mer-
cury absorbing less heat than the water in being raised to the same
temperature; or, as usually expressed, water compared with mercury
has twice the capacity for heat.
In comparing the capacity for heat of different substances, it is usual
to take equal weights of each rather than equal volumes.
A pound of distilled water takes a certain amount of heat to raise it a
certain number of degrees; to this standard other substances are reduced:
if a pound of mercury requires .033 of the same degree of heat to attain
the same temperature, the specific heat of w r ater is to that of mercury
as 1,000 to 33. According to the same standard, the specific heat of
various substances has been ascertained to be as folloAvs:
Specific heat
Substances. of equal weights.
Water 1,000
Ice 513
Iron 113.8
Copper 95.15
Zinc 95.55
Glass 198
Mercury 33.32
Lead 31.4
Air 3,705
The great capacity of air for heat has already been shown.
The aqueous vapor suspended in the atmosphere, though it may ex-
tend only a few feet from the surface of the earth, constitutes a moist
stratum which as effectually retards the nocturnal process of cooling as
the whole atmosphere. Indeed, the most striking facts in connection
•with the temperature of the atmosphere are the absorption of heat which
accompanies the transformation of water into vapor, and the part played
hy the vapor in maintaining the temperature of the earth's surface. This
is latent heat. It is so named because it does not affect the sense of touch
or the thermometer: it mav be thus illustrated: —
60 HEAT.
If a vessel be filled with ice in a melting state and subjected to the-
application of heat, a thermometer placed ia it will not show a temperature
above 32° F. till the Avhole of the ice is melted: not till then will the tem-
perature of the water begin to rise. The heat thus absorbed by the ice
in passing from a solid to a liquid state has therefore become latent. It
is the effect of the same property — the absorption of heat by liquefaction —
which chills the air during a thaw: the ice and snow absorb heat from
the surrounding air during the process of melting.
Evaporation from the surface of water proceeds at all temperatures,
and goes on gradually and insensibly: the particles of water rise in the
air, and are mixed with it, and, unless they exist in large amount, are
invisible. Whenever evaporation takes place, heat is absorbed from
some contiguous substance to supply the amount which becomes
latent in the conversion of the liquid into vapor; hence evaporation
is always a cooling process. From the same cause, ether, which
evaporates rapidly at a low temperature, applied to the surface of
the skin produces the sensation of cold; and the evaporation of water
from the surface of a porous jar cools its contents; but in this case
the water of the vessel exudes through the pores and forms on the outside
of the vessel like dew, and this is rapidly taken up by the surrounding-
air, just as the moisture is taken up, as so beneficially experienced, in hot
climates.
The insensible evaporation of water and the diffusion of its vapor in the
atmosphere may be proved by exposing a shallow vessel out of doors filled
with water: in a few days of dry weather, it will be found emptied of its
contents. If the surface of the vessel be of a given area, say one square
loot, and the water be weighed from time to time, the loss of weight will
give the rate of evaporation for the temperature and locality for one
square foot of the surface.
The process of evaporation is always going on on a large scale over the
surface of the ocean, seas, lakes, rivers, and moist surfaces everywhere.
The absorption of the heat by the moisture evolved in the process of
evaporation has the effect of cooling the surfaces from which it is taken,
but the temperature of the vapor is not thereby sensibly increased, it is
latent; but at such a degree of temperature as to sustain the vapor. The
heat which is thus absorbed is destined to be transported to the most
distant latitudes, and to establish and maintain in the atmosphere an
equality of temperature which would not otherwise be produced; it is
restored to the earth again in its entirety when it returns to the liquid
state as rain. The quantity of heat which thus passes from the equa-
torial to the polar regions is beyond conception.
The smaller the amount of moisture which the atmosphere contains,
the more easily it is traversed by heat. At an altitude of 4,000 feet and
upwards, the increase of heat in the sun's rays relative to the temperature
of the surrounding air becomes a marked feature, insomuch that, at an
HEAT. CI
altitude of from 6,000 i<> L 0,000 Feel above the level of the sea, the ther-
mometer exposed to I he raya of I lie sun usually registers about one-1 bird
higher than when in ( he Bhade.
The difference between the indications of the thermometer in the
Bhade and in the sun augments with elevation, on the authority of Dr.
Charles Denison, " one degree greater difference between the temperature
in the sun and shade for each rise of 235 feet." 1
But the general result of altitude shows that the temperature 'In-
creases about l c Fahrenheit for every 345 feet elevation.
Humbolt observed that the decrease of temperature by altitude va-
ries in different countries, as follows :
"The decrease in a southern atmosphere was 1° F. to 344 feet in the
mountains, and 440 feet upon the table-lands. A series of places in
Southern India gave 320 feet; in the north of Hindoostan, on the other
hand, the decrease was 1° in 410, about the same as upon the table-
lands of America. Everywhere analogous differences of level are re-
marked; in "Western Siberia, 1° in 450 feet is the result arrived at, and
this number is converted into 440, if the comparison includes the ele-
vated regions of Northern India. In the United States, the decrease is
1" to every 400 feet. The configuration of the country seems to be the
most important element in the calculation. If there is a gentle rise in
the ground, or if the country is made up of successive gradients, the de-
crease in the temperature is much more gradual than upon the sides of
steep mountains. In the first case, 1° may be taken to represent a dif-
ference in level of 420 feet; in the second, of 350 only." 3
Although what is above stated in regard to the decrease of temperature
on the increase of altitude is a general law, in hilly regions and a general
prevalence of calm weather the law is reversed; the cold air, by reason
of its greater density, descending into the valleys, and the warmer rising to
the top of the hills. If there be wind enough, however, to create a dis-
turbance and intermixture of the higher and lower strata, this excep-
tion to the general law does not occur. These facts are familiar to
observant people in hilly regions generally, but they have not been
sufficiently taken account of in meteorological observations to render
them popular for the protection of health. The damp and chilly val-
leys, with their attendant ills, arc more frequently chosen as building
places than the dryer, warmer, and healthier hills.
That the atmosphere under the influence of heat is in a per-
petual state of circulation; that the sea which covers three-fourths of
the earth's surface is distributing temperature to every shore; that there
are general winds which periodically traverse the different regions of
the globe from the equator to the poles, which temper at once the cold
1 "Rocky Mountain Health Resorts," etc., p. 70.
8 " Aspects of Nature."
62 HEAT.
high latitudes in their course and the heat of the tropical regions,
warming the former and cooling the latter, are all so many evidences-
of the insignificance of mere latitude as an indication of temperature. No-
where in the world does this insignificance of latitude appear more marked
than in the eastern portion of the United States, nor is it anywhere more
deserving of attention with relation to the public health.
Seasoning upon latitude as an index of the general course of clima-
tological differentiation, the natural inference would be that the ex-
tremely high temperature which occasionally occurs in this region is due
to the transfer of the heated atmosphere from a more southern latitude.
But the facts are quite the contrary — the temperature in that direction
at such times is frequently lower. It is not at all uncommon when the
temperature reaches the nineties in Baltimore, Philadelphia, New York,
and Boston, that of Charleston and Savannah will be from three to five
degrees lower; and in the interior of the Carolinas and Georgia, and.
along the Gulf coast it is sometimes, at corresponding periods, still lower.
And no less striking is the opposite extreme — the Arctic severity of
some winter days or weeks, when the temperature is several, and some-
times many, degrees higher than it is several parallels of latitude fur-
ther north. The reader who carefully studies the records and charts in
subsequent chapters will not fail to observe that there is a general move-
ment of the climatic changes in the United States from west to east,
rather than from the south to the north, or in the opposite direction.
Yet these excesses of heat and cold rarely or never occur in the regular
line of the westerly current south of the 42d parallel. During the
summer, the Lake district and British America north of the 42d parallel
are warmed by the westerly winds, but south of the 38th parallel they
seem to exercise no regular influence.
How far this difference is due to floating masses of ice on the north-
ern sea-coast at one season and the warmth of the Gulf Stream uninflu-
enced by this condition at another, is mere matter of conjecture. The
fact, however, is of very great importance in its sanitary bearings, espe-
cially during the period of high temperature, as possibly accounting for
the rise and spread of epidemic diseases, particulary of yellow fever, in
extraordinary localities equalized for the time being with the usual habi-