288 
into its place from the sides, which again, in its 
turn, becoming heated, ascends in the same man- 
ner, and thus constitutes an ascending current 
or upward draft from the place where it is heated. 
Were it not for this no fire could continue for 
any length of time, since it must necessarily be 
extinguished as soon as the surrounding oxygen 
were absorbed, but the air being heated, at the 
same time it becomes lighter, and ascends, and 
thus causes an afflux of fresh air from the sides 
or from below, by which the combustion is main- 
tained. The different commotions of the air, 
such as wind, gales, and hurricanes, owe their 
origin also to the same cause, the primary move- 
|| ment being always an ascending current, pro- 
duced by the heated and expanded air, over some 
spot rising in a vertical column, and the denser 
and colder air rushing towards this point, and 
_ thus producing the above horizontal currents on 
| the surface of the earth. The following table ex- 
hibits the rate of expansion of air from 32° to 
212°, according to Mr. Dalton :— 
Degrees of Degrees of Dal- 
Fahrenheit. Bulk of Air, ton’s Thermometer, 
32° 1000 32 
39°3 1017°9 42 
47 1036°1 52 
55 1054°7— 62 
63°3 1073°5 72 
UD 1092°7 82 
8l 1112°3 92 
90°4 S22 102 
10071 1152°4 112 
110 11731 122 
120°1 1194 132 
130°4 1215°4 142 
141°1 1237°1 152 
152 1259°2 162 
163°2 1281°8 172 
175 1304:7 182 
186°9 1328 192 
199°2 1351°8 202 
212 1376 212 
359°1 1643 312 
539°8 1962 412 
754:7 2342 512 
1000 2797 612 
1285 3339 712 
The reader will observe, that the expansion of air 
in the second column of the table constitutes a 
geometrical progression, the ratio of which is 
1:0179. The third column exhibits the corre- 
sponding degrees of a Fahrenheit’s thermometer 
graduated, according to Mr. Dalton’s notion of 
the expansion of mercury, according to the square 
of the temperature. 
The temperature of the atmosphere is greatest 
near the earth, because the air transmits the rays 
of light without decomposing them, and cannot, 
therefore, be heated by them before they reach 
the surface of the earth, where they are decom- 
posed and their heat set free. The air, which 
thereby becomes warmed, ascends and mixes with 
the upper colder strata. This is the reason why 
the atmosphere becomes colder as we ascend into 
it, until, even in the hottest summer season, its 
temperature sinks below the freezing point. In 
| the lower strata this decrease of temperature has | 
ATMOSPHERE. 
been found to amount to about 1 degree for every 
302 feet of ascent, although at greater elevations 
the decrease is probably less rapid. This is also 
the reason why the temperature decreases in as- 
cending high mountains until, even in the warm- 
est climates, the region of perpetual congelation 
may be attained. Thus the summits of the Andes 
under the equator are covered with perpetual 
snow and ice. The height at which this line of 
perpetual congelation is attained has been fixed, 
at 15,207 feet under the equator, but decreases 
progressively in higher latitudes, being 3,818 feet 
at 60° latitude, and only 1,016 at 75° latitude. 
Climate—The comparative warmth of any lo- 
cality greatly, though by no means wholly, de- 
pends on the quantity of sunbeams which it re- 
ceives. In consequence of the sphericity of the 
world, only the portions of it close to the equator 
receive the cylinders of the sun’s rays upon a 
level base, and all other portions, in the degree 
in which they lie distant from the equator, re- 
ceive these upon an increasingly oblique base; so 
that a district or patch of ground of any given 
extent near the equator basks under a far denser 
play of sunbeams than a district or patch of equal 
extent situated at a distance from the equator. 
For the same reason, though on quite a mimic 
scale, when the southern and the northern sides 
of a hill are simultaneously under sunshine, the | 
southern side enjoys a far denser play of rays, and 
is therefore far more acted on by both light and | 
heat, than the northern side. A southern ex- 
posure in a farm is thus a mighty element in | 
local climate ; and, provided the declivity be con- 
siderable, and the conditions of soil, drainage, 
and ventilation be equal, it may amount, in prac- 
tical value, to a superiority of several degrees 
southward over a neighbouring farm of northerly 
exposure. 
Yet the character of the upper strata of the 
terrestrial surface, the dryness, siliceousness, and | 
porosity of the soil, the state of cultivation, the | 
absence of morasses, and particularly the altitude | 
above sea-level, and the character of the neigh- | 
bouring countries and seas, exert, both individu- 
ally and combinedly, a strongly modifying influ- 
ence upon the atmosphere, and impart a distinc- 
tive character or important peculiarities to local 
climate. When much damp ascends into the air 
from prevailing marshiness of surface, it rarefies 
the atmosphere, decreases its calorific capacity, 
and, in consequence, renders the local climate 
cold, chilly, and replete with ungenial vapour. 
But by a beautiful law of the almighty and all- 
benevolent Governor of the universe, the physical 
constitution of the atmosphere combines with 
the operations of husbandry to reduce and even- 
tually to destroy such local causes of noxiousness. 
That not much cold air and damp air shall ac- 
cumulate in any one place, is provided for by a 
decrease in the atmosphere’s capacity for watery 
vapour proportionately to the fall of its tempera- 
ture; and that chilliness and dampness shall be 
