with air, or feel the supple yieldings of an air-
cushion; when a warm and gentle breeze fans
our cheek, we say to ourselves, "How light the
air is!" but when we have to meet it in a
storm, or a hurricane, we perceive at once that
it has something besides velocity, and that
it must have also weight to give it the
momentum which it is capable of exerting with
such crushing effect. It is the pressure of the
air which enables the calf to suck its mother's
milk, the leech to gorge itself with blood, and
the fly to walk up the pane of glass. The
pressure of the atmosphere even adds heartiness
to the application of a kiss.

And yet the ponderousness of air was denied
by the ancients! It was regarded as an illusion
of the senses. The boldest free-thinkers
ventured no further than to entertain their doubts.
Galileo first discovered the weight of air,
inferring it from the fact that water, in pumps,
remained arrested and suspended at a height of
about thirty-three feet. After him, Torricelli,
and then Boyle (the inventor of the air-pump),
confirmed his ideas by further experiment. In
the seventeenth century, Pascal had
demonstrated the material existence of air by the
mere force of reasoning, showing, in his Abrégé
sur le Vide, that it is a tangible and heavy
body.

During the first half of the last century, and
before it, the notions about air were excessively
crude. Satanas was the Prince of Air, as the
Queen's eldest son is the Prince of Wales; and
his subjects, the witches, mounted on brooms,
rode through his realms to attend his drawing-
rooms. All gases were air, and the same; their
differences were not known. There was merely
the trifling distinction that some were respirable,
and others fatal to the animals breathing
them. No suspicion was entertained of the
absorption and assimilation of the different
gases by animals and vegetables. The air
contained everything—the whole fossil (mineral),
vegetable, and animal kingdoms; and nothing
was ever taken out of it. For, an animal,
when dead, being exposed to the air, is, in a
little time, carried off, bones and all;
vegetables, by putrefaction, become volatile; and all
kinds of salts, sulphurs, and metals, convertible
into fume, are thus capable of being mixed with
the air. Well might Boerhaave take it for an
universal chaos or colluvies of all the kinds of
created bodies.

Air—elementary air it was called—was made
of the vapour of the sun (and perhaps it is); it
was condensed and thickened ether; it was
mechanically producible, being nothing else
than the matter of other bodies altered, so as to
become permanently elastic. For, solid bodies,
unsuspected of elasticity, being plunged in
corrosive unelastic menstrua, would, by a
comminution of their parts in the conflict, afford a
considerable quantity of permanently elastic air,
To account for its elasticity, some believed the
corpuscules of air to be branched; others held
them to be so many minute flocculi, resembling
tufts of wool; others conceived them round.
like hoops; or curled in corkscrews, like
shavings of wood; or coiled in spirals, like
steel watch-springs; all which ingenious
hypotheses are yet unconfirmed by the microscope,
or otherwise. But, into what sloughs of
absurdity does the human mind straggle when it
follows fancy instead of observation! Air is here
imaged, as its exactest types, by thickets of
bushes, wool mattresses, sacks of shavings, and
spring cushions.

Newton put the thing another way, attributing
the elasticity of air to a repulsive force
between its particles, after their original separation
by heat. And indeed, of the two forces,
attraction and repulsion, the latter would seem
to reign amongst gases, the former amongst
liquids. The particles which compose elastic
fluids avoid each other as much as circumstances
allow; those which make up liquids, hug each
other as closely as they can; it requires the
superior force of heat to cause a divorce between
them.

"The atmosphere," from ατμος, vapour,
means the sphere of vapours. Maury likens it
to another ocean enveloping the entire world
over the ocean of water, which covers only two-
thirds of it. All the water of the one weighs
about four hundred times as much as all the air
of the other.

As to the height of the atmosphere, the
received opinion is that its upper surface—if it
has a surface—cannot be nearer to us than fifty,
nor more remote than five hundred miles. But
it is impossible to fix any precise limit, by reason
of its growing tenuity, as it is released from
the pressure of its own superincumbent mass.

It is something to know that more than three-
fourths of the entire atmosphere is below the
level of the highest mountains. The other
fourth is rarefied and expanded, in consequence
of the diminished pressure, until the height of
many miles be attained. From the reflexion of
the sun's rays after he has set, or before he rises
above the horizon, it is calculated that the upper
fourth part must extend at least forty or forty-
five miles higher. Sir John Herschel has shown
that, at the height of eighty or ninety miles,
there is a vacuum far more complete than any
which we can produce by any air-pump. In
1783, a meteor, computed to be half a mile in
diameter, and fifty miles from the earth, was
heard to explode. As sound cannot travel
through a vacuum, it was inferred that the
explosion took place within the limits of the
atmosphere. Herschel thence concludes that
the aerial ocean is at least fifty miles deep.

Maury's illustration of the way in which the
atmosphere is packed, and of its diminished
density as we ascend in it, is admirable.

If we imagine the lightest down, in layers of
equal weight, and ten feet thick, to be carded
into a pit several miles deep, we can readily
perceive how that the bottom layer, though it
might have been ten feet thick when it first fell,
yet with the weight of the accumulated and
superincumbent mass, it might now, the pit
being full, be compressed into a layer of only a