Hw affect

ed thereby

Heat how conveyed.

What fol

By this accession of heat it is expanded, and becoming lighter, mounts to the higher regions, while colder and heavier air sinks into the place vacated.

The warm ascending currents carry with them the heat which they have received from the earth, communicating a portion thereof to every particle of air with which they come in contact.

Thus they gradually become cooler and cooler lows hence as they ascend, and the uppermost strata receive but very little heat.

What this explains.

composed.

This is one reason why the tops of mountains are much colder than their bases; another being that the air expands as it ascends, from its being relieved of pressure, and being expanded, absorbs heat instead of communicating it.

SECTION II.

THERMAL SCALES OF VAPOUR AND AIR. (An evidence of divine wisdom and goodness.)

Air how The atmosphere, it must be remembered, is composed of two parts, dry air and aqueous vapour, mechanically mixed together.

how grad

uated.

Their heat The temperature of each diminishes as it ascends, but at a very different rate; that of dry air decreasing about five times as fast as that of vapour.

Exemplify

How then.

Suppose, for example, both to be 80 degrees when they start from the earth, when they have ascended 5000 feet, the dry air will have lost fifteen degrees, but the vapour only three.

In another 5000 feet of perpendicular height, the dry air will have lost 30 degrees of its

original heat, but the vapour only six; and so on as they continue to ascend.

lows hence.

Hence, if these two gaseous fluids exist what fol together, they must modify each other; the warm vapour must impart heat to the cold air, making the upper regions of the atmosphere warmer than they would otherwise be.

Again, as the aqueous vapour retains Another its heat longer, it must ascend faster than principle. the dry air, which being quickly condensed, soon becomes too heavy to continue its upward motion.

lows hence.

Hence the vapour soon outstrips and passes What folby its laggard companion, leaving it free to absorb moisture elsewhere.

The benefits resulting from this wise pro- what the vision are very numerous and varied :

benefits.

(1.) It promotes evaporation. As the vapour 1st what. mounts up quickly, leaving the particles of dry air in the rear, those particles are free to absorb moisture.

wise what.

If this were not the case, evaporation would If otherbe very greatly checked, the earth would be burdened with moisture, and a thick haze or impenetrable mist would at all times brood over the face of the whole earth.

result.

(2.) Another result is, we have less rain. For Another the air not being at all times saturated with vapour, may frequently be cooled down several degrees without being obliged to part with any of its moisture.

wise what.

If however, vapour and dry air had cooled If otherdown at equal ratios, and ascended precisely at the same rate, the air would at all times have been saturated; and whenever the shifting sun

What then

A third result.

What fol

had diminished its temperature in the least, rain must have fallen.

It is needless to add, what a change for the worse would such a showery drizzling atmosphere have been for the present inhabitants of the animal and vegetable kingdoms of the world!

(3.) Again, in consequence of the present wise law, the clouds are less dense, and the sunshine more vivid and constant.

As the air is left free to absorb moisture, it lows hence often absorbs it from the clouds, which in consequence, either diminish in magnitude, or wholly disappear.

What if otherwise.

Anotherre salt what.

What then

But if the gaseous air had been always at den-point, it would never have been able to relieve or dissipate the clouds, which would at all times have shrouded the earth and veiled from it the bright invigorating sun.

(4.) Lastly, if the vapour had cooled down in its ascent as fast as dry air, it would have been reduced so low as to freeze before it reached the summit of our high mountains.

It is scarcely possible to calculate all the mischief which would have resulted from such a state of things; but we see and know quite enough to admire the wisdom and goodness of that Being, who so wonderfully prevented these calamities by such a simple arrangement.

CHAPTER XIV.

LIGHT.

SECTION I.

WHAT IS LIGHT.

what.

According to Sir Isaac Newton, light resem- Light bles the perfume of flowers, and the perception of light is analogous to the perception of smell. Odorous substances emit very small particles How so. of their own material essence, and these particles or effluvia, as they are termed, upon reaching the nose, produce the effect called odour or smell.

to light.

So visible objects, according to Newton, emit Apply this luminous particles from their surface, and these particles or corpuscules, as they are called, striking upon the eye, produce the impression of vision.

ory is pre

This theory of light, called the corpuscular, What theis now generally discarded, and the theory ferred. of Huygens, called the undulatory, is almost universally adopted.

According to Huygens, light bears no analogy Explain it. to odour, but a very striking analogy to sound.

As sound is an effect produced upon the ear How so. by vibrations of air, so light, by this theory, is an effect produced upon the eye by vibrations in what is termed the luminiferous ether.

this ether.

This luminiferous ether is a fluid much more What is subtile than air, penetrating between its fine atoms, as water between grains of sand.

be granted.

By either theory a vast concession must be What must made. If the Newtonian theory be accepted,

What if

we must grant that all visible bodies throw off from their surface certain material substances or corpuscules in order to produce vision.

If, on the other hand, the theory of Huygens Huygens' be adopted, we must allow the existence of a luminiferous ether; that all objects which we see impress this ether with undulations; and that the ether transmits these undulations to the eye. These are large admissions to make, but then the theory explains in a most satisfactory manner all the optical phenomena with which we are acquainted; and the Newtonian theory fails in many points when put to the same test.

Can these be granted

What are its uses.

SECTION II.

USES OF LIGHT.

Without light the eye would be useless; no object would be visible; and although animal life could exist, yet what a difference would it make, if there were no light to dispel the "blackness of darkness" from the earth. What else. All the pleasures of sight would be at once exterminated. All the information we receive from reading and observation would be cut off. All the activity of business would be at a stand-still.

How with

plants.

Why so.

Plants would grow, but would never thrive or propagate their species. Their leaves would lose their healthy green colour, and their shoots would be feeble and watery.

It is the stimulating influence of day-light which enables the leaves of plants to absorb the carbonic acid gas of the air; to appropriate the carbon to the use of the plant, and restore the oxygen to the air again.