5. Two cannon-balls hung near together by long ropes exert mutual attraction, both ropes being drawn out of the perpendicular.

6. The sun attracts the earth the earth, the moon-the moon and sun, the waters of the earth, producing the tides; accounts for the revolution of the earth, planets and their satellites; for "Attraction is the hidden force

On which Creation's law depends."

7. It is gravity which gives weight to bodies. Discovered by Sir Isaac Newton by the falling of an apple to the ground in his garden at Woolsthorpe, in 1666.

III. Apparent Contradictions.

Notice that some substances appear not to gravitate; e. g.:

a. Smoke, steam, and balloons rise in a direction opposite to the force of gravity. They are forced upward by the pressure of the air, being lighter, volume for volume,-as cork, wood, or oil rise through water, and stones, lead, or iron through quicksilver.

b. Birds rise in the air; they are animate; supported by the action of their wings; descend like stones when shot, their specific gravity being greater than that of air.

Note. "The intensity of gravitation varies from local causes as well as from the form of the earth; it is feeble at Bordeaux, and increases to Clermont, Ferrand, Milan, and Padua,-this increase being probably caused by dense masses underground."

IV. Specific Gravity.

Specific gravity: this is the proportion of the weight of a body to the weight of an equal portion of another substance of a chosen standard, which is distilled water at the temperature of 60° of Fah

renheite. g., if the specific gravity of a mineral equal three, the weight of any volume of it equals three times that of the same volume of distilled water. In comparing specific gravities, equal bulks must be taken.

To find the specific gravity, "weigh the body in air, in the usual way, and then in water, in the latter case it will be lighter; subtract the weight of the body in water from the weight of it in air, the remainder will give the weight of an equal volume of water; state the following proportion to complete the process,-As the weight of the water is to the weight of the solid, so is 1000 to the number expressing the specific gravity of the solide. g., if a body weigh in the air 960 grains, and in water 650; then, 960-650 = 310 grains; then,

As 310 960 :: 1000: 3097 (nearly), the specific gravity of the solid." The specific gravity of liquids is readily discovered by the hydrometer.

V. Lesson.

A consideration of this wonderful, invisible force leads us to admire the power of Almighty God, "Who appointed the sun his place, launched the planets in the depths, obedient to the law which has preserved the family compact originally established unbroken through the long series of ages:

"Who plann'd, and built, and still upholds, a world
All clothed with beauty for rebellious man."

LESSON LII. — ICE.

I. Formation of Ice.

If possible place before the children a lump of ice, and elicit how it is produced. Heat abstracted

from the water till its temperature is reduced to thirty-two degrees, the freezing-point of water; it then becomes solid. formed into hail, snow, or

ice:

"A crystal pavement, by the breath of Heaven
Cemented firm."

A bottle filled with water, and securely corked,
is shattered while its contents are freezing; the
effect of expansion; nine cubic inches of water be-
come ten of ice; why? the fluid particles of water
fit closer than the solid crystals of ice; hence:—
a. On melting a block of ice, a much smaller
bulk of water is left.

b. Glass and earthen vessels are often broken in frosty weather by the freezing of their contents. c. Tiles, slates, bricks, rocks, &c., are occasionally cracked by the frost; their moisture frozen, expanded, and the solid split.

d. Water-pipes frequently burst from the same cause. Notice the effect of frost upon rocks, reducing them to sand.

II. Waters affected by Frost.

Show that all waters are not equally affected by frost. Shallow water freezes sooner than deep; why? the whole body must be reduced to the temperature of the freezing-point before it begins to congeal; a deep bed takes longer to cool than a shallow one. The surface first covered with ice, which floats and thickens as the cold continues,the heat of the water passing off through its pores; hence, a frost is warmer than a thaw. Notice that some waters rarely freeze; e. g. :

1. Sea-water. The mass being so great, requires a long time to cool; tides also prevent it, but chiefly its saline properties, for salt freezes only at a much

lower temperature. Common sea-water congeals at about twenty-seven degrees.

2. Running water. The motion of the water in rivers and brooks retards the freezing process; yet occasionally their surfaces are covered with a rough, uneven coating of ice.

3. Lakes.-Generally of great depth; frequently supplied at the bottom by springs; these supplies prevent the necessary reduction of the tempe

rature.

III.* The Frozen Regions.

In high latitudes, in the vicinity of the Poles, ice is perpetual; the Arctic and Antarctic called the Frozen Oceans; these parts of the earth the Frigid (from frigere, "to be cold") Zones. Here, "the surface of the ocean is converted into one solid mass every tree fringed-every stream stopped; the severity of the cold appears in icicles pendent from the eyelashes of every Russian boor and the conversion of his beard into a lump of ice, from the congelation of the vapour of his breath." Thus are formed (a) icebergs, (b) icefields or floes, and, at great altitudes, (c) glaciers.

a. Icebergs. Masses of ice of immense size; sometimes two miles in circumference and two hundred feet high :

"Icebergs vast,

With heads all crown'd with snow,

Whose green roots sleep in the awful deep,
Two hundred fathoms low!"

They are broken from the glaciers of these regions, and carried by the polar currents into the open water of the Atlantic and Southern Oceans; most common

*It may be found convenient to divide these notes into two parts, taking sub-divisions III. and IV. for a separate lesson.

in the latter; why? the ice of the South less hemmed in by land than that of the North Frigid Zone; the climate of the southern hemisphere is said to be affected by the cold they give off on entering a warmer atmosphere. Gradually dissolved. Sometimes reach as low as the Azores and the Cape of Good Hope, Beautiful in appearance; the white part as brilliant as silver, the colours of the other portions as varied and splendid as those of the rainbow. Shapes very irregular; resemble palaces, cathedral towers and pinnacles, castles, obelisks, pyramids, &c. A number seen together appear like a mountainous country. Extremely dangerous to ships.

"Far as our wandering eyes could reach

Uprose their summits clear;

Like cities on a distant shore
We saw them floating near.
Cathedrals, pinnacles and towers,
And palaces of cold :
Rose-tinted, amber, opal, blue,
Alight with living gold."

b. Icefields and floes. Large sheets of ice detached and borne by the same currents into the temperate regions; sometimes a hundred square miles in extent. Upon one Captain Parry was carried southward faster than he could travel northward. The polar bear and arctic fox are often found upon them. They frequently destroy whalers and merchantmen:

"On the frozen deep's repose

"Tis a dark and dreadful hour

When round the ship the icefields close
To chain her with their power."

c. Glaciers.-Bodies of ice formed in the valleys of mountainous districts. Not confined to the frozen regions; very common in the Alps; occur in the