had just set with a placid appearance, closing a beautiful afternoon, and the usual mirth of the evening watch was proceeding, when the captain's order came to prepare with all haste for a storm. The barometer had begun to fall with alarming rapidity; and yet the oldest sailors had not perceived even a threatening in the sky, and were surprised at the extent and hurry of the preparations; but the required measures were not completed, when a more dreadful hurricane burst upon them than the most experienced had ever witnessed. Nothing could withstand it; the sails, already furled and closely bound to the yards, were riven away in tatters; even the bare yards and masts were in great part disabled; and at one time the whole rigging had nearly fallen by the board. Such, for a few hours, was the mingled roar of the hurricane above, of the waves around, and of the incessant peals of thunder, that no human voice could be heard, and, amidst the general consternation, even the trumpet sounded in vain. In that awful night, but for that little tube of mercury which had given warning, neither the strength of the noble ship, nor the skill and energy of the commander, would have saved one man to tell the tale."-Dr. Arnott's Elements of Physics.

LESSON L.-DEW.

I. Formation.

SELECT a suitable day, and direct the attention of the children to the dim appearance of the schoolwindows, and draw from them the reason. The

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air filled with vapour; the quantity in proportion to its temperature; when it comes in contact with any cooler substance, it deposits a part of this moisture, thus, the glass being cooler than the air of the room, condenses the vapour that comes in contact with it.

Illustration.-A glass filled with cold water, or a bottle of wine brought from a cold cellar into a hot room, is soon covered with moisture.

During the day, plants, the earth, and other objects absorb heat from the sun-when he sets they radiate, or give off a portion of it, and so cool down; the air in contact is cooled, becomes less solvent, and deposits some of its moisture in the form of dew. Dew does not descend as rain, (as was formerly thought); it is found on the under and side surfaces of plants and objects which nothing rising or falling could reach; forms into globules of extreme beauty- dewdrops clear and sparkling, emblems of beauty and purity. "Resting in luminous beads upon the down of leaves, pendent from the finest blades, or threaded upon the floating lines of gossamer, its orient pearl varies in size from the diameter of a small pea to the most minute atom that can be imagined to exist." When frozen, dew is changed into hoar-frost-the ice of dew:

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"Then fields with glist'ning dew-drops bright
Seem changed to sheets of silver white."

II. Deposition.

The deposition of dew is very irregular; affected by (1) the state of the weather, (2) the nature of the object exposed, and (3) the locality.

1. Cloudless nights after hot days most favour

able to the deposition; the heat, radiated, ascends at once to the higher regions, and the surface of the earth is rapidly cooled. What follows? Little or none on cloudy nights; the radiated heat is reflected back to the earth by the clouds; effectsurface not cooled sufficiently.

Illustration. - Gardeners protect tender plants and trees by means of awnings; these act in the same manner as the clouds, and return the heat.

Winds are generally unfavourable; in Egypt a north wind (from the Mediterranean) brings much; from the south, very little; why? What wind in England is most favourable? why the west? Copiously deposited in Great Britain during spring and autumn. Annual deposition in England equals five inches.

2. Some substances part with their heat less rapidly than others, as water, rocks, stones, and metals-these therefore receive little dew; grass, low plants, &c., cool quickly, and so receive much more. Hence gravel walks, stones, buildings and water receive little, while the surrounding vegetation is covered.

Experiments.-a. Take equal quantities of wool of a given weight, place them respectively on gravel, grass, and glass; weigh them in the morning; the increase of weight will show the amount of moisture deposited on each.

b. Place thermometers upon a garden walk and upon grass-turf; that upon the latter will be found some sixteen degrees lower than the former.

3. Dew is most plentiful in Western Asia and other places where the rains are periodical; to them it is a substitute for rain; of great importance in nourishing vegetation and protecting it from the excessive heate. g., very copious in Palestine :

"Our tents were as wet with dew as if it had rained all night."

III. Uses.

In France it is employed to split blocks of stone into horizontal layers for mill-stones: wooden wedges introduced; these absorb the dew, expand, and split the stones.

Of great utility in affording nourishment to plants and vegetables, which, without it, would. suffer from the drought and heat.

Notice also the beauty of early morning, when—

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Morning shines

Serene, in all her dewy beauty bright;

And hung on every spray, on every blade
Of grass, the myriad dew-drops twinkle round."

IV. Scriptural and Liturgical References.

Frequent allusions to it, on account of its importance, in Holy Scripture; e.g. :—

a. Moses, in blessing Joseph's inheritance, numbers it with "the precious things of heaven.” (Deut. xxxiii. 13.)

b. The Psalmist compares brotherly love to "the dew of Hermon." (Ps. cxxxiii. 3.)

c. David mentions its absence as a curse. (2 Sam. i. 21.)

d. In the Collect for clergy and people, it typifies grace: "The dew of Thy blessing.'

"The dew of heaven is like Thy grace,

It steals in silence down;

But where it lights the favour'd place
By richest fruits is known."

LESSON LI.— GRAVITATION.

I. The Force of Gravity.

ETYMOLOGY-Latin, gravis, "heavy, weighty;' called also the force of gravity—the force by which particles of matter mutually attract each other-the force by which all things are drawn towards the centre of the earth. Universal; all things subject to its influence; acts in all directions; its power in proportion to the mass or quantity of matter contained in the attracting body; some bodies of equal size contain more matter than others-e. g., pieces of lead and cork—to exert an equal attraction, the latter must be about twelve times larger than the former, as it contains about twelve times less matter than a piece of lead of equal dimensions. Distance likewise determines the force of gravity; it is inversely as the squares of the distances between the two bodies If at 2 ft. distance, it = 4 lbs., then at 4 ft. it will be but 1 lb. ; or, if 3 lbs. at 6 ft., then 27 lbs. at 2 ft.

e. g.:

II. Examples and Illustrations.

Draw from the children examples of this force, as seen daily around us, and lead them to others; e. g.:

1. Unsupported substances fall to the ground; the earth exerts the greatest force, and attracts them towards its centre.

2. Small bodies floating upon water are drawn together into heaps or clusters.

3. A plummet suspended over a precipice is drawn towards the rock.

4. Leaden balls suspended by wires-the smaller attracted by the larger when brought near each other.