FROM THE PLEASURES OF MEMORY.

TWILIGHT'S Soft dews steal o'er the village green,
With magic tints to harmonize the scene.
Stilled is the hum that through the hamlet broke,
When round the ruins of their ancient oak
The peasants flocked to hear the minstrel play,
And games and carols closed the busy day.
Her wheel at rest, the matron thrills no more
With treasured tales and legendary lore.
All, all are fled; yet still I linger here!
What secret charms this silent spot endear?

Mark yon old mansion frowning through the trees,
Whose hollow turret woos the whistling breeze.
That casement, arched with ivy's brownest shade,
First to these eyes the light of Heaven conveyed.
The mouldering gateway strews the grass-grown court
Once the calm scene of many a simple sport,
When nature pleased, for life itself was new,
And the heart promised what the fancy drew.
Childhood's loved group revisits every scene,
The tangled wood-walk and the tufted green!
Indulgent memory wakes, and lo, they live!
Clothed with far softer hues than light can give.
Thou first, best friend that Heaven assigns below,
To sooth and sweeten all the cares we know;
Whose glad suggestions still each vain alarm,
What nature fades and life forgets to charm;
Thee would the muse invoke!-To thee belong
The sage's precept, and the poet's song.
When softened views thy magic glass reveals,
When o'er the landscape Time's meek twilight steals!
As when in ocean sinks the orb of day,

Long on the wave reflected lustres play;
Thy tempered gleams of happiness resigned,
Glance on the darkened mirror of the mind.

The school's lone porch, with reverend mosses gray,
Just tells the pensive pilgrims where it lay.
Mute is the bell that rang at peep of dawn,

Quickening my truant feet across the lawn:
Unheard the shout that rent the noontide air,
When the slow dial gave a pause to care.
Up springs, at every step, to claim a tear,
Some little friendship formed and cherished here;
And not the lightest leaf, but trembling teems
With golden visions and romantic dreams.

Down by yon hazel copse, at evening, blazed
The gipsy's fagot-there we stood and gazed;
Gazed on her sunburnt face with silent awe,
Her tattered mantle and her hood of straw;
Her moving lips, her caldron brimming o'er;
The drowsy brood that on her back she bore,
Imps in the barn with mousing owlets bred,
From rifled roost at nightly revel fed;

Whose dark eyes flashed through locks of blackest shade,
When in the breeze the distant watch-dog brayed:
And heroes fled the sybil's muttered call,
Whose elfin prowess scaled the orchard wall.
As o'er my palm the silver piece she drew,

And traced the line of life with searching view,

How throbbed my fluttering pulse with hopes and fears, To learn the colour of my future years!-Samuel Rogers.

CHANGES IN THE ATMOSPHERE.

THERE are several causes which tend constantly to produce changes in the atmosphere. We have already noticed, that the air which we breathe is composed of several different dry gases, that it also contains a great quantity of the vapour of water in an invisible state, besides the vapour which exists in the visible form of clouds and mists; and that currents of wind are always moving some parts of the air over the ocean, and others over large tracts of land, by which they become heated or cooled, and raise greater or less quantities of water

by evaporation. Besides these causes there are others -for instance, the action of electricity, the effects of which upon the air are less known but very great. Thus we might expect from the combined action of all these causes, that the atmosphere should be in a state of constant change.

The real wonder is that, in a fluid so subtile as the air, yielding to every pressure, and expanding or contracting with every alteration of temperature, the changes of the air should be confined within such moderate limits as to be scarcely ever injurious.

The principal changes in the atmosphere are those which affect its heat, its weight, and its moisture.

The changes of heat are those of which we are the most sensible. But our own feelings give us a very imperfect measure of heat and cold. A simple experiment will shew this,-suppose a person puts one of his hands into snow, or into very cold water, and the other hand at the same time, into water as hot as he can bear it; and after suffering them to remain in that state for a few minutes, puts both his hands into water moderately warm, this water will convey a sensation of warmth to the hand which has been plunged into the snow, but will feel cold to the hand which has been in the hot water. As long, then, as we trust merely to our own sensations, we can have but a very uncertain estimate even of the sensible heat and cold of the air, or of any other substance. Much less can we estimate the sensible heat of bodies which part with their heat differently.

If a piece of wood, a piece of marble, and a piece of iron are all placed in a room heated to a temperature much higher than that of the human body, and the hand is then laid upon each, although each of these substances have the same actual temperature, the iron will feel the hottest, the marble not so hot, and the wood still less hot; and the reverse will be the case if each is first exposed to the action of a temperature much colder than that of the human frame. It becomes, then, highly desirable to have some

instrument which shall measure exactly the changes of heat in the atmosphere, or in any other body.

Such an

instrument is called a thermometer, a word which implies heat-measurer.

A

G

THE THERMOMETER.

The principle upon which a thermometer is constructed is very simple. All fluids, when heated, swell out, so as to take up more room; and again shrink when they are cooled. Hence, if we can measure the quantity of expansion or contraction, we can measure the quantity of heat which has been added or taken away, provided that equal additions of heat always cause equal quantities of expansion.

F

Mercury (or quicksilver) is the most convenient fluid for this purpose; since, as far as can be ascertained, it does expand equally for all equal additions of heat, within the limits which it is required to measure. Suppose, then, a certain quantity of mercury to be put into a tube A B, having a very small uniform bore from A to B, and a bulb at the end B. While the end A remains open, let the mercury in the bulb B, be violently heated. The mercury will expand, so as to fill up the whole length of the tube, and drive out any air which is in it. When the mercury has reached A, the end of the tube at A must B be closed by suddenly heating it by means of a blow-pipe. We have now the bulb and the tube filled with heated mercury. But as the mercury is left to cool, it shrinks back into the bulb, leaving a part of the tube towards A quite empty, except, indeed, that a very fine vapour of mercury still remains, the effects of which may not be neglected.

Now suppose the bulb of the thermometer to be plunged into melting ice, and that the mercury sinks to the point F. That point is called the freezing point of water, which gives one natural point from which temperature may be measured. Again, let water be made to boil, when the pressure of the air is in its mean state, or when the barometer (which we shall afterwards describe) stands at a certain height, and suppose the mercury in the tube of the thermometer then to have expanded as far as the point G. This gives us a second natural point for measuring temperature. The space between F and G may be divided into such a number of equal parts, as may be thought convenient. In Fahrenheit's thermometer, which is commonly used in England, the space between the freezing and boiling points of water is divided into 180 equal parts, the freezing point being 32 degrees, and the boiling point 212 degrees. In Réaumur's thermometer, the freezing point is 0, and the boiling point 80; in Celsius's thermometer, which is now most frequently used on the Continent, the freezing point is 0, and the boiling point 100. An easy rule reduces the degrees of one of these scales to either of the others;* but it would be a great convenience if all thermometers were constructed to the same scale. When a thermometer is graduated, or has its scale divided into equal parts, we have an accurate measure of the sensible heat of the atmosphere, or of any other body to which it can be applied; and thus we can know precisely what changes take place in the temperature of the air.

* To convert degrees of Réaumur into those of Fahrenheit, above freezing point, multiply by 24, and add 32; below freezing point, multiply by 24, and subtract from 32; thus,

To convert degrees of Celsius into those of Fahrenheit, multiply by 1, and add 32, if above freezing point; subtract 32, if below freezing point.

S. VI,

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