air is within about three miles from the earth. Above this height, the air is unable to support any considerable quantities of vapor. Hence we see the reason why clouds rise no higher, and why the thinnest and lightest rise highest.

To an attentive observer the clouds present many interesting subjects of contemplation. Their ever-varying forms, their beautiful and richly variegated colors, and their silent motion, varying often in velocity and direction, while they furnish the poet with a field in which his fancy may rove delighted, also afford to the student of nature many an interesting theme for reflection. At one time, dark and portentous fancy might easily imagine them the ruins of some ancient castle or time-worn tower; at another, they gather in beautiful and glorious forms around the path of the descending sun, and seem to vie with that luminary itself in splendor. Sometimes they move swiftly over the face of heaven, and soon recede from our view; sometimes they seem to meet each other, and soon, like hasty travellers, pass each other by, without a sign of recognition. At one time, while we gaze upon them, they vanish; at another, they gather into darker and heavier masses of settled gloom. Now they collect, now they disperse, and now they change form and color with surprising rapidity. To the inquiring mind the question naturally occurs, What is the cause of all these varied appearances? The inquiry leads to careful observation; and though, in many instances, that cause defies our search, yet, in many others, we are enabled to arrive at general principles and uniform relations, which enable us to anticipate the storm, and predict the time of its termination.

The principal circumstances which influence the form of clouds are the motion of the air, and the formation and condensation of vapor. Substances so light as clouds readily change form, when subjected to greater atmospheric pressure on one side than on the other. Different portions of the

air move with different degrees of velocity. Hence clouds situated in these portions of air divide, collect and change form, according to the force acting upon them. Waterspouts are usually attended by a thick, black cloud, formed, probably, by the vapor condensed by opposite currents of air meeting. New accessions of vapor often change the form of clouds; also the dissolving of vapor, or a diminution of its density. Sometimes, probably, a cloud meets with a stratum of air sufficiently warm to dissolve it. In this case, it will vanish by degrees. Different parts of a cloud may be in strata of air of different warmth or density. The cloud will then partly dissolve, and the part dissolved will, perhaps, rise and become visible in a higher portion of the air, where the heat is not sufficient to render it invisible. In the spring, it is often cloudy in the morning, and clear towards noon. The heat of the sun dissolves the moisture which arose in great quantities from the damp earth in the morning.

Clouds near the horizon generally appear to extend much farther horizontally than perpendicularly. This is, probably, in part, an illusion. If the lower surface of a cloud is nearly parallel to the surface of the earth, its extent towards the zenith will appear much less than it really is, while this will not be the case with its extent in the direction of the horizon. As it approaches the zenith, it will appear more nearly of its true dimensions.

Hence clouds

in the zenith seldom or never appear to be of this form. Clouds often move in opposite directions. Different portions of air often move in different directions above one another, on account of their being unequally rarefied by heat. They, of course, carry the clouds with them. This may be readily illustrated. If, in cold weather, the door of a warm room be opened a little, and a candle held near the bottom of the opening and another near the top, the flame will often be blown in opposite directions. The cold air rushes in at the bottom, and the warm air, being lighter, goes out at the top.

The color of clouds depends on the rays of light which

*

they reflect. Dark clouds often precede wind. But, although they are seen before the wind is felt, they are not the cause, but the effect, of the wind. As the wind moves on, it presses upon that portion of the air which has a velocity less than its own, and by this pressure, and perhaps, also, by its greater coldness, condenses the vapor contained in it, and thus forms a cloud. This cloud, being so dense that little or no light can pass through it, appears black. And the degree of darkness depends on the density of the vapor, or, in other words, on the velocity of the wind, and the quantity of water in the portion of air compressed. The beautiful colors that often adorn the sky at sunset, are caused by the clouds reflecting the sun's light. That redness of the sky in the morning, which is often regarded as the precursor of a storm, probably results from the red rays of the sun passing through the vapor collected in the air. Light is composed of seven differentcolored rays, possessing different degrees of force. These may be seen separate from each other in the rainbow. Of these the red rays have the greatest force or momentum. Hence, when the air is very full of vapor, the red rays have sufficient power to penetrate it, while the others have not. Many of the red rays, however, do not come directly from the sun, but are scattered in various directions on striking the vapor, and thus the redness is diffused over a considerable space.

Thunder clouds exhibit an appearance peculiarly striking. To many they are objects of terror; in a greater or less degree they arrest the attention of almost every one. These clouds are collections of vapor strongly electrified. They are generally very dense, and very near the earth. Frequently two clouds rise in different parts of the horizon, and move towards each other till they meet, at the same time rising up towards the zenith. When clouds in different electrical states approach near each other, or when a strongly electrified cloud approaches near to the earth, the electricity is discharged in vast quantities and with tremendous violence, thus constituting what is termed

lightning; while the concussion given to the surrounding air by its force, and the rushing together of the portions of air separated by its motion, causes thunder. This sound, reflected and reverberated among the clouds, produces the long-continued and solemn roll, which forms one of the sublimest characteristics of a thunder-storm.

It is often imagined that lightning always moves towards the earth. But there is reason to suppose that discharges are sometimes made from the earth to the clouds, as well as from the clouds to the earth. It is not difficult to measure the distance of thunder-clouds from the earth. Sound moves at the rate of eleven hundred and forty-two feet in a second; light at the rate of about two hundred thousand miles in a second. The time in which light traverses so small a space as that between a thunder-cloud and any place from which the thunder can be heard, is so short that it need not be estimated. If, then, we multiply the number of seconds between the flash and the thunder by eleven hundred and forty-two, we have the distance of the cloud in feet. Hence, when a very short time elapses between the flash and the thunder, the discharge is very near.-There is a peculiar sublimity attend-. ing thunder-storms in mountainous regions. The traveller among the Andes frequently hears the thunder roll, and sees the lightning flash from the clouds that gather around the hills far beneath him, while around his path, and on the heights above him, the sun is shining with unclouded splendor.

LESSON CV.

Character of Professor Playfair.-FRANCIS JEFFREY.

THE same admirable taste which is conspicuous in his writings, or rather the higher principles from which that taste was but an emanation, spread a charm over his

whole life and conversation, and gave to the most learned philosopher of his day the manners and deportment of the most perfect gentleman. Nor was this in him the result merely of good sense and good temper, assisted by an early familiarity with good company, and a consequent knowledge of his own place and that of all around him. His good breeding was of a higher descent, and his powers of pleasing rested on something better than mere companionable qualities. With the greatest kindness and generosity of nature, he united the most manly firmness and the highest principles of honor,-and the most cheerful and social dispositions with the gentlest and steadiest affections. Such was the fascination of the perfect simplicity and mildness of his manners, that the same tone and deportment seemed equally appropriate in all societies, and enabled him to delight the young and the gay with the same sort of conversation which instructed the learned and the grave.

There never, indeed, was a man of learning and talent who appeared in society so perfectly free from all sorts of pretension or notion of his own importance, or so little solicitous to distinguish himself, or so sincerely willing to give place to every one else. Even upon subjects which he had thoroughly studied, he was never in the least impatient to speak, and spoke at all times without any tone of authority; while, so far from wishing to set off what he had to say by any brilliancy or emphasis of expression, it seemed generally as if he had studied to disguise the weight and originality of his thoughts under the plainest form of speech, and the most quiet and indifferent manner; so that the profoundest remarks and subtlest observations were often dropped, not only without any solicitude that their value should be observed, but without any apparent consciousness that they possessed any.

Though the most social of human beings, and the most disposed to encourage and sympathize with the gaiety and joviality of others, his own spirits were in general rather