is the stronger attraction of the particles of water towards the air than towards each other, those that are already dis solved and taken up will be raised still higher, by the attrac tion of the dry air, which lies over them, and thus will dif fuse themselves, rising gradually higher and higher, thereby leaving the lower air not so much saturated, but that it will still dissolve and take up fresh particles of water; which process is greatly promoted by the motion of the wind.

When the vapours are thus raised into the higher and colder parts of the atmosphere, some of them will coalesce into small particles, which, slightly attracting each other, and being intermixed with air, will form clouds; and these clouds will float at different heights, according to the quan tity of vapour borne up, and to the degree of heat in the upper part of the atmosphere. The clouds, therefore, are generally higher in summer than in winter; in the former season they are from one mile to three miles high, and in the latter from a quarter of a mile to a mile.

When the clouds are much increased by a continual addition of vapours, and their particles are driven close together by the force of the winds, they will run into drops heavy enough to fall down in rain. If the clouds are frozen before their particles are gathered into drops, small pieces of them being condensed, and made heavier by the cold, they fall down in flakes of snow. If the particles are formed into drops before they are frozen, they become hailstones. When the air is replete with vapours, and a cold breeze springs up which checks the solution of them in the air, clouds are formed in the lower parts of the atmosphere, and these compose a mist or fog: this usually happens in a cold morning; but the mist is dispersed when the sun has warmed the air, and made it capable of dissolving the watery particles of which the mist is composed.

Southerly winds generally bring rain, because, being commonly warm, and replete with aqueous vapours, they are cooled by passing into a colder climate; and therefore part with some of them, and suffer them to precipitate in rain: northerly winds, on the contrary, being cold, and acquiring heat by coming into a warm climate, take up or dissolve more vapour than they before contained; and therefore are dry and parching, and usually attended with fair weather. GREGORY.

QUESTIONS.-1. What is a cloud? 2. Describe the process by which the watery particles are supposed to become visible clouds. 3. What is the process by which other or fresh particles of water are taken up? 4. Why are clouds generally higher in summer than in winter? 5. When will clouds fall down in rain?-in flakes of snow? 6. When do they become hailstones? 7. How and when is mist or fog produced? 8. Why do southerly winds generally bring rain? 9. Why are north winds usually attended with fair weather.

LESSON 134.

Invention and Progress of Printing.

Glutinous, gluey, viscous, tenacious.

THE art of printing deserves to be considered with attention and respect. From the ingenuity of its contrivance, it has ever excited mechanical curiosity; from its intimate connexion with learning, it has justly claimed historical notice; and from its extensive influence on morality, politics, and religion, it is now become a very important speculation. Coining, and taking impressions in wax, are of great antiquity, and the principle is precisely that of printing. The application of this principle to the multiplication of books, constituted the discovery of the art of printing. The Chinese have for many ages printed with blocks, or whole pages engraved on wood; but the application of single letters or moveable types forms the merit of the European art.

The honour of giving rise to this method has been claimed by the cities of Harlem, Mentz, and Strasburg; and to each of these it may be ascribed in some degree, as printers resident in each made successive improvements in the art.

It is recorded by a reputable author, that Laurens Koster of Harlem, walking in a wood near that city, cut some letters upon the rind of a beech-tree, which for fancy's sake, being impressed upon paper, he printed one or two lines for his grand-children; and this having succeeded, he invented a more glutinous ink, because he found the common ink sunk and spread; and then formed whole pages of wood, with letters cut upon them, and (as nothing is complete in its first invention) the backsides of the pages were pasted together, that they might have the appearance of manuscripts written on both sides of the paper. These beechen letters

he afterwards exchanged for leaden ones, and these again for a mixture of tin and lead, as a less flexible and more solid and durable substance. He died in 1440, and by some his first attempt is supposed to have been made about 1430, but by others as early as 1423.

From this period printing made a rapid progress in most of the principal towns of Europe, superseded the trade of copying, which, till that time, was very considerable, and was in many places considered as a species of magic. In 1490 it reached Constantinople, and was extended by the middle of the following century to Africa and America.

During the period since its invention, what has not the art of printing effected? It has blunted the edge of persecution's sword, laid open to man his own heart, struck the sceptre from the hand of tyranny, and awakened from its slumber a spirit of knowledge, cultivation, liberty. It has gone forth like an angel scattering blessings in its path, solacing the wounded mind, and silently pointing out the triumphs of mortality and the truths of revelation to the gaze of those whom the want of precept or good example had debased, and whom ignorance had made skeptical.

QUESTIONS-1. The application of what principle to the multiplication of books constitutes the discovery of the art of printing? 2. What is said of Harlem, Mentz, and Strasburg? 3. What is related of Laurens Koster? 4. What is said of the progress of printing in the world? 5. Of its effects? [NOTE. The fourth Centennial Anniversary of the Invention of Printing was observed at Harlem in Holland on the 10th and 11th of July, 1823, with great rejoicing and a splendid festival.]

LESSON 135.

Hope.

THERE is no happiness which hope cannot promise,-no difficulty which it cannot surmount,- -no grief which it cannot mitigate. It is the wealth of the indigent, the health of the sick, the freedom of the captive. As soon as we have learned what is agreeable, it delights us with the prospect of attaining it; as soon as we have lost it, it delights us with the prospect of its return. It is our flatterer and com

forter in youth; it is our flatterer and comforter in years which need still more to be flattered and comforted. What it promises, indeed, is different in these different years; but the kindness and irresistible persuasion with which it makes the promise are still the same; and while we laugh, in advanced age, at the easy confidence of our youth in wishes which seem incapable of deceiving us now, we are still, as to other objects of desire, the same credulous, confiding beings, whom it was then so easy to make happy. Nor is it only over terrestrial things that it diffuses its delightful radiance. The power which attends us with consolation, and with more than consolation, through the anxieties and labours of our life, does not desert us at the close of that life which it has blessed or consoled. It is present with us in our last moment. We look to scenes which are opening on us above, and we look to those around us, with an expectation still stronger than the strongest hope, that, in the world which we are about to enter, we shall not have only remembrances of what we loved and revered on earth, but that the friendships from which it is so painful to part, even in parting to Heaven, will be restored to us there, to unite us again in affection more ardent, and in still purer adoration of that Great Being, whose perfections, as far as they were then dimly seen by us, it was our delight to contemplate together on earth, when it was only on earth that we could trace them, but on that earth which seemed holier, and lovelier, and more divine, when thus joined in our thought with the Excellence that made it.

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BROWN.

APPENDIX.

EXPLANATION OF THE ENGRAVINGS.

LESSON 17.

Centre of Gravity. Engraving I.—If the centres of gravity of two bodies, A and B, fig. 12. be connected with the right line A B, then the common centre of gravity, C, will be as much nearer to A than to B, as the ball A is heavier than the ball B. If the ball A weigh 12 pounds, and the ball B only 4 pounds, and the length A B be 20 inches, then, because the ball A is three times heavier than the ball B, the distance AC will be three times less than the distance BC, that is, A C will be 5 inches and B C 15 inches; the point C, therefore, is the common centre of gravity of the two bodies A and B, and if supported by this point they will balance each other. As 12+4=16 is to 20, so is 4 to 5, or so is 12 to 15.

The inclining body A B C D, fig. 4. whose centre of gravity is E, stands firmly, because the line of direction EF falls within the base. But if the body A B G H be placed upon it, the centre of gravity will be raised to L, and then the line of direction L D will fall out of the base towards I: the centre of gravity, therefore, is not supported, and the whole body must fall.

LESSON 19.

Compound Motion.-The body A, fig. 1. acted upon by a force in the direction A B, and at the same time by another force in the direction A C, will move in the direction A D. If the lines A B and A C be made in proportion to the forces, and CD and DB be drawn parallel to them, then A D, the diagonal, will represent the force with which the body will move; and this force will be as much greater than either of the two forces by which it was impelled as AD is