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From this period printing made a rapid progress in most of the principal towns of Europe, superseded the trade of copying, which, till that time, was very considerable, and was in many places considered as a species of magic.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 Hol. land 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.

25

BROWN.

APPENDIX.

➡9044

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 B C, 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 LD 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

longer than A C, or any other single side of the parallelogram. This is called the composition and resolution of motion. [NOTE. Several things in this Lesson may be obscure to some students: the teacher should explain and illustrate them by familiar "verbal instructions," and by such figures and diagrams as he may have in his possession, or may easily draw upon paper or a slate.]

LESSON 20.

Levers.-First kind, fig. 7. CE is the lever, and B the prop. A the stone to be raised 1000 pounds, and the strength of a man at C = 100 pounds. Since the strength of the man is only one tenth the weight of the stone, that the power and weight may balance each other, the arm of the lever B C must be ten times as long as the arm B E. Second kind, fig. 9. If the hand C be nine times as far from A as the point X, then one pound at C will balance nine pounds at B. Fig. 5. a burden on a pole. Weight W three times nearer to a than to b, a then will bear three times as much of the weight as b. Third kind, fig. 10. Distance PF 3 inches; W F 12:-then 20 pounds at W will require the force of 80 at P in order to balance it, for 12 is four times 3. Fig. 2. man's arm,--D centre of motion,--the power is the muscle inserted at C,-A the weight-now as the distance D C is one tenth part of C A, the muscle, therefore, must exert a power equal to 100 pounds in order to raise 10 pounds.

LESSON 21.

Pulley. Fig. 13. single moveable,-in order to raise the weight W one inch, the power P must draw the strings B and C one inch each: the whole string, therefore, is shortened two inches, while the weight is raised only one. Fig. 15. System of pullies. While the weight W rises one inch, each of the four ropes must be shortened an inch, and P, therefore, must move four inches: 5 pounds at P will balance 20 at W. Wheel and Axle, Fig. 11. If the diameter of the wheel be 4 feet, and that of the axis only 8 inches, then the power P of 100 pounds will balance the weight W of 600 pounds; for 6×8-48 inches which make 4 feet, the diameter of the wheel. Inclined Plane. Fig. 8. If B C

4 AC, then W will be supported by a power of its weight. Example. If a wagon with its load weigh 40 cwt. and may be drawn on level ground by a force equal to 8 cwt., in drawing it to the top of a hill which rises 20 yds. in a 100, the horses will have to pull with an additional force =} of 40 cwt., that is, 8 cwt. more than on level ground, or with double their former force.

LESSON 22.

The

The Wedge, fig. 6.—A BCD may be divided into two inclined planes, A DC and BDC, which may be used separately, and will gain advantage as such; therefore, when united at DC, the advantage gained will be in the same proportion as when they were used in different parts. Screw, fig. 3. A must turn once round before the resistance can be moved from one spiral winding to another, as from x to z= an inch. If the lever A 36 inches, then the circle described by its end a will be about 226 inches or 452 half inches; therefore one pound at a will balance a resistance of 452 pounds. [NOTE. Since the lever 36 inches, the diameter of the circle will be 72 inches, and the circumference of a circle is 3.1416 times the diameter, there. fore, 72×3.1416 the circumference 226 inches or 452 half inches.]

LESSON 23.

Pressure of Fluids.-In the vessel A B, fig. 25. Engr. II. the bottom C B does not sustain a-pressure equal to the quantity of the whole fluid, but only of a column, whose base is C B, and height C F. In the vessel F G, fig. 24. the bottom sustains a pressure equal to what it would if the vessel were as wide at the top as bottom. If to the wide vessel A B, fig. 23. a tube CD be attached, and water poured into either of them, it will stand at the same height in both; of course the small quantity in CD balances the large quantity in AB. This has been called the hydrostatical paradox, because any quantity, however small, may be made to counterpoise any quantity, however large, but it is no paradox, when we consider that the particles of a fluid press against each other in every direction, not only downwards, but upwards and sideways.