MECHANICS.-XVI. DYNAMICS. DEFINITIONS-THE THREE LAWS OF MOTION. We have now to pass on to the second and more difficult part of Mechanics; but as we have already acquired a knowledge of some of the fundamental principles of the science, the difficulty will not be great, and by a little thought and application will easily be overcome. Hitherto we have had to deal with forces which acted on a body and produced equilibrium. If any of these forces be now altered or modified in any way, so that one or more remain unbalanced, some motion will take place, and the nature of this motion will, of course, depend upon the forces. It is the object of dynamics to inquire what these motions will be, and what are the laws that govern them; and though at first they may appear comparatively unimportant, we shall find as we advance that an acquaintance with them is of great practical use for many purposes. The investigation of the action of the earth's attraction, of the motion of bodies projected with any given velocity, and of many other common things, depends on the principles of dynamics, and the laws we discover by examining these are found to apply on an infinitely more grand and glorious scale in nature, for by their action all the stars and planets are kept in their orbits and made to perform their varied revolutions. By these laws astronomers can not only explain and account for their varying distances and motions, but can foretell with the utmost accuracy eclipses and other phenomena of the heavenly bodies. Calculations like these require, indeed, a far deeper acquaintance with the higher branches of mathematics than we can acquire from these lessons; but still the principles we shall investigate are those on which all such calculations are based, and the subject will, we hope, be pursued by many far beyond the point to which we can advance here. There is one important difference between statics and dynamics, and that is, that the latter is one of the inductive sciences, though perhaps the simplest of them. Some sciences, like arithmetic and geometry, are called deductive, their principles being deducible from abstract truths without reference to experiment, though that is sometimes resorted to as a corroborative evidence or a simpler mode of proving their truth. To this class statics belongs, for all its fundamental truths can be mathematically proved. Not so with dynamics, many of the truths of which can only be ascertained by experiment, and in order to ensure accuracy in these experiments they must be repeated again and again, for slight errors are likely to creep in, and it is only by taking the average of many different experiments that we can arrive at accurate results. Many, however, of its principles can be ascertained by deduction, and it thus approaches much more nearly to the deductive sciences than the other branches of natural philosophy, to which we shall turn our attention shortly. As previously stated, we have in dynamics to introduce a fresh idea, that of time. In statics force was considered only as producing pressure, and therefore this element did not enter into our calculations; but it is clear that, in treating of motion, the time occupied is an important thing to consider. It is needful at starting that we should have some mode of measuring the degree and intensity of motion, that is, the velocity of any body, and, as we saw, two quantities are needed to determine this-the space passed over, and the time occupied in passing over it. We may know that a force applied to a body causes it to move over a certain space, but to form a correct idea of the force, we must also know how long it takes to travel this distance. When we speak of a speed of 12 miles an hour, we mean that if the motion continued uniform through that space of time the body would have travelled 12 miles. It does not, however, imply that the body actually passes over 12 miles, but merely that it moves with that degree of speed. Great inconvenience often results from thus requiring two numbers to represent a velocity, and hence it is usual to express it by the number of feet passed over in one second. If a body moves a mile in 8 minutes, it passes over a furlong, or 660 feet, in one minute, and therefore over 11 feet in one second, and it is said to have a velocity of 11. When, therefore, we represent a velocity by a number, it is always to be understood as the number of feet passed over by the body in one second. The motion of any body may be either uniform or variable. It is uniform when equal spaces are always passed over in equal times, and its velocity is then measured by the number of feet actually passed over in a second. When this number is not constant, the motion is variable, and the velocity at any point of time is measured by the space it would pass over in one second if it continued during the whole second to move at the same rate as at the given moment. A variable motion may be either accelerated or retarded, and if the gain or loss of velocity in equal times be equal, it is said to be a uniformly accelerated or retarded motion. A railway train when first started affords an illustration of accelerated motion. The power of the engine is more than sufficient to overcome friction and the resistance of the air, and therefore the speed increases; but the resistance increases in a greater ratio, till, after a time, it exactly equals the power of the engine, and then equilibrium ensues, and the train continues in a state of uniform motion. The actual measurement of the space passed over in a given time is often a difficult thing, especially as there are always counteracting forces which impede the motion in a greater or less degree. There are, however, various ways in which this may be accomplished, some of which we shall see as we proceed. Now there are two modes in which we may regard force; one is, by considering merely the velocity imparted without any reference to the quantity of matter moved; force considered thus is called accelerating force. The other mode is by taking into account the quantity of matter moved as well as the velocity, and this is called moving force. These are not two different kinds of force, but merely two ways of regarding the same force. It is clear that a different amount of force is required to impart the same speed to two bodies of different weights. The impulse that would impart a very great velocity to a pistol-bullet may scarcely be able to move a large cannon. ball. The quantity of matter or mass of a body is thus an important element in measuring the force required to produce motion in it. Now we cannot determine exactly what the mass of a body is, as we do not know the ultimate particles of which it consists; but we can always measure it by the weight of the body, for gravity may be considered to act equally on all particles, and therefore two substances on which it acts equally-that is, which have the same weight-may be considered to contain the same quantity of matter. Hence, when we want to find the quantity of motion or momentum of any body-that is, the force which would be required to gene rate in it a motion equal to its own, or which it would exert against any obstacle which obstructed it-we have to multiply its velocity by its weight. This is usually given as a definition: The momentum of any body is its mass multiplied by its velocity. If, for example, a body weighing 100 lbs. be moving with a velocity of 15 feet per second, its momentum is 1,500. After thus much by way of definition, we pass on to the laws of motion; but we shall have to return to momentum. The most important principles of motion were drawn up by Newton in the shape of three general laws. These have since been altered in their form, but assert nearly the same facts The first teaches that every body will continue in its state of rest or uniform motion in a straight line unless acted upon by some external force or forces. This law merely asserts the inertia of matter, that is, its inability of itself to alter or modify in any way any motion which has been imparted to it We can easily understand that a body at rest will remain so unless some force be applied to it, as we see constant illustra tions of the fact. It is, indeed, one of the earliest truths which we acquire from observation, but the other part of the law seems more at variance with experience. In fact, almost every motion we observe seems at first sight to point out the inaccuracy of the law; but it is only at first sight, and a little examination will show its truth. Let a stone be rolled along the ground with great speed, it comes to rest in a very short time; so, too, a boat when rapidly rowed along soon stops if the man ceases to ply the oars. The true reason, however, why in these and similar instances the motion ceases, is, that other forces neutralise that which has been acquired. In the first case, these forces are friction along the ground and the resistance of the air; in the second, the resistance of the water, for the boat as it advances must displace some of the water, and all the momentum it had acquired is thus soon dispelled. If all such counteracting causes could be removed, the body would move on for ever. This cannot, of course, be proved directly by experiment, but we can easily assure ourselves of its truth, for, in proportion as we remove these obstructions, the motion continues for a longer period. If, instead of rolling the stone along the ground, we send it on smooth pavement, the motion will continue to a much greater distance; and if we try the experiment on a good surface of ice, it will move farther still, the simple reason being that the force of friction which before overcame its motion has been greatly removed. In a similar way we can carefully construct a pendulum so as to swing with as little friction as possible, and having started it from a given point in the arc, note how long it takes to settle to rest. Now remove it to the receiver of an air-pump, exhaust the air, and set it vibrating as before, it will be found that the motion will continue for a much greater length of time, the resistance of the air being in a great degree removed. From experiments like these we can ascertain the truth of the law, and it is important to bear it in mind, since the neglect of it has often led to great mistakes. Force, then, is not required to maintain motion, but only to produce or alter it, either by increasing or diminishing its speed, or by changing its direction. We now turn to the second law of motion, which may be stated as follows:-When any number of forces act on a particle, each produces its full effect in producing or altering motion, exactly as it would if it acted singly on the body when at rest. Of this we have many simple proofs. Let a stone be dropped from the mast-head of a ship, it will fall exactly at the foot of the mast, just as if the vessel were perfectly at rest. B C We now pass on to the third law of motion, which was stated by Newton as follows:-Reaction is always equal and contrary to action, or the mutual actions of two bodies upon each other are always equal and in opposite directions. When a carriage is drawn by horses, they are pulled back with the same force as the carriage is drawn forward; so, if a boat in a stream be pushed off from another, the quantity of motion produced in each is the same. If both be of the same weight they will move with the same velocity; but if one be heavier, its motion will be so much less than that of the other. We see, thus, that motion is never lost, it always produces motion in other things; but as this is shared among all bodies in proportion to their mass, it soon becomes so small as to be unnoticed. Now if we consider the pressure on a body to be the action, the quantity of motion produced is the reaction, and this law asserts that these are equal. But the quantity of motion is measured by the product of the mass and the velocity, that is, by the momentum generated. The momentum produced is therefore proportional to the pressure. Hence the law is frequently stated thus:-When pressure produces motion in a body, the momentum generated is proportional to the pressure. Momentum, then, is the measure of moving force, as velocity is of accelerating force. From this we find a way of comparing these two. The latter is measured by the velocity, irrespective of the mass, and as the pressure, which is the moving force, imparts the velocity to the body, it is equal to the mass multiplied by this velocity. That ismoving force mass accelerating force. Hence, if we divide the moving force by the mass, we obtain the accelerating force. = From this we can calculate the dynamical unit of force, that is, the force required to cause the unit of mass to move one foot per second, which force we stated in our second lesson to be 7.85 grains. The unit of mass is one cubic inch of distilled water, and this weighs nearly 253 grains. Now the accelerating force of gravity which produces this weight is 32.2, that being, as we shall shortly see, the velocity a falling body acquires in one second. But the velocity we want is only 1 or 32 of this. Hence the unit of moving force is 253 gr. X or 7.85 gr. 1 32-2 1 The proof of the third law we must defer to the next lesson. ANSWERS TO EXAMPLES IN LESSON XV. 2240 x 20 x 400 3. It would raise it from a depth of 663 feet. If gravity alone acted upon it, it would reach the deck some distance in the rear of the mast, for in the interval which it has occupied in falling, the vessel has been moving onwards, and the point from which the stone fell is, when the stone reaches the deck, vertically over a place some distance behind the mast; but another force was also acting on the stone, and that was the onward motion which, like the vessel, it had acquired. This motion was exactly equal to that of the vessel, as both were moving at the same rate; and each of these forces produces its full effect. The stone falls in exactly the same time as it would take if the vessel were at rest; it moves through the same horizontal space that it would if it were not falling; and at the end of the time occupied in falling is in the same place as if each force had acted singly during that length of time, the only difference being that then it would have passed over two sides of a parallelogram, whereas now it has travelled down the diagonal. Another good illustration of this is afforded by a boat crossing a river when the stream is running down rapidly. Suppose the stream to be flowing in the direction of the arrow. A boatman at A wants to cross to a point в some distance lower down; he does not, however, steer directly for it, since, if he did, the force of the stream would carry him to some point much lower down, but he makes for a point almost opposite him. If the current be so rapid that it would carry him THE THIRD DECLENSION (continued). down from C to B in the time it takes him to row from A to C, he THE adjectives which follow the nouns of the Third Declension must steer directly across to c. There will be then two forces given in the last lesson, arе-1, 8, ǹ ажαтæр, тo anаTop, fatheracting on the boat-his own force impelling it from A to c, and less, aunTwp, aunтop, motherless, the genitive ends in opos: 2, the force of the stream from c to B, and under the joint actions, appny, To apper, manly; gen. appevos: 3, adjectives in wv (m. of these two forces it will move from A to B in the same time that it would take him to row to c. If, now, he wants to cross again to D he must steer for some point higher up than A, for as BA is longer than A c, the tide will have more time to act upon the boat and carry it down. More commonly, however, he rows from B towards c along the shore, where the current has less force, and then crosses as at first. But it is clear that in either case each force produces its full effect. D Fig. 95. In our lessons on statics we learnt the parallelogram of forces, and found that if two forces acting on a body be represented by two adjacent sides of a parallelogram, the resultant will be represented by the diagonal. We may now extend this principle to velocities, thus: If any two velocities impressed on a particle be represented by two sides of a parallelogram, the diagonal will represent the resulting motion in direction and velocity. 5. About 5 days by means of a windlass, or 3} days by ascending a ladder and allowing his own weight to raise it. LESSONS IN GREEK.-VIII. and f.) and ov (n.), as 8 ǹ evdar, to evdarov, happy; and the comparatives in ων, ον, ίων, τον. These comparatives, after dropping the v, suffer contraction in the accusative singular, and in the nominative, accusative, and vocative plural. The vocative is the same as the nominative neuter. Happy.' Singular. то δ, ἡ δ, ἡ Greater. ΤΟ μείζων, μείζον. μείζονος. μείζονι. μείζονα, μείζον. (μείζω). μείζον. ῥητορ-οιν. Ν.Α.Τ. ποιμεν-ε. δαιμον-ε. λεοντ-ε. αιθερ-ε. ρητορ-ε. G.D. ποιμεν-οιν. δαιμον-οιν. λεοντ-οιν. αιθερ-οιν. Δαηρ, a husband's brother, makes in the vocative δαερ; Αμφίων (ovos) makes ω Αμφίον ; also Αγαμεμνων (ovos) vocative Αγαμεμνον. The following in ων (ovos) in some cases drop the r and undergo contraction: ἡ αηδων, the nightingale, genitive αηδονος, contracted into αηδούς, dative αηδοῖ; ἡ χελίδων, swallow, genitive χελίδονος, dative χελιδοῖ. VOCABULARY. Αγέλη, ης, ή, a flock, | Εικω (dat.), yield; της όδου, get out of the way of. Ηγεμων, ονος, δ, a herd. Αδικος, -ον, unjust (α priv., and δικη, justice). Ανευ (genitive), without. Γερων, -οντος, δ, an old man. Δημος, -ου, δ, the people (Lat., populus). Όλβιος, -α, -ον, happy. Σωφρων, or, gen. ovos, sound-minded. Υπερφρων, ὑπερφρον, genitive-ovos, highminded, too high. minded, proud (ύπερ, over). Φρην,-evos (pl.φρενες), the heart, soul. Φυλαττω, I watch, guard, keep. Ναιω,I inhabit,dwell. 1. Τον γεροντα θεραπευε. 2. Σεβου τους δαίμονας. 3. Οἱ ποιμένες αγέλας φυλαττουσιν. 4. Τον κακον φευγε ὡς κακον λιμενα. 5. Ανευ δαίμονος δ ανθρωπος ουκ ολβιος εστιν. 6. O θεος εν αιθερι καιει. 7. Πολλακις χαλεπαι μεριμναι τείρουσι τας των ανθρώπων φρενας. 8. Επου, ω φιλε, αγαθοις ἡγεμόσιν. 9. Εικε, ω νεανια, τοις γερουσι της όδου. 10. Πολλακις δημος ἡγεμονα έχει αδικὸν νοῦν, 11. Ο θεος κολαστής εστι των αγαν ὑπερφρονων. 12. Εχε νοῦν σωφρονα. 13. Ω δαιμον, παρεχε τοις γερουσι καλην ευτυχίαν. 14. Οἱ θηρευται τους λέοντας ενεδρεύουσιν. EXERCISE 22.-ENGLISH-GREEK. 1. Good boys honour old men. 2. Old men are honoured by good boys. 3. Sound-minded young men get out of the way of old men. 4. Follow, Ο friends, a good leader. 5. We have good leaders. 6. The people often follow bad leaders. 7. God affords prosperity to the sound-minded. 8. Lions are hunted by huntsmen. 9. We worship the divinity. To the previous examples belong the following substantives in ηρ—namely, ὁ πατηρ, the father; ἡ μητηρ, the mother; ἡ θυγάτηρ, the daughter; ή γαστήρ, the belly ; ἡ Δημητηρ, Demeter (Ceres in Latin); and δ ανηρ, the man; differing, however, from them in the omission of e in the genitive and dative singular The word αστήρ, -ερος, a star, which otherwise retains the e of the stem, belongs to this class in consequence of having its dative plural in αστρασι. VOCABULARY. Εχθαίρω, I hate. Περσεφόνη, ης, ή, Persephone (per-sefo-ne), Proserpine. Σοφος, -η, -ον, wise. Στεργω, I love. Χαίρω (dative), I rejoice at, delight in. Χαριζομαι, I show favour, gratify. EXERCISE 23.--GREEK-ENGLISH. 11. Η Δημητρος 1. Στέργετε τον πατέρα και την μητέρα. 2. Μη δουλευε τη γαστρι. 3. Χαίρε, ω φιλε νεανια, τῳ αγαθῳ πατρι και τη αγαθή μητρι. 4. Μη συν κακῳ ανδρι βουλευον. 5. Δημητρι πολλοί και καλοι νεῳ ησαν. 6. Η αγαθη θυγατηρ ἡδέως πείθεται τη φίλη μητρι. 7. Οἱ αγαθοι ανδρες θαυμαζονται. 8. Πολλακις εξ αγαθόν πατρος γιγνεται κακος υἱος. 9. Εκθαίρω τον κακον ανδρα. 10. τοις αγαθοις ανδρασι λαμπρα δοξα έπεται. θυγατηρ ην Περσεφόνη. 12. Ω φιλη θυγατερ, στεργε την μητέρα. 13. Η αρετη καλον αθλον εστιν ανδρι σοφῷ. 14. Οἱ αγαθοι νέοι τους πατέρας και τας μητέρας στεργουσιν. 15. Οι Έλληνες Δημητερα σεβονται. 16. Πείθεσθε, ω φιλοι νεανίαι, τοις πατρασι και ταις μητρασιν. 17. Χαρίζου, ω φιλε πατερ, τη αγαθή θυγατρι EXERCISE 24.-ENGLISH-GREEK. 1. Ο young men, love your father and mother. 2. Good daughters obey their (the) father and mother. 3. The citizens worship Ceres. 4. Persephoné follows Ceres. 5. We admire the star. 6. Be not ye, O huntsmen, slaves to the belly. 7. A good mother loves a good daughter. 8. O mother and father, love your children. 9. The man is hated. 10. They hate the man. 11. They obey wise men. 12. I follow Ceres. 13. Often bad sons arise from a good father and mother. Note that the Greek article has frequently the force of an English possessive pronoun, when, from the nature of the sentence, no mistake as to the meaning can arise. Consequently, in such cases, when you translate into English, give the possessive pronoun for the Greek article, and when you translate into Greek, give the article for the possessive pronoun. KEY TO EXERCISES IN LESSONS IN GREEK.-VII. 1. Temples are built to the gods. 2. It is not easy to walk on ropes. 3. We hunt hares. 4. Androgeus was the son of Minos. 5. Hares are hunted by huntsmen. 6. Pray to the merciful God. 7. 8. Reverence the merciful divinities. 9. The Eagles capture hares. brave receive deathless praise. 10. Pray that you may have (hind) God merciful. lead away most people as captive. 13. The Samians support beautiful 11. The gods are propitious to the good. 12. Pleasures peacocks in honour of Juno. 14. The peacock has beautiful wings. EXERCISE 16.-ENGLISH-GREEK. 1. Τοις θεοις νεως κτίζεις. 2. Κτίζονται νεφ τοις θεοις. 3. Νέων του Θε κτίζω. 4. Επι καλῶν βαινουσι. 5. Τους λαγως θηρευομεν. 6. Οἱ λαγῳ θηρεύονται, 7. Οι Σάμιοι καλους τα ως σέβονται. 8. Τον ίλεων Θεον σέβονται. 9. Ο Θεός Έλεως εστι τοις αγαθοίς. 10. Οἱ θηρευται θηρεύουσι τους λαγως. 11. Ο Μενέλεως λαμβάνει αγήρων επαινον. EXERCISE 17.-GREEK-ENGLISH. On the western side of the Pacific Ocean are the seas of Japan and Okhotsk, and the Yellow Sea and China Sea; and on the eastern side are the inlets called the Gulf of California and Queen Charlotte's Sound. 1. Peacocks were sacred to Hera (Juno). 2. We admire Menelaus for his valour. 3. The poets call the morning rosy-fingered. 4. Truth (ʼn andel) often does not satisfy the people. 5. Helen was the wife of Menelaus. 6. Babylon produces many peacocks. 7. In the temples of the gods are many pillars. 8. Hares are timid animals. 9. The voyage round (Mount) Athos was dangerous. 10. The palace has fine by the equator, the one being called the North Atlantic Ocean, chambers. EXERCISE 18.-ENGLISH-GREEK. 1. Μενέλεως θαυμάζεται επί τη αρετη. 2. Θαυμάζομεν την ροδοδάκτυλον 'Ew. 3. Πολλοι τα εν Βαβυλωνία τεκτονται. 4. Εν τῷ της Ήρας νεφ εστι καλος ταως. 5. Οἱ θηρευται τα ως ενεδρεύουσι. 6. Oi Taw eveдpevovтаι UTO των θηρευτών. 7. Οι αγαθοι πολίται τον ανοητον λέων φευγουσι. EXERCISE 19.-GREEK-ENGLISH. 1. Avoid wild beasts. 2. A hand washes a hand. 3. Keep from the wasp. 4. The meadows bloom. 5. The soldiers sing their war song. 6. We know (try) gold and silver in (by) fire. 7. Many become friends at the goblet (over their cups), but most (a greater number become) enemies. 8. Men are delighted with the harp and banqueting and dances and songs of victory. 9. The Greeks worship Apollo and Poseidon (Neptune). 10. Industrious scholars read the works of Xenophon with pleasure. EXERCISE 20.-ENGLISH-GREEK. 1. Φεύγε τους θήρας. 2. Θηρα φεύγουσι. 3. Τας χειρας νιζε. 4. Απέχεσθε των μηνων. 5. Στρατιώτης τῳ παιανι τέρπεται. 6. Ο παιαν τους στρατιωτας τέρπει. 7. Ω σπουδαιοι μαθηται, τα του Ξενοφωντος βιβλια αναγιγνώσκετε. 3. Τα του Ξενοφώντος βιβλια αναγιγνώσκονται ύπο των σπουδαίων μαθητών. 9. Τερπόμεθα τοις καλοις λειμοσι. 10. Οἱ λειμωνες θαλλουσι. 11. Οἱ ποιηται τον Απολλω σεβονται. 12. Τον Ποσειδώ σεβεται ὁ ποιητης. LESSONS IN GEOGRAPHY.-XXI. NATURAL DIVISIONS OF THE EARTH'S SURFACE (continued). WITH regard to the natural divisions of the water, the sea which surrounds the land is divided into three great sections, called oceans, exclusive of the comparatively small portions lying within the polar circles, which are denominated the Arctic and Antarctic Oceans. These three sections are:-1st. The Atlantic Ocean, extending from the Arctic Circle to the Antarctic Circle, a distance of 9,188 miles, and from the western coasts of the Old World to the eastern coasts of the New World, varying in breadth from 1,818 miles, the distance between Sierra Leone and Cape Roque, to 4,135 miles, the distance between the Cape of Good Hope and Cape Horn. 2nd. The Pacific Ocean, also extending from the Arctic Circle to the Antarctic Circle, and from the eastern coasts of the Old World to the western coasts of the New World, varying in breadth from sixty miles at Behring Straits, to about 11,000 miles at the equator, and then tapering to 5,277 miles, the distance between Cape Horn and Tasmania. 3rd. The Indian Ocean, extending from the Tropic of Cancer to the Antarctic Circle, a distance of 6,214 miles, and from the eastern coasts of Africa to the western coasts of Australia, varying in breadth from 3,491 miles at the equator, to 6,126 miles, the distance between the Cape of Good Hope and Van Diemen's Land. The ocean which rolls between Asia and America, called the Pacific, from the smoothness of its waves, and sometimes the Great South Sea, from its vast extent, exceeds the whole surface of the dry land. It is usually divided into two parts by the equator, the portion which lies in the northern hemisphere being called the North Pacific Ocean, and that in the southern hemisphere the South Pacific Ocean. It is bounded on the east by the western and north-western shores of America, and on the west by the eastern coasts of Asia and Australia. Towards the eastern side, and in the torrid zone, the face of this ocean is studded with innumerable groups of islands, all remarkably small. These consist generally of coral reefs, rising up like a wall from unfathomed depths, and emerging but a little way above the level of the sea. The most noted of these groups is that called the Society Islands, the chief of which is Otaheite or Tahiti (for an engraving of Otaheite see Vol. I., page 237); but all of them are the works of insects, both minute and innumerable, whose incessant labours are gradually forming new groups at the bottom of the ocean. The situation of these islands is such that, although lying between the tropics, the temperature of their atmosphere is so moderated by the surrounding ocean that they enjoy the most delightful climate in the world. The ocean which rolls between Europe and America, and also between Africa and America, is usually divided into two parts and the other the South Atlantic Ocean. The whole ocean receives the name Atlantic, from its washing the shores of that part of Africa where the mountains of Atlas were situated, which the poets feigned were employed to support the heavens. The Atlantic Ocean is bounded on the cast by Europe and Africa, and on the west by America; that part of it between Europe and America is called, from ancient times, the Western Ocean. The Atlantic Ocean, taken between the limits of the Arctic Circle and the latitudes of 35° S. on the one side, and 55° S. on the other, is only about half the size of the Pacific Ocean. The South Atlantic Ocean contains few islands of any size, and no inlets of consequence. The North Atlantic Ocean abounds in large islands, of which Great Britain and Ireland are the most noted; and in deep and numerous inland seas, which penetrate far into the interior of both the Old and New Worlds, and which have rendered the nations which possess its seaboard the most commercial and enterprising people on the face of the globe. The Mediterranean Sea and the Baltic Sea are but arms of the North Atlantic Ocean, on the east; and the Caribbean Sea, the Gulf of Mexico, Hudson Bay, and Davis Strait, arms of the same an the west. On the eastern shores, few large rivers, except the Niger, discharge themselves into its waters; but on the western shores it receives the great rivers La Plata, the Orinoco, the Maranon or Amazons, and the Mississippi, the largest water-ways on the surface of the globe. The Indian Ocean rolls between the Atlantic and Pacific Oceans, washing the eastern shores of Africa, the southern shores of Asia, and the western shores of Australia; whence its western, northern, and eastern boundaries are manifest; on the south it is bounded by the Pacific and Antarctic Oceans. This ocean contains many islands, the most important of which are Madagascar and Ceylon; and several bays and gulfs, such as the Bay of Bengal, the Arabian Sea, the Persian Gulf, the Red Sea or Arabian Gulf, etc. The ocean (from the Greek oke avos, o-ke'-a-nos, the great outward sea surrounding the world) means collectively, all the water which surrounds the earth; or, individually, any very large expanse of water. The term sea (from Saxon, sæ) is used in the same sense, both collectively and individually; but it is also applied to a smaller portion of water, and is often synonymous with the term gulf, from the Italian golfo, which is a bay, or opening of the sea into the land, either by a wide or a narrow opening. When the mouth of the opening into the land is wide, it is more usually called a bay, from the French baie; and when narrow, a gulf. When the sea penetrates far and wide into the land, the collection of water is then called an inland sea; such are the Mediterranean Sea and the Baltic Sea, the one in the south and the other in the north of Europe. The Arctic Ocean is the sea that surrounds the north pole, or rather that lies within the Arctic Circle; its boundaries are not exactly known, that is, it is not yet ascertained how much land lies within this zone, and, consequently, the extent of sea is equally unascertained. Whether Greenland extends to or falls short of the north pole has not yet been discovered; and the limits of North America have not quite been determined. This sea, besides the greater part of Greenland, contains Nova Zembla, the extreme north of Europe, the Liakhov Islands or New Siberia, and others, and some north of Baffin Bay. The White Sea is on the borders of the Arctic Ocean. The Antarctic Ocean, though considered as being likely to contain more land, is still less known than the Arctic Ocean; and if both were equally free of land, they would be of the same size within the Arctic and Antarctic Circles. Lakes are large or small portions of water wholly surrounded by land; some of these are so large as to be called seas, such as the Caspian Sea, the Sea of Aral, etc. A channel is a narrow passage between two seas, or two parts of the same sea; as, the English Channel, between the North Sea or German Ocean and the Atlantic. As appropriate illustrations to the present lesson on the |