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cannot pull a bone in two opposite directions; if, for example, it bends the arm, it cannot also extend it. For this latter purpose a second muscle is required. The same thing obtains in all the limbs, and in every movable part of the body. Not a finger can be bent and straightened without the contraction of two antagonistic muscles. We, therefore, find that most of the muscles are arranged in opposing groups, which pull against each other like sawyers in a pit. Not only the faculty of motion, but even the symmetry of the body is preserved by this opposition. The mouth is kept in the middle of the face by two sets of muscles drawing against and balancing each other, and when, from disease or accident, one of them becomes powerless, the other, having nothing to counteract its influence, draws the mouth awry.
We can never sufficiently admire the variety, quickness, and precision of which muscular motion is capable. Let any one observe his own hand while he is writing, and notice how many different muscles must be brought to bear upon the pen. The joint and accurately adjusted operation of several tendons is concerned in every stroke, yet, by a rapid writer, many hundreds of these strokes are drawn in a minute. Not a letter can be formed without more than one, or two, or three separate contractions, each of them definite both as to the choice of the muscle, and the extent of its exertion. Yet how easily, how currently does the work proceed! And, when we look at the result, how faithful have the muscles been to their duty, how true to the order which endeavour or habit has inculcated!
QUESTIONS FOR EXAMINATION.
Of what use is the skeleton? How are the bones of the skull joined? Why is the skull connected with the back-bone? What joint enables us to nod? to turn the head round? Where are they respectively situated? How do they differ? What purposes are served by the spine? Describe a vertebra. What gives the spine its flexibility? its stability? Where are the ribs? the breast-bone? the pelvis? With what bone are the arms articulated? Why are there two bones in the forearm? Why so many in the wrist? How does the shoulderjoint differ from the elbow? What sort of joints have the fingers? Show that the skeletons of the leg and arm are similarly constructed. Where is the collar-bone? Of what use is it? What means are taken to lessen the friction
of the joints? Of what use are the muscles? What peculiar property have they? What are tendons? Why are they necessary? How is the direction of their motion changed? Give examples. How are two opposite motions provided for? Give an example of the celerity and exactness of muscular action.
THE WAY OF THE WIND.
Ir is scarcely necessary to remark that wind is nothing but the air in motion. We may learn our first lesson respecting it from the breezes that are frequently noticed on the shores of our own country in the hot summer weather, and which occur with greater constancy in tropical climates. Many of us may have observed how a cool sea-breeze sets in during the day, and a warmer land-breeze begins to blow shortly after sunset. The cause of this is easily explained. The land absorbs the heat from the sun to a much greater extent than the water does, and becoming thus hotter, it naturally causes the air above it to be warmer than that which overlies the sea. Now, air expands by heat, as other things do, and thus becomes lighter, and ascends, its place being supplied by colder air, which, in the instance before us, must come from the sea, thus forming the refreshing seabreeze which moderates the heat of the summer noon. But at night the reverse is the case. The land cools much more rapidly than the water, so, after a while, it becomes actually the colder of the two, and the breeze now sets from the land to the sea. It will be evident, that when the air is sweeping along near the surface of the earth from the ocean to the shore, the space it is leaving must be filled from elsewhere, and what so ready to fill it as the air which has just risen from the heated ground. Accordingly, we always find that when there is a wind of this sort blowing in one direction, there is a contrary current in the upper regions of the atmosphere.
From this we may learn what is constantly taking place on a grand scale over the whole surface of the globe. The great heat near the equator causes a constant upward current which draws the air from those regions that are
colder. Hence there are two gigantic circles of wind, one in each hemisphere: the air rising, passing through the upper parts of the atmosphere, descending as it nears the poles, and then sweeping again over the surface of the earth towards the equator. But there are several things which interfere with the regularity of this action. In the first place, this globe of ours is always revolving rapidly from west to east; and although this would not affect air when at rest, or blowing around the same latitude, yet it must be remembered that the air starting from the equator has a more considerable impetus than is necessary to accompany the earth in its rotation in higher latitudes; and hence the stream which blew at first from the south, appears presently to be coming from the south-west; and when it arrives near the pole, it will have become almost a due west wind. The reverse takes place with the stream from the north, for the opposite reason, it is soon found to be blowing from the east of north, and eventually changes in like manner into a regular east wind.
Again, the configuration of the land, the direction of mountain-chains, local alternations in the temperature of the earth, and a thousand other circumstances, are always interfering, and thus render the problem of the winds an extremely difficult one. The wind, too, meets with resistance in passing near the earth, and thus flows less quickly than it does in the higher regions, just as the water at the bottom and sides of a river never has so great a velocity as that in the middle of the stream.
This great circle above mentioned, between the poles and the equator, is observed in the trade-winds, which blow steadily from the eastward on each side of the "region of calms" that marks the hottest part of the earth's circumference. The returning “trade” in those latitudes is at such a height above the ground that it is only indicated by light films of cloud, which are often seen to move in the opposite direction to the prevailing current below. In our own climate the upper stream frequently descends, and, as a south-west wind, brings to us some of the warmth and
moisture of the tropics; though, perhaps, we more frequently experience the lower current-the north-east wind, which will often blow steadily for weeks together, especially in spring, brings us cold from the pole and the frozen plains of Russia.
When two currents, blowing from opposite directions, meet, they must slide, as it were, past one another, but at their junction a whirlwind will be produced, such as we have often seen on a small scale in the eddies caused by a house, or some such opposing body. We all know how such little whirlwinds catch up dust or straws into the air, or sometimes play strange pranks with the dress of the unwary traveller; and we can readily understand how, when a larger vortex than these is sweeping unobstructed over the sea, the clouds above and the waters below should be caught into it, and meeting in mid-air, should form those water-spouts which are so dangerous to small vessels. The sailors are accustomed to disperse them, as they do human enemies, by firing cannon at them, and so breaking the circle of wind. The most severe storms are produced when the great southwest and north-east currents oppose one another. The gigantic whirlwind then formed may be scores of miles in breadth, and will often sweep right across the Atlantic, traverse our own country, and then pass on to Denmark and the continent of Europe. Through the great attention lately paid to the "circular theory of storms," captains of ships are enabled, by observing the direction of the change of wind, to sail right away from the danger. For instance, if sailing eastward in the northern hemisphere, he observes the wind veering from south to west, and then to north, the centre of the storm is on his right hand, and he, of course, will only have to turn his vessel towards the left, and the tempest may spend its fury harmlessly in the distance. Just in the same way the landsman may judge, by the rapid shifting of the weathercock, what is the course of a hurricane, which may be at that very moment tearing up trees, overthrowing hayricks, and unroofing houses.
Thus, however much we may have been accustomed to
view the wind as an emblem of all that is fickle and capricious, we now see that, whether it be a West-Indian tornado, at the speed of seventy miles an hour, devastating whole islands, or a gentle breeze, which scarcely stirs the petals of a flower, it is as subject to fixed and regular laws, and as much a matter of scientific investigation, as the growth of a tree, or the course of a planet.
J. H. GLADSTONE,
[PERCY BYSSHE SHELLEY, the eldest son of Sir Timothy Shelley, was born in Sussex in 1792. He was sent to Eton, whence, owing to his eccentricity of character, he was removed to Oxford, much before the usual period. Here a repetition of youthful irregularity, deeply aggravated by the open avowal of his atheistical opinions, caused his expulsion, and an imprudent early marriage cast him off from his family. In search of health he repaired to Switzerland, d thence to Italy. He was drowned in the Gulf of Spezzia in 1822. His principal poetical works are "Prometheus Chained," "Alastor or the Spirit of Solitude," "Queen Mab," and "Cenci."]
I bring fresh showers for the thirsting flowers,
From the seas and the streams;
I bear light shades for the leaves! when láid
In their noon-day dreams;
From my wings are shaken' the dews that wáken
When rocked to rest! on their mother's breast,
I sift the snow on the mountains belów,
While I sleep' in the arms of the blast.