Professor Paxton observes :Often as the thirsty traveller pursues his journey, a broad expanse of water, clear as crystal, seems to open to his view; and, faint and weary under the fierce sunbeam, he gazes on the unexpected relief with ineffable delight, and fondly anticipates a speedy termination to his present distress. He sees the foremost camels enter the lake, and the water dashed about by their feet. He quickens his pace, and hastens to the spot; but to his utter disappointment the vision disappears, and nothing remains but the dry and thirsty wilder ness. Rae Wilson remarks: About three o'clock I perceived the turrets and sycamore trees of Rosetta, at which time I found myself greatly exhausted from oppressive heat and fatigue; and, like other travellers, was deceived by the mists and apparitional lake* so celebrated under the name of the mirage or al serab, the illusory lake of the desert, which, even at a very short distance, had the most perfect resemblance to a vast sheet of water, with trees planted in it at certain distances, and reflecting every surrounding object as a mirror. We fancied this watery wilderness to be an insurmountable barrier to our reaching Rosetta, and that our guide had mistaken the proper track through the desert; but as we advanced, the supposed lake and its objects vanished: so powerful was the optical delusion. This prospect is at first sight cheering, but ultimately is most delusive. The traveller quickens his steps to reach the place where he hopes to quench his thirst, and feels the bitterness of disappointment; in truth, an ignis fatuus is not more tantalizing. Even swallows in great numbers swim over these imaginary pools. This singular phenomenon is in all probability that which is alluded to by the prophets and psalmist ;† and it may serve to point out how false are the objects pursued by men of the world, and how like these streams of the desert. Another writer says:— In sanscrit the phenomenon of the seraub or mirage is called mriga-trichna, thirst or desire of the antelope, no doubt because this animal, compelled by thirst, (trichna,) approaches those barren plains, where, from the inflection of the rays, he thinks he perceives the undulating surface of the waters. THE PLANETARY MOTIONS. IN our first volume, page 113, we gave an account of the planetary system, showing that each of the planets described elliptical orbits about the sun as a primary, placed in one of the foci of the ellipse as a centre, and also that the moon described a similar path (or orbit) about the earth. We also stated that, according to the Newtonian philosophy, the revolution of the planets in their orbits was occasioned by the combined operation of two forces (termed, by scientific men, centrifugal and centripetal forces) impressed upon them by the Creator at the commencement of time. We shall now endeavour to explain, in a familiar way, the manner in which the two forces operate in producing so regular and beautiful an effect. Centrifugal force must be understood to be that by which a body revolving about a centre, or about another body, endeavours to recede or fly off from it; as when a stone is whirled round at the end of a string, the faster it is whirled the greater is its tendency to fly off in a straight line from the circle (or orbit) in which it is moving; and centripetal force is that by which a moving body is perpetually urged towards a centre and, acting against the centrifugal force, causes the body to revolve in a curve, instead of moving in a straight line: the force exerted by the string above mentioned in retaining the stone in its orbit may represent the centripetal force; the two forces when thus combined are called central forces. It is one of the established laws of nature that all motion is of itself rectilinear, (or in straight lines,) and that the moving body never recedes from its first right line, till some new impulse be superadded in a different direction. If a stone be carried to a great height, and then set at liberty, it will immediately take a downward direction, and fall straight to the earth, with a continually accelerated velocity: the direction of its fall would be towards the earth's centre, that is, perpendicular to the horizon (or the surface of still water.) If the stone be thrown upwards into the air, the velocity of its motion will be continually retarded, and at length its upward motion will cease, and it will descend in the same path that it rose, with a continually accelerated velocity. If the stone be thrown obliquely to the horizon, the impulse it re- | ceives would give it a tendency to move in a straight line; but the instant it leaves the hand, it begins to fall perpendicularly, as well as move forward in the direction thrown, and its path, from the combined action of the two forces, will then become a curve till it reaches the earth. What we have now stated of falling bodies is the effect of the attraction of the earth, which, at its surface, will cause a body to fall about fifteen feet in one second of time, and like the attraction of a magnet, whose power upon a needle decreases as the needle is removed farther from it, so the attractive power of the earth decreases as the body is removed farther from its surface (or, to speak more technically, from its centre). This diminution of attractive power is found to be in the inverse ratio of the squares of the distances; that is, if a body be removed to double any distance, it would only be attracted with one quarter the force that acted upon it when at the distance itself; this explains why the stone above mentioned will fall with a continually accelerated velocity as it approaches nearer to the earth. C simultaneously, cause the stone to move in the curve A D; it will readily be understood that these forces represent respectively the centrifugal and centripetal forces before spoken of. The orbits of the planets are not circles, but ellipses, (or ovals,) which differ but little from circles; and if, in the foregoing account of the effects of central forces, we substitute the sun for the earth, and a planet in the place of the stone, we shall be able to understand how a planet is retained and continues to move in its orbit. Let the annexed oval figure represent the path Suppose, in the above figure, the line A B to be 15 feet, and that a stone, let fall from the point A, would, by its own weight, (or gravity,) fall to B, in one second of time, but, instead of letting it fall, we throw it obliquely in the direction A C, with such a force that it would reach the point C in the same time that it would take to fall to B, namely, one second; then the instant it leaves the hand (as before stated) it would depart from the straight line A C, and describe the curve A D, so that at the end of one second (which is the time it would take to fall from A to B, or pass from A to C) it will be at D, the distance C D being equal to A B, namely, fifteen feet, the effect being the same as if it had fallen from the point C on the line in which it was thrown. The two forces, namely, the impulse given to the stone at A, in the direction A C, and the attraction of gravitation, in the direction A B, acting P or orbit of a planet; let S represent the sun, placed in one of the foci of the oval; the longest diameter, A P, is called the line of the apses, A being the higher apsis or aphelion, and P the lower apsis or perihelion. The planet when at A is at its greatest distance from the sun, and is therefore least affected by its attractive power; and when the planet is at P, the least distance from the sun, its attractive power is the greatest. Suppose, in the first instance, the planet was conveyed to the point A, and there set at liberty, without giving it an impulse in any direction, it would immediately, by the force of gravity, commence its descent to the sun in the direction A S; but if, at the instant it was set at liberty, an impulse was given it, which, if acting alone, would have caused it to move in the direction A B, then (from what has been before shown, with respect to the stone) the planet would move in the path A CD &c., so that it would arrive at C in the same time it would take to fall from A to E, the distance A E being equal to F C, and it will also arrive at D in the same time it would have taken to fall from A to S; but the planet at D, being so much nearer the centre of attraction S than it was at A, is more affected by it; and the centripetal force would appear likely to cause it to fall to the sun; but this is counteracted by the centrifugal force, the planet having acquired a greater velocity in falling from the line A B, in which it was first impelled, through a space equal to the distance A S; and as before observed, that falling bodies move with a continually accelerated velocity, this increase of velocity would incline it to fly off from its orbit, (or recede from the centre of attraction,) which exactly counterbalances its greater inclination to fall to the sun. In like manner, when it arrives at P, (its perihelion,) the attractive force of the sun is then greatest; but the planet having fallen through a space equal to A P, its velocity also is greatest, and the two forces again counterbalance each other. The velocity the planet has now acquired is capable of carrying it through the other half of its orbit, up again to its aphelion A, with a velocity continually diminishing, and as it is receding from the sun, its attractive force equally diminishes, and thus a balance of power between the two forces being continually maintained, the planet will be impelled in its orbit; for having again arrived at A, its velocity again is on the increase, and the same circuit will continue to be performed. The planet passing through the lower half of its orbit with a greater velocity than the upper, it is evident, that it must perform that half in less time than the other; and being winter when the planet we inhabit (the earth) is in that part of its orbit, will account for our winter half-year being shorter than the summer: the difference is about seven days. If the centrifugal force of the planets was to be destroyed, each of them would instantly commence falling to the sun, and the moon would fall to the earth. We have annexed the times which each would occupy in thus falling from their mean distance to their primaries. Days Hrs. Mercury would fall to the sun in 15 13 Venus 17 The Earth Mars Jupiter 3.9 The Moon would fall to the earth in 4 21 We have stated that the sun attracts each of the planets, but it is also true that the planets attract the sun, and also one another, and that, in proportion to the quantity of matter each body contains. It is not, therefore, strictly true that one body will revolve about the other as a centre, as they are each of them under the influence of the gravitating power of the other, and they therefore must revolve about the common centre of gravity of the two bodies: just as two stones of unequal magnitude, connected together with a string, if thrown into the air, will revolve about a certain point (or place) in the string, which will be so much nearer the larger stone as that stone exceeds the other in weight. This point of the string will be the common centre of gravity of the two stones: but in the solar system, where there are so many planets revolving with the sun about their common centres of gravity, each exerting its own influence upon the sun, often attracting it in different directions, as they are situated in different parts of the heavens, as well as acting upon each other; it must be obvious that the whole planetary system, and the sun, revolve around the common centre of gravity of the entire solar system, which centre must be continually varying with the change in the position of the planets in their orbits. On account of the superior magnitude of the sun with respect to the planet, the common centre of gravity of the solar system is not far removed from the centre of the sun. "We can form no idea of the nature of that power called gravity, by which distant bodies are made to act upon each other without any apparent connexion; and yet we know that all bodies in our system are retained in their courses by such a power. And it is a very singular instance of the unerring wisdom of the Creator, that the law which this power observes is such, that, notwithstanding the mutual attraction of the bodies, the system will never fall into ruin, but is capable of preserving itself until it shall please the Creator to make any change. "Moreover, the mutual attraction which takes place between distant bodies, could not, of itself, either produce their motion about the sun, nor the rotation about their axis; it required an external impulse to operate in conjunction with it, to produce these effects; an act which nothing but the arm of Omnipotence could accomplish. And the power which thus connects the distant bodies, operates also on the constituent particles of the same body, and preserves its figure; for, without attraction, the particles must have been dissipated by 1 1 their rotation. An invisible power pervades the whole system, and preserves it. In the effects produced by man, we see the operation of the cause; but the ways of the Almighty are past finding out. "The sun, that great and only fountain of light and heat, is placed in the centre of the system; and whilst, by its influence, it retains the planets in their orbits, it pours forth its rays and gives life to the creation: the formation of such a glorious body, and its arrangement, are circumstances which afford the clearest evidence of design, and if of design, there must be a Designer. "Hence, in whatever point of view we take a survey of our system, we trace the power, wisdom, and goodness of the Creator: his power in its formation; his wisdom in the simplicity of the means to produce the ends; and his goodness in making those ends subservient to our use and enjoyment. Thus we are led by our inquiries into the structure of the universe, to the proofs of the existence and attributes of a Supreme Being, who formed and directs the whole. Arguments of this kind produce conviction which no sophistry can confound. Every man may see it; man may behold it afar off.' Let not, therefore, the ignorant declaim against those pursuits which direct us to a knowledge of our Creator, and furnish us with unanswerable arguments against the infidel and atheist." (Pea ock Butterfly and its Chrysalis.) INSECTS. No. XXV. (Form, Colour, and Age of Chrysalises or Pupa.) THE form of a pupa is peculiar. If, for instance, that of the lappet-moth be disengaged from the cocoon, it has much the appearance of an Egyptian mummy, or an infant in the old-fashioned swaddling bands. The feet are crossed over the breast, and folded closely down, but the wings are compressed into a very small compass. This appears the more remarkable, as the wings of the moth are large and conspicuous, and so like the withered leaf of an oak, both in form and colour, that the in sect would readily impose on a careless observer. By opening one of these pupa, Reaumur discovered that various sheaths were appropriated to the feet, the antennæ, and the wings. At first, interiorly, all pupae consist of a milky fluid, in which the unformed members of the future perfect insect may be said to float, and in which they may be discerned, and separated with the point of a pin. As these acquire consistency, and are more and more developed by the absorption of the surrounding fluid, they occupy its place, while the rest passes off by transpiration. Reaumur regards this matter as analogous to the white of an egg. When the membranous covering, which is thinner, but more firm and elastic than India paper, has been carefully removed from the chrysalis of the peacock butterfly, selecting for this operation an advanced period of it, the legs, antennæ, and sucker, may be seen folded down longitudinally on the breast; but so firm are these, that the insect can move about its legs, coil up its sucker, and play its antennæ; while the wings are still covered with moisture, so that they have not yet assumed their beautiful colours and elegant markings, but are of a dusky ash grey, and the powdering down which clothes them is scarcely visible. The membrane which covers the more prominently exposed parts, such as the legs, is considerably thicker than the other por tions. | There is much less variety in the colour of pupe than in that of larvæ. Some are white, or whiteish; others are brown, of various shades, often verging on black or red; but many are gaily decorated. Some are of a greenish yellow, marked with spots of black; others are of a uniform green; others reddish; and others again red, with black spots. A still greater number shine as though gilded with burnished gold, either applied in partial streaks, or covering the entire surface. It was from this gilded appearance in some pupa that the terms chrysalis and aurelia, derived from the Latin and French names for gold, were applied to the whole. The alchemists mistook this for real gold; and referred to the fact as an argument in favour of the transmutation of metals. But Reaumur has satisfactorily shown that this case the proverb is applicable, "All is not gold that glitters." He found that the gay appearance is owing to the shining white membrane immediately below the outer skin, which, being of a transparent yellow, gives a golden tinge to the former; in the same way that tinfoil, when covered with a yellow varnish, assumes the metallic appearance which we see in gilt leather. Ĥe mentions, too, that for the production of this effect, it is essential that the inner membrane be moist; whence may be explained the disappearance of the gilding as soon as the butterfly is ready to escape from the pupa. The shade of colours in these gilded chrysalises is various. Some are of a rich yellow, like pure gold, others much paler; some are nearly as white as silver, and one is red, with silver spots. One chrysalis is of a glaucous blue, another of a lilac colour, and another of a lively blue, caused by a kind of bloom, like that of a plum, spread upon a brow ground. A similar bloom is found in another, which was observed to be renewed when rubbed off. Many pupa have the sheaths of the wings of a different colour from that of the rest of the body; a few are variegated with paler streaks or bands; and that of the common gooseberry and currant moth, which may be found in every garden, has alternate rings of black and yellow. Almost all, at their first assumption of the pupa state, have a different colour from that which they take a few days afterwards. This last they retain until the disclosure of the perfect insect, except in the case of some that have transparent skins, which, a few days prior to this period, exhibit the colours of the included animal. It is a general rule, that one pupa-case incloses only one insect; but a German entymologist asserts that he had once two specimens of one insect produced from one pupa, which was large, being full two inches long and one thick. There is as great a variety in the length of the age of insects in their pupa, as in their larva state. Some continue in it only two or three days; others, as many weeks, or months, or even years. Each, however, has in general a stated period, which, in ordinary circumstances, it neither much anticipates nor exceeds. Many remarkable variations are dependent merely on temperature. In the month of January, Reaumur placed the chrysalises of several moths and butterflies, which would not naturally have been disclosed until the following May, in a hothouse: the result was, that the perfect insects made their appearance in less than a fortnight, in the very depth of winter; and by other numerous and repeated experiments, he ascertained that, in this heated atmosphere, five or six days hastened their maturity more than many weeks would have done in the open air. The disclosed insects were in every way perfect, and the females, after pairing, laid their eggs, and then died, just as if they had not thus prematurely been forced into existence. converse of this experiment equally succeeded. By keeping the pupa the whole summer in an ice-house, Reaumur caused them to produce the fly one full year later than their ordinary period. The The only general rule that can be laid down is, that small pupa continue in that state a shorter time than those of larger bulk: but to this, there are many excep |