c.o.c.ks crowed, birds flew low or fluttered about uneasily, but every object far and near was well defined to the eye.
"A singular broadway of light stretched north and south for upwards of a quarter of an hour; from about 12.54 to 1.10 P.M."
[Ill.u.s.tration: Fig. 38.]
[Ill.u.s.tration: Fig. 39.]
If the annular eclipse of the sun be a matter for wonderment, the total eclipse of the same is much more surprising; no other expression than that of _awfully grand_, can give an idea of the effects of totality, and of the suddenness with which it obscures the light of heaven. The darkness, it is said, comes dropping down like a mantle, and as the moment of full obscuration approaches, people"s countenances become livid, the horizon is indistinct and sometimes invisible, and there is a general appearance of horror on all sides. These are not simply the inventions [Page 28] of active human imaginations, for they produce equal, if not greater effects, upon the brute creation. M. Arago quotes an instance of a half-starved dog, who was voraciously devouring some food, but dropped it the instant the darkness came on. A swarm of ants, busily engaged, stopped when the darkness commenced, and remained motionless till the light reappeared. A herd of oxen collected themselves into a circle and stood still, with their horns outward, as if to resist a common enemy; certain plants, such as the convolvulus and silk-tree acacia, closed their leaves. The latter statement was corroborated during the annular eclipse of the 15th of March, 1858, by Mr. E. S. Lane, who states, that crocuses at the Observatory, Beeston, had their blossoms expanded before the eclipse; they commenced closing, and were quite shut at about one minute previous to the greatest darkness; and the flowers opened partially about twenty minutes afterwards. A "_total eclipse_" of the sun has always impressed the human mind with terror and wonder in every age: it was always supposed to be the forerunner of evil; and not only is the mind powerfully impressed, as darkness gradually shuts out the face of the sun, but at the moment of totality, a magnificent corona, or glory of light, is visible, and prominences, or flames, as they are often termed, make their appearance at different points round the circle of the dark ma.s.s.
This glory does not flash suddenly on the eye; but commencing at the first limb of the sun, pa.s.ses quickly from one limb to the other. Our ill.u.s.tration shows "the corona" and the "rose-coloured prominences,"
whose nature we shall next endeavour to explain. Professor Airy describes the change from the last narrow crescent of light to the entire dark moon, surrounded by a ring of faint light, as most curious, striking, and magical in effect. The progress of the formation of the corona was seen distinctly. [Page 29] It commenced on the side of the moon opposite to that at which the sun disappeared, and in the general decay and disease which seemed to oppress all nature, the moon and the corona appeared almost like a local sore in that part of the sky, and in some places were seen double. Its texture appeared as if fibrous, or composed of entangled threads; in other places brushes, or feathers of light proceeded from it, and one estimate calculated the light at about one-seventh part of a full moon light. The question, whether the corona is concentric with the sun and moon, was specially mooted by M. Arago, and Professor Baden Powell has produced such excellent imitations of the "corona" by making opaque bodies occult, or conceal, very bright points, that it cannot be considered as material or real, although it ought to be remembered that the best theory of the zodiacal light represents it to be a nebulous ma.s.s, increasing in density towards the sun, and yet no portion of this nebulous ma.s.s was seen during the totality. But by far the most remarkable of all the appearances connected with a "total eclipse" are the rose-coloured prominences, mountains, or flames, projecting from the circ.u.mference of the moon to the inner ring of the corona; and, although they had been observed by Vaserius (a Swedish astronomer) in 1733, they took the modern astronomers entirely by surprise in 1842, and they were not prepared with instruments to ascertain the nature of these strange and almost portentous forms. In 1851, however, great preparations were made to throw further light on the subject. Professor Airy went to make his observations, and he says, "That the suddenness of the darkness in 1851 appeared much more striking than in 1842, and the forms of the rose-coloured mountains were most curious. One reminded him of a boomerang (that curious weapon thrown so skilfully by the aborigines of Australia); this same figure has been spoken of by others as resembling a Turkish scimitar, strongly coloured with rose-red at the borders, but paler in the centre. Another form was a pale-white semicircle based on the moon"s limbs; a third figure was a red detached cloud, or balloon, of nearly circular form, separated from the moon by nearly its own breadth; a fourth appeared like a small triangle, or conical red mountain, perhaps a little white in the interior;" and the Professor proceeds to say, "I employed myself in an attempt to draw roughly the figures, and it was impossible, after witnessing the increase in height of some, and the disappearance of another, and the arrival of new forms, not to feel convinced that the phenomena belonged to the sun, and _not_ to the moon."
Still the question remains unanswered, what are these "rose-coloured prominences?" If they belong to the sun, and are mountains in that luminary, they must be some thirty or forty thousand miles in height.
M. Faye has formally propounded the theory, that they are caused by refraction, or a kind of mirage, or the distortion of objects caused by heated air. This phenomenon is not peculiar to any country, though most frequently observed near the margin of lakes and rivers, and on hot sandy plains. M. Monge, who accompanied Buonaparte in his [Page 30]
expedition to Egypt, witnessed a remarkable example between Alexandria and Cairo, where, in all directions, green islands appeared surrounded by extensive lakes of pure, transparent water. M. Monge states that "Nothing could be conceived more lovely or picturesque than the landscape. In the tranquil surface of the lake, the trees and houses with which the islands are covered were strongly reflected with vivid and varied hues, and the party hastened forward to enjoy the refreshment apparently proffered them; but when they arrived, the lake, on whose bosom the images had floated--the trees, amongst whose foliage they arose, and the people who stood on the sh.o.r.e, as if inviting their approach, had all vanished, and nothing remained but the uniform and irksome desert of sand and sky, with a few naked and ragged Arabs."
If M. Monge and his party had not been undeceived, by actually going to the spot, they would, one and all, have been firmly convinced that these visionary trees, lakes, and buildings had a real existence. This kind of mirage is known in Persia and Arabia by the name of "serab" or miraculous water, and in the western districts of India by that of "scheram." This illusion is the effect of unusual refraction, and M.
Faye attempts to account for the rose-coloured mountains by something of a similar nature.
It is right, however, to mention, that learned astronomers do not consider this theory of any value.
Lieutenant Patterson, one of the observers of the eclipse of 1851, says, that "It is very remarkable that the flames or prominences correspond exactly (at least as far as he could judge) with the spots on the sun"s surface." Taking this statement with that of M. Faye, it may be a.s.sumed, as a new idea, and nothing more, that these prominences are, after all, mere aerial pictures of these openings in the sun"s atmosphere, or what are called "sun spots." In the "Edinburgh Philosophical Journal," it is said, that although it has lately been shown in the Edinburgh Observatory that it is possible to produce, by certain optical experiments, red flames on the sun"s limb of precisely the rose-coloured tint described, yet, on weighing the whole of the evidence, there does seem a great preponderance in favour of the eclipse flames being real appendages of the sun, and in that case they must be ma.s.ses of such vast size as to play no unimportant part in the economy of that stupendous...o...b..
During the last eclipse great disappointment was felt that the darkness was so insignificant, although, when we consider the enormous light-giving power of the sun, and know that it was not wholly obscured, we could hardly have expected any other result. There can be no doubt that a decided change in the amount of light is only to be observed during a total eclipse of the sun, one of which occurred on the 7th of September, 1858; but, unfortunately, it was only visible in South America; we must therefore content ourselves with the descriptions of those astronomers who can be fully relied on. From the graphic account given by Professor Piazzi Smyth, the astronomer-royal for Scotland, [Page 31] of a total eclipse as seen by him on the western coast of Norway, we may form some notion of the imposing appearance of the surrounding country when obscured during the occurrence of this rare astronomical phenomenon.
The Professor remarks, "To understand the scene more fully, the reader must fancy himself on a small, rocky island on a mountainous coast, the weather calm, and the sky at the beginning of the eclipse seven-tenths covered with thin and bright cirro-strati clouds. As the eclipse approaches, the clouds gradually darken, the rays of the sun are no longer able to penetrate them through and through, and drench them with living light as before, but they become darker than the sky against which they are seen. The air becomes sensibly colder, the clouds still darker, and the whole atmosphere murkier.
"From moment to moment as the totality approaches, the cold and darkness advance apace; and there is something peculiarly and terribly convincing in the two different senses, so entirely coinciding in their indications of an unprecedented fact being in course of accomplishment. Suddenly, and apparently without any warning (so immensely greater were its effects than those of anything else which had occurred), the totality supervenes, and darkness _comes down_. Then came into view lurid lights and forms, as on the extinction of candles. This was the most striking point of the whole phenomenon, and made the Norse peasants about us flee with precipitation, and hide themselves for their lives.
"Darkness reigned everywhere in heaven and earth, except where, along the north-eastern horizon, a narrow strip of unclouded sky presented a low burning tone of colour, and where some distant snow-covered mountains, beyond the range of the moon"s shadow, reflected the faint mono-chromatic light of the partially eclipsed sun, and exhibited all the detail of their structure, all the light, and shade, and markings of their precipitous sides with an apparently supernatural distinctness.
After a little time, the eyes seemed to get accustomed to the darkness, and the looming forms of objects close by could be discerned, all of them exhibiting a dull-green hue; seeming to have exhaled their natural colour, and to have taken this particular one, merely by force of the red colour in the north.
"Life and animation seemed, indeed, to have now departed from everything around, and we could hardly but fear, against our reason, that if such a state of things was to last much longer, some dreadful calamity must happen to us all; while the lurid horizon, northward, appeared so like the gleams of departing light in some of the grandest paintings by Danby and Martin, that we could not but believe, in spite of the alleged extravagances of these artists, that Nature had opened up to the constant contemplation of their mind"s-eye some of those magnificent revelations of power and glory which others can only get a glimpse of on occasions such as these."
It can be easily imagined, that under such peculiar and awful circ.u.mstances, the careful observation of these effects must be somewhat difficult, [Page 32] and the only wonder is that the astronomical observations are conducted with any certainty at all.
In the eclipse of 1842, it was not only the vivacious Frenchman who was carried away in the impulse of the moment, and had afterwards to plead that "_he was no more than a man_" as an excuse for his unfulfilled part in the observations, but the same was the case with the grave Englishman and the more stolid German. In 1851, much the same failure in the observations occurred; and on some person asking a worthy American, who had come with his instruments from the other side of the world expressly to observe the eclipse, what he had succeeded in doing? he merely answered, with much quiet impressiveness, "_That if it was to be observed over again, he hoped he would be able to do something, but that, as it was, he had done nothing: it had been too much for him._"
This is not quite so bad as the fashionable lady who had been invited to look at an eclipse of the sun through a grand telescope, but arriving too late, inquired whether "it could not be shown _over again_."
With this brief glance at the science of astronomy, we once more return to the term "gravity," which will introduce to us some new and interesting facts, under the head of what is called "centre of gravity."
CHAPTER IV.
CENTRE OF GRAVITY.
_That point about which all the parts of a body do, in any situation, exactly balance each other._
The discovery of this fact is due to Archimedes, and it is a point in every solid body (whatever the form may be) in which the _forces_ of _gravity_ may be considered as _united_. In our globe, which is a sphere, or rather an oblate spheroid, the centre of gravity will be the centre. Thus, if a plummet be suspended on the surface of the earth, it points directly to the centre of gravity, and, consequently, two plummet-lines suspended side by side cannot, strictly speaking, be parallel to each other.
If it were possible to bore or dig a gallery through the whole substance of the earth from pole to pole, and then to allow a stone or the fabled Mahomet"s coffin to fall through it, the momentum--_i.e._, the force of the moving body, would carry it beyond the centre of gravity. This force, however, being exhausted, there would be a retrograde movement, and after many oscillations it would gradually come to rest, and then, unsupported by anything material, it would be suspended by the force of gravitation, and now enter into and take part in the general attracting force; and being equally attracted on every side, the stone or coffin must be totally without weight. _Momentum_ is prettily ill.u.s.trated by a series of inclined planes [Page 33] cut in mahogany, with a grooved channel at the top, in imitation of the famous Russian ice mountains: and if a marble is allowed to run down the [Page 34] first incline, the momentum will carry it up the second, from which it will again descend and pa.s.s up and down the third and last miniature mountain.
[Ill.u.s.tration: Fig. 40. F. The centre. A B C D E. Plummet-lines, all pointing to the centre, and therefore diverging from each other.]
[Ill.u.s.tration: Fig. 41. P P P. Inclined planes, gradually decreasing in height, cut out of inch mahogany, with a groove at the top to carry an ordinary marble. B B B. Different positions of the marble, which starts from B A.]
In a sphere of uniform density, the centre of gravity is easily discovered, but not so in an irregular _ma.s.s_; and here, perhaps, an explanation of terms may not be altogether unacceptable.
_Ma.s.s_, is a term applied to solids, such as a ma.s.s of lead or stone.
_Bulk_, to liquids, such as a bulk of water or oil.
_Volume_, to gases, such as a volume of air or oxygen.
[Ill.u.s.tration: Fig. 42. A B D, The three points of suspension. C, The point of intersection, and, therefore, the centre of gravity. P, The line of plummet.]
To find the centre of gravity of any ma.s.s, as, for example, an ordinary school-slate, we must first of all suspend it from any part of the frame; then allow a plumb-line to drop from the point of suspension, and mark its direction on the slate. Again, suspend the slate at various other points, always marking the line of direction of the plummet, and at the point where the lines intersect each other, there will be the centre of gravity.
[Ill.u.s.tration: Fig. 43.]
If the slate be now placed (as shown in Fig. 43) on a blunt wooden point at the spot where the lines cross each other, it will be found to balance exactly, and this place is called the _centre of gravity_, being the point with which all other particles of the body would move with parallel and equable motion during its fall. The equilibrium of bodies is therefore much affected by the position of the centre of gravity.
Thus, if we cut out an elliptical figure from a board one inch in thickness, and rest it on a flat surface by one of its edges (as at No.
1, fig. 44), this point of contact is called the point of support, and the centre of gravity is immediately above it.
In this case, the body is in a state of secure equilibrium, for any motion on either side will cause the centre of gravity to ascend in these directions, and an oscillation will ensue. But if we place it upon the smaller end, as shown at No. 2 (fig. 44), the position will be one of [Page 35] equilibrium, but not stable or secure; although the centre of gravity is directly above the point of support, the slightest touch will displace the oval and cause its overthrow. The famous story of Columbus and the egg suggests a capital ill.u.s.tration of this fact; and there are two modes in which the egg may be poised on either of the ends.
[Ill.u.s.tration: Fig. 44. The point of support. C, The centre of gravity.]
The one usually attributed to the great discoverer, is that of sc.r.a.ping or slightly breaking away a little of the sh.e.l.l, so as to flatten one of the ends, thus--
[Ill.u.s.tration: Fig. 45. A Represents the egg in its natural state, and, therefore, in unstable equilibrium; B, another egg, with the surface, S, flattened, by which the centre of gravity is lowered, and if not disturbed beyond the extent of the point of support the equilibrium is stable.]
The most philosophical mode of making the egg stand on its end and without disturbing the exterior sh.e.l.l is to alter the position of the yolk, which has a greater density than the white, and is situated about the centre. If the egg is now shaken so as to break the membrane enclosing the yolk, and thus allow it to sink to the bottom of the smaller end, the centre of gravity is lowered; there is a greater proportion of weight [Page 36] concentrated in the small end, and the egg stands erect, as depicted at fig. 46.
It is this variable position of the centre of gravity in ivory b.a.l.l.s (one part of which may be more dense than another) that so frequently annoys even the best billiard-players; and on this account a ball will deviate from the line in which it is impelled, not from any fault of the player, but in consequence of the ivory ball being of unequal density, and, therefore, not having the centre corresponding with the centre of gravity. A good billiard-player should, therefore, always try the ball before he engages to play for any large sum.
[Ill.u.s.tration: Fig. 46.--No. 1. Section of egg. C. Centre of gravity. Y.
The yolk. W. The white.
No. 2. C. Centre of gravity, much lowered. Y. The yolk at the bottom of the egg.]
The toy called the "tombola" reminds us of the egg-experiment, as there is usually a lump of lead inserted in the lower part of the hemisphere, and when the toy is pushed down it rapidly a.s.sumes the upright position because the centre of gravity is not in the lowest place to which it can descend; the latter position being only attained when the figure is upright.
[Ill.u.s.tration: Fig. 47.--No. 1. C. Centre of gravity in the lowest place, figure upright.
No. 2. C. Centre of gravity raised as the figure is inclined on either side, but falling again into the lowest place as the figure gradually comes to rest.]