Abbildungen der Seite
PDF
EPUB

N

[ocr errors]

F

D

SUMMER SOLSTICE.

exceedingly slow, that it has taken about two thousand years The question of day and night depends upon this prilifor the equinoctial points to recede through one sign, or ciple, that one half of a globe, and no more, is illuminated 30°, from the point where it was fixed by Hipparchus, who by one source of light; as we see in the annexed figure. was called the father of astronomy.” The vernal equinox, If the candle is placed in the plane of the equator, for or the 21st of March, now occurs when the Sun is actually instance, then, as the globe is turned round, one hemiin the constellation Pisces; but still, for the convenience of sphere is constantly illuminated from north to south. maintaining the same nomenclature, the Sun is said to But, now suppose we have a miniature representation of enter the first point of Aries on the 21st of March : it the Earth, and suppose the Sun at o to be shining on the therefore results that the sign Aries does not correspond parts at a from some distance on the right. We then find with the constellation Aries. We now understand what is meant by the tropic of

Fig. 24. Cancer, and the tropic of Capricorn; and we shall also see in what way the signs of the Zodiac become symbols, as it were, of the kind of season which we experience, by merely

sitä vode considering that the Sun is between us and those constel

34395ed sig lations. For instance, when Spring is about verging towards Summer, the effect is thus alluded to:

anigai At last from Aries rolls the bounteous Sun,

- #ಸ ು And the bright Bull receives him. Then no more

s, Yound Buda Th' expansive atmosphere is cramped with cold; But, full of life and vivifying soul, Lifts the light clouds sublime, and spreads them thin,

Fleecy and white, o'er all-surrounding Heaven. In the middle of a northern Summer the Sun is in the one-half of the globe illuminated, and the other half sign Cancer, and at that time the inhabitants of that portion shrouded in darkness. If ns be the axis of the globe, of the Earth where the tropic of Cancer is situated, have round which it may be supposed to revolve, then e Q is the the noon-day Sun precisely over their heads; that tropic equator, B a the ecliptic, and A d, Bb, the two tropics

. being the most northern situation in which such a circum- The line 1 D separates the dark from the illuminated stance occurs. From thence the Sun gradually travels in hemisphere, and is therefore the real horizon. Let the a spiral manner down towards Capricorn, during which spot a represent any given place, and let us see what will time Autumn's milder heat succeeds to Summer's glow,- occur if the globe revolve on its axis ns. In its present When the bright Virgin gives the beauteous days,

position, the spot a receives a large and continuous amount And Libra weighs in equal scales the year;

of light from the Sun; because he is not invisible until a From Heaven's high cope the fierce effulgence shook

arrives at the position c: when, however, the globe has Of parting Summer, a serener blue,

turned half-round on its axis, we find the place in question With golden light enlivened, wide invests The happy world. Attempered suns arise,

at a, where it is shrouded in darkness. These effects, it Sweet-beamed, and shedding oft through lucid clouds will be perceived, are brought about by the motion of the A pleasing calm; while broad, and brown, below

globe on its axis. Furthermore, we see that the North Extensive harvests bang the heavy head.

Pole has constant day, and the South Pole constant night; Again, when the Sun is in the sign Capricorn, the inha- for, as the globe revolves upon Ns, L N must always be in bitants of the line called the tropic of Capricorn have their the light, and s p in the shade. Again, suppose the three noon-day Sun vertical. For ourselves and the northern lines A A, E Q, B 6, to be the parallels of latitude of certain hemisphere, it may be remarked that at this season, when places; at A, E, and B, it is midnight,-at c, at the centre, the cheerless empire of the sky

and at c, it is sunrise; and at a, Q, and 6, it is mid-day. To Capricorn the Centaur Archer yields,

As our figure represents the time of the Summer solstice, And fierce Aquarius stains th' inverted year;

we see that, at the North Pole, there is no night; at the Hung o'er the farthest verge of Heaven, the Sun

tropic of Cancer, there is a short night and long day; at Scarce spreads through ether the dejected day.

the equator, the day and night are equal, as is always the Faint are his gleams, and ineffectual shoot His straggling rays, in horizontal lines,

case ; at the tropic of Capricorn the night is long, and the Through the thick air ; as clothed in cloudy storm,

day short, and at the South Pole there is continual night. Weak, wan, and broad, he skirts the southern sky;

We will now take the case of the Winter solstice, wben And, soon descending to the long dark night,

the former positions will be reversed, as we see in the Wide-shading all, the prostrate world resigns.

accompanying figure. The globe, which is illuminated as If, in astronomical works or almanacs, the reader should meet with such expressions as “the Moon is in Aries," or

Fig. 25. “a conjunction will take place in Libra," &c., he will at once understand that he must look for this event in the

NR cluster of stars forming such a constellation, although he may feel quite certain that the Sun, or Moon, or planets, are much nearer to him than the constellations to which he refers them as a background, and to which they seem, at the time, to belong.

L

E

b

B

WINTER SOLSTICE.

N

DAY AND NIGHT. We will now explain the causes of the different amount of light which we receive at different hours of the day, and at different seasons of the year. Fig. 23.

to one hemisphere by the Sun at o, revolves on its axis us, so that I n is constantly in the dark, and sd constantly in the light. The places on the northern parallel a a. have a short night and long day; at E Q they have the day and night equal; and at the southern parallel b B, a long day and a short night; at s there is constant day. At the equinoxes, the real horizon passes through the poles.

Nov, the surface of the Earth, to a spectator standing on it, does not seem to form part of a globe, because the cur

rature is so very gradual: it seems to be a flat surface, the S

boundary of which is called the visible horizon: it follows,

therefore, that every different spot on the Earth's surface The Sun is the great source of light to the solar system ; and without him, all would be dark and invisible: we must rise, we mean that the Sun first makes his appearance

has a different horizon. When, therefore, we speak of suntherefore direct our special attention to the position of the above the horizon of the place we are in. If, then, each Sun, in order to understand the changes from light to separate place has a different horizon, it follows that tee darkness.

absolute time of sunrise is different

at the different places

Fig. 26.

R

P

In the above figure, ABC is a segment of the Earth's portions of the atmosphere a power of refracting rays of

surface. The horizon of A is in the direction PA; that of B light more than the upper portions. t in the direction Q B; and that of c in the direction CR. The atmospheric air, then, is most dense at the surface of

it is Earth ; and power , evident, by inspection, that the people at a must see him when he shall have arrived at p; that the inhabitants at B disturb the direct progress of a ray of light. When a ray must wait till he shall have arrived up at Q; and that the falls upon a medium of varying density, such as the air, it people at c will only begin to see him at R; or, more will be turned gradually out of its path so as to form a strictly speaking, the Earth revolves in the direction CBA, curved line, as shown in the figure, where we have the and the horizon of each place rolls towards the Sun. Earth surrounded by an atmosphere of varying density, the

density at c being greatest, and least at A. Now, if h h'

be the horizon of an observer, and s be the Sun actually REFRACTION.

below his horizon, the ray s r will proceed in a right line

until it meets with the atmosphere A, when it will begin to From what has been stated, and from the circumstance turn out of its previous path; and were the refracting methat the Sun is the source of light to the Earth, it would dium of the same density throughout, the new path of the seem that the moment the Sun set, or became concealed ray would be in a right line also; but not in the same direcfrom us, we should at once be immersed in darkness; a tion as the first. But, as we have seen that the density of transition so sudden, that the presence of the stars, or even the refracting medium increases as we approach the Earth, of the Moon, would not be sufficient to recompense us for the ray is consequently more refracted on entering the the deprivation of the Sun's light. All this would occur, strata B and c, so that an observer at o receives this refracted if there were no atmosphere surrounding the Earth. The ray in a curved line; but, as he can only see an object in atmosphere, which supplies us with the means of respira- straight lines, it follows that he seems to receive the Sun's tion, and with a medium for the communication of sound, rays at rl, and that an image of the Sun is seen by him which supports combustion, which is an aërial ocean in at's' which birds can float,—this same atmosphere, wrapped Hence it is inferred that the heavenly bodies are never round us like an invisible garment, serves to soften the seen in their true places, except when in the zenith; intensity of the noon-day heat of the Sun, to gire him his refraction having no effect on a vertical ray. Refraction is ruddy colour at rising and setting, and also io furnish us greatest at the horizon, and decreases upwards, as may be with that subdued light which, under the name of twilight, noticed in the case of the star a, which is seen at a' by an moderates the change from light to darkness, and prepares observer at o'; but, being higher up, its rays are not so ns by degrees for the still, cold, calmness of ni ht. We much refracted as in the former case. The altitude of the must explain how this occurs.

heavenly bodies seems greater, therefore, than it really is, in When a ray of light is passing through a transparent proportion as we come down to the horizon, where the air is body, it moves in a straight line, so long as the medium thicker and more humid. The effects of refraction were remains of the same density ; but if any change occur in first noticed by Ptolemy, the Egyptian astronomer, about the density, or compactness, of this medium, the rays | the year A. D. 140. become bent out of the straight line, the bending taking While the Sun is within 18° of the horizon, or for an place towards the denser portion : this bending is called hour and twelve minutes before sunrise and after sunset, refraction. Now, the atmosphere which surrounds the we have some light from the Sun; and for many nights Earth consists of a light elastic fluid, not gögth part so heavy about the time of the Summer solstice, the Sun being as water; so that the upper portions press on those which within 18° of the horizon, we have no actual night. are lower, and make them more dense or compact than they It follows, therefore, curiously as it may seem, that, when otherwise would be; this compression gives to the lower I we seem to see the Sun just above the horizon at sunrise

[graphic][subsumed][subsumed][subsumed][subsumed][ocr errors]

and sunset, he is not actually above the horizon, but only nearest to the Moon, in consequence of her attraction of scems so, in consequence of the bending round of his rays them, and that they would thus be drawn up into a protatowards the parts where we may be. Again, the tickness berance by which an additional quantity of water would be of the stratum of horizontal air makes the Sun and Moon collected on the side b, which could only occur by a depriappear larger than when their rays come down to us less vation of the proper quantity elsewhere, which is indicated obliquely; and, owing to the difference of the density of by the diminution of the width of the aqueous ring at the the air at the upper and lower parts of the disks of the other parts, and chiefly at a. Sun and Moon, these bodies are oiten seen of a compressed or oval form.

Fig. 28.

[ocr errors]

2

TIDES.

[ocr errors]
[ocr errors]
[ocr errors]

enormous.

We must now recur to the consideration of the direct influence of the Moon upon our globe, and give our attention to the third class of phenomena which we enumerated in the former paper, as resulting from the joint operation of the Sun, Moon, and Earth-the tides.

In our first paper we stated that one of the properties of matter is to attract other matter towards it: for example, But here, against all this, we have a remarkable circumthat a stone drops to the Earth, by virtue of the Earth stance to notice. We actually find in nature that not only attracting the stone; that the planets retain their wonted is the water nearest to the Moon drawn towards it in the distances from one another, and from the Sun, by virtue way just described, and is therefore elevated, but the partly of the operation of this law.

water on the opposite side of the Moon is elevated in like Now it will not be difficult to conceive that, if attraction manner. Now it is plain that the Earth itself, the solid part has reference to all matter, and if the Earth and Moon (especially in a central direction,) must feel the attractive attract each other reciprocally, the Moon's attraction, if not or drawing influence of the Moon in a greater degree than felt to influence the solid matter of the Earth, may produce the portion of water which is most reinote from the Moon. a great effect on that form of matter which is most easily The real form of the liquid surface therefore becomes somemoved, i. e., the liquid form. Now, if we exert an attrac- thing like that of an egg, the small ends being formed by tive or moving power of any kind, on a solid mass, such as the protuberant water at each extremity, and the flattened of iron, the whole mass will be moved at once, if any part be sides being due to the diminution of the quantity of liquid moved; because ihe particles of which it is composed are so which would otherwise belong to them: the real nature of closely and intimately bound together, that a pressing or this rise of the waters is therefore better represented by pulling force will not separate them; unless indeed it be very the annexed figures, where it will be seen the waters are

With a liquid, however, we know that such is collected in greatest quantity at the sides nearest to, and not the case. If we move the finger rapidly to and fro in farthest from, the Moon, that is, at the zenith and nadir. a basin full of water, we shall separate some of the water In fig. 29 the Moon is new, or in conjunction; in fig. 30 from the remainder, and propel it over the edge of the the Moon is full, or in opposition. Hence the same basin; we can dip a vessel into the water, and take out a

effects are produced twice in the lunar month-at new portion of it, without altering the nature or appearance of and full moon. that which remains. A solid body may be immersed in We have hitherto considered the action of the Moon only the liquid, so as to be completely surrounded by it, thus on the waters of the Earth; we must now refer to that of showing the ease with which the bulk of water may be the Sun. Although the Sun is so much larger than the separated, so as to allow other bodies to insinuate themselves Moon, yet his distance exceeds that of the Moon in a still between its particles. Nothing of this kind happens with greater ratio; so that his specific attraction for the waters of solid bodies, and this difference is the sole cause of the the Earth is less than that of the Moon; the attractive production of Tides, which we proceed to explain.

force of the Sun being to that of the Moon (as is generaliy The reason why the Earth revolves round the Sun, considered) 1 to 5. We have therefore to add, or to subtract, instead of round any of the planets, is, that the large mass the Sun's attraction, according as it assists or opposes that of the Sun exerts a stronger attraction than any of the of the Moon. Suppose (fig. 31.) that the Moon is in quadplanets. Again, the reason why the Moon revolves round rature; that is, in the first or last quarter. In this state, ihe Earth, is, that she is much nearer to the Earth than

as before explained, the Sun s, the Earth E, and the to any other planet; while at the same time she is much Moon m, form a right-angled triangle. The Sun acts on smaller than the Earth.

a part of the Earth 6, which is a quarter of a circle removed The distance between the Earth and the Moon is regu- from the spot a, on which the Moon acts. In fact the Sun lated by the amount of the attraction of one for the other, and acts precisely on the part depressed through the Moon's by the velocity of the Moon's motion; and as those quantities action. As the Moon, therefore, would cause the waters to are equal year after year, there is no perceptible disturbance rise at the two parts a and a' in the proportion of 5, while exerted by one on the other. This is, however, only true the Sun's intluence at b and b' is in the proportion of l, with reference to the Earth, considered as a whole: we

we have by these means four protuberances instead of two. must bear in mind that three-fourths of her surface is covered We have represented these as separated by indentations, with water, a substance more likely to be influenced by for the sake of clearer illustration. Hence by subtracting an attractive force than a solid body, owing to the ease with the Sun's effect 1 from the Moon's effect 5, we bare which the particles of water glide over one another. It is remaining 4, which expresses the highest tide in this posifound that, although the Moon's attraction of the Earth, as tion of the Sun, Moon, and Earth. a whole, is neutralized by the counteracting attraction of But now suppose again that the moon is either full or the Earth for the Moon; yet the waters on the surface of new: the three bodies in question are then in a line, and the Earth, hanging loosely together as it were, suffer a per- the action of both Sun and Moon will be concentrated to ceptible inclination towards that quarter where the Moon the same points, and the rise will be greater than if the may be situated.

Sun had not been present.

It matters but little whether Let us suppose, while discussing the Theory of the Tides, the Moon be new, as at m, fig. 29, or full, as at m, fig. 30; that the whole of the exterior of the Earth consists of water. because in either case the effect is to raise the waters at This will make our illustrations clearer. We will also first aa', and to depress them at bb'; for the attractive force of suppose that there is no Moon to act on this watery surface the two bodies, when the Moon is new, operates most at of the Earth; then the waters would maintain their wonted

a, and least at a'-partially at 6 and 6'. In the sinking level, which we may represent by the shaded ring ab sur- down of the water at and ', the greater portion is carried rounding the Earth E. (No. 1, Fig. 28.) But suppose now off towards a, and a smaller portion goes round to help to (No. 2) the Moon, m, to exert her influence on the surface of raise the parts at a", where the water is least affected by the Earth; then we must suppose that the waters on the the attracting bodies. The like reasoning may be used in Earth's surface would tlow towards the side 6 of the Earth reference to the full Moon (fig. 30,) where the rise at a ani

a' is followed by the subsidence at b and b'. * It was in consequence of this effect of refraction that on the 20th of April, last year, the Moon rose eclipsed, before the Sun had

It should, however, be observed in addition, that the Bet, which, but for refraction, could

not possibly have happened; curious circumstance of high water at the zenith and nadir or the three bodies would not have been in a line.

at the same time, has been accounted for by reference to + See Suturduy Magazine, Vol. XII. p. 126.

the centrifugal force of the water of the Earth; which force

[merged small][ocr errors][ocr errors][merged small][merged small][merged small][ocr errors][merged small]

is generated by the rotation of the Earth on its axis, and is Earth, the Earth must make rather more than one revolugreatest at the contrary side of the Earth to the Moon: tion on her axis, before two ebbs and two flows can be so that high water at the side nearest to the Moon is due to completed : hence high water is about three-quarters of an her positive attraction; but on the side farthest from the hour later on each succeeding day. The tides at new and Moon to uninterrupted centrifugal force.

full moon, which are grea'er than those when the Moon is These fluctuations of the waters of the ocean constitute in quadrature, are called spring tides, and the latter neap Tides. As the Earth revolves on its axis the protuberanttides. The tides suffer a variation in amount from another waters travel in the opposite direction, being chietly influ- cause. We have seen that the Moon revolves in an ellipenced by the Moon; and to a certain extent by the Sun. tical orbit; so that at one time she is nearer to the Earth The Moon comes to the south of us every day later than on than at another time. Now if the spring tide happen when the preceding day and the time of flux or high tide at the Moon is at her shortest distance from the Earth, the any place is also later every day than on the preceding, in tide will be greater from that cause also than if she were the same proportion; which is a sufficient indication of the more remote; and if at the same time the Earth be at its superior attraction of the Moon. As there are two opposite nearest distance from the Sun at the time of spring tide, protuberances, or high tides, on the surface of the Earth at the time will then be the greatest that can occur. one time, it follows that, in its turning once on its axis, It is not, however, high water at the parts immediately erery part of the Earth experiences two high tides and two under the Moon: the attraction of the Moon takes time to low tides intermediate between them. These alternations in produce its full effect; so that three hours elapse, even in the open sea are at intervals of about six hours asunder; the open ocean, before the completion of the highest tide: but in rivers great irregularities occur; where, very that is, if the Moon came over any given spot at twelve generally, owing to the impediments of the banks, the tide o'clock, it would not be high water there until three. An rises higher than in the open seas. In order to supply the analogous circumstance is observed in the hottest month additional quantity of water to those two portions of the of the year, and the hottest hour of the day. The Sun Earth which are in a line with the centres of the Moon and sheds most heat on our portion of the Earth in June; yet we Earth, the rivers and seas on each side must part with a find July and August to be hotter: again, he sheds more portion of their waters, which is returned to them about six heat at twelve o'clock than at any other hour; yet we find hours afterwards; but this flux and retlux, or ebb and flow, one and two o'clock to be hotter. This arises from the fact is much retarded by the nature of the channels through that the Sun's influence is felt as if it were accumulated, which the water has to flow; so that high tide occurs at very after his greatest direct effect is passed, by reason that the various times in parts not much distant from each other. earth is already saturated with heat. In like manner, the

If the Moon were stationary, an ebb and flow would occur spring tide is not until about three days after new, or full every twelve hours; but as the Moon revolves round the moon; and the greatest spring tide not until February and

[graphic]
[ocr errors]

9PRING TIDE AT THE FULL MOON.

[merged small][merged small][ocr errors][merged small]

October. The Moon's effect is felt, in fact, after it has ceased In the subjoined figure, we have a representation of the to operate, on the principle in Mechanics, that the effect of terrestrial globe. The North and South Poles, P and Q, are a force continues even after the motive power has subsided. the extremities of the imaginary axis; or those points

When the Moon is above the horizon more than twelve which do not revolve at all. The equator, E MBR, is equihours in one day, one of the tides is higher at that place than distant from these points. Lines parallel to the equator the other on that day, because the Moon is acting upon that are called parallels of latitude ; of which as many niay be spot for a longer period: but if the Moon be above and drawn as we choose; but it is usual in maps and globes below the horizon for about equal periods, the two tides are to draw them through every 10° northward and southward : also about equal. The tides are very unequal at different the space between every two parallels being then divided places, chiefly on account of the nature of the channel in into ten parts, each of which is a degree. From P or Q to which the water is confined. At Plymouth the difference B or R are reckoned 90°, which is one quadrant of the globe. between the highest and lowest tide, on the same day, is sometimes twenty-one feet; while at Mount St. Michael it is sometimes eighty or ninety feet.

As was observed in the second paper on Navigation, the ancients, whose naval pursuits were long limited to the Mediterranean Sea, were ignorant of tides; this sea, as likewise the Baltic, not being apparently so affected by the solar and lunar intluence, as to exhibit an ebbing and flowing of their waters. These seas have inlets so narrow, as not to allow of a free passage of the water; and the attractive influence has passed away before any very sensible effect is produced. In the case of these seas, and also of lakes, the Moon's influence is equally exerted ; and, there being no swell, is less perceptible. Though we learn that the cause of the tides was first suspected by Pytheas, a navigator, who flourished 400 B. c.*, yet Newton was the first to demonstrate that the tides were the result of the attraction of the Sun and Moon; and thus did he add another to the long list of benefits which he conferred on posterity, by explaining those sublime and beautiful laws, according to which it has been ordained that the world shall be governed, by Him who

It is usual for the principal nations in the world, which Gives and guides the Sun's attractive force,

have cultivated science, and made themselves respectable in And steers the planets in their silver course;

the opinion of fellow-nations, to reckon their longitude from With heat and light revives the golden day,

the meridian of their own capital. Thus, the English And breathes his spirit on organic clay;

reckon longitude from the meridian of Greenwich, which With hand unseen directs the general cause,

is, as it were, part of London, and is the site of the royal By firm, immutable, immortal laws.

observatory. The French count degrees of longitude from Air, being lighter than water, and being likewise nearer to the meridian of Paris. the moon, must be more within her influence: hence there

Longitude is reckoned eastward and westward; and the are doubtless aërial tides; of which if the nature were meridians are usually drawn on globes and maps 15o apart, well understood, the subject of Meteorology and of the because the Sun passes over this extent of the Earth's weather generally, would be more decidedly a science than surface in one hour; or all round the globe in twenty-four it now is.

hours: so that there are twenty-four meridians, or lines of LATITUDE AND LONGITUDE.

longitude, such as we see drawn from p to Q, where the line

PA Q is the meridian of A. Each degree of longitude is, For the origin of the terms latitude and longitude, we refer at the equator, about sixty-nine and a half English miles ; our readers to Vol. XII. page 208.

and each degree, becomes less as we recede from the In the modern acceptation of the terms, latitude is the equator towards the poles, in consequence of the circles distance of any place north or south from the equator; getting smaller. and longitude its distance east or west of any given meridian. Thus, the line of latitude is always at right angles to the line of longitude; and the spot, denoted by orbits are exterior to the orbit of the Earth.

In our next paper, we shall describe those planets whose a certain latitude and longitude, is situated where these lines cross each other. This mode of marking out the position of a place on the surface of the Earth, is referable

LONDON: to the motion of the Eartla on its axis; which motion is the JOHN WILLIAM PARKER, WEST STRAND. most constant and equable in nature.

PUBLISHED IN WEEKLY NUMBERE, PRICE ONE PENNY, AND IN MONTALT PASTE * See Saturday Magazine, Vol. XII., p. 204.

Sold by all Booksellers and Newsreuders in the Kingdom.

[graphic]

PRICE SIXPEXCE.

« ZurückWeiter »