« ΠροηγούμενηΣυνέχεια »
great circle passing through the poles of another For the methods of calculating the latitude is called its secondary. That vertical circle, and longitude, see LATITUDE and LONGITUDE. which has its plane perpendicular to the plane Sect. III. OF THE DIFFERENT POSITIONS OP of the meridian, is called the prime vertical. The meridian and prime vertical, by their intersec
THE SPHERE, THE Zones, CLIMATES, &c. tions with the horizon, divide it into four equal If we suppose an inhabitant of the earth parts: the points of their intersection are called living at either of the poles, he will there have the cardinal points. The meridian cuts the ho- one of the celestial poles always in his zenith rizon at right angles in the north and south and the other in his nadir, the equator coinciding points, and the prime vertical cuts it at right an- with the horizon. Hence all the celestial paralgles in the east and west points. Lesser circles lels are also parallel to the horizon; and hence of the sphere parallel to the horizon are called a person, or people, are said to live in a parallel parallels of altitude or olmacantar.
sphere, or to have a parallel horizon. The armillary sphere was a machine formerly Those who live under the equator have both in use, which represented the principal circles poles in the horizon, all the celestial parallels above described, the poles of the earth, &c.; but cutting the horizon at right angles; whence they since globes which contain all those circles have are said to live in a right sphere, or to have a been more general, this machine is become ex- right horizon. ploded.
Those who live between either of the poles The degrees of longitude are not equal like and the equator are said to live in an oblique those of latitude, but diminish in proportion as sphere, or to have an oblique horizon; because the meridians incline, or their distance contracts the celestial equator cuts their horizon obliquely, as they approach the pole. Thus in 60° of lati- and all the parallels in the celestial sphere have tude, a degree of longitude is but half the length their planes oblique to that of the horizon. In of a degree of the equator. We therefore here this sphere some of the parallels intersect the add
horizon at oblique angles, some are entirely A Table, showing the Number of Miles con- above it, and some entirely below it; all of tained in a Degree of Longitude in each Parallel them, however, so situated, that they would of Latitude from the Equator.
obliquely intersect the plane of the horizon extended.
The largest parallel which appears entire above the horizon of any place, in north latitude, is called by ancient astronomers the arctic circle of that place. Within this circle, i. e. between it and the arctic pole, are comprehended all the stars which never set in that place, but are carried perpetually round the horizon, in circles parallel to the equator.
The largest parallel which is hid entirely be1 59 96 31 51 43 61 29 04
low the horizon of any place, in north latitude, 2 59
was called the antarctic circle of that place by 3 59 92 33 50 32 63 27 24
the ancients. This circle comprehends all the 4 59 86 34 49 74 64 26 30
stars which never rise in that place, but are 5 59 77 35 49 15 65 25 36
carried perpetually round below the horizon, in 6 59 67 36 48 54 66 24
circles parallel to the equator. 7 59 56 37 47 92 67 23 45 In a parallel sphere, however, the equator 8 59 40 38 47 28 68 22 48
may be considered as both arctic and antarctic 9 59 20 39 46 62 69 21 51
circles; for, being coincident with the horizon, 10 59 08 40 46 00 70 20 52 all the parallels on one side are entirely above 11 58 89 41 45 28 71
it, and those on the other entirely below it. In 12 58 68 42 44 95 72
55 an oblique sphere, the nearer any place is to 13 58 46 43 43 88 73 17 54
either of the poles the larger are the arctic and 14 58 22
16 74 16 53
antarctic circles, as being nearer to the celestial 15 58 00 45 42 43 75 15 52 equator, which is a great circle.
In a right 16 57 60 46 41 68 76 14 51 sphere, the arctic and antarctic circles have no 17 57
30 47 41 00 77 13 50 place, because no parallel appears either entirely 18 57 04 48 40 15 78 12
above or below it. 1956 73 49 39 36 79 11 45
By the ancients the arctic circle was called 20 56 38 50 38 57 80 10 42 maximus semper apparentium, and circulus per21 56 00 51 37 73 81 09 38 petuæ apparationis; the antarctic circle, on the 22 55 63 52 37
82 08 35
other hand, being named maximus semper oc23 55 23 53 36 18 83 07 32 cultorum, and circulus perpetuæ occultationis. 241 54 81 26 84 06
By the arctic and antarctic circles, however, 25 54 33 55 34 41 85 05 23 modern geographers in general understand two 26 54
56 33 55 86 04 18 fixed circles, at the distance of 23° 30' from the 27 53 44 57 32 67 87 03 14 poles. These mark out the space all round the 28 58
02 09 globe where the sun appears to touch the horizon 29 52 48 59 30
at midnight in mid-summer, and to be entirely 30 51 96 60 30 00 90 00 00
sunk below it in winter.
According to the different positions of the
Degrees of Latitude.
Degices of Latitude.
100th parts of a mile.
Degrees of Latitude.
100th parts of a mile.
og 100th parts of a mile.
8 | Miles.
globe, with regard to the sun, the celestial bodies his apparent annual revolution never removes exhibit different phenomena to the inhabitants. farther from the equator than 23° 30', none of Thus, in a parallel sphere, they appear to move those who live without that space, or beyond in circles round the horizou; in a right sphere the tropics, can have that luminary vertical to they appear to rise and set as at present, but them at any season of the year. 2. All who always in circles cutting the horizon at right live between the tropics have the sun vertical angles; but, in an oblique sphere, the angle twice a year, though not all at the same time. varies according to the degree of obliquity, and Thus, to those who live directly under the equathe position of the axis of the sphere with re- tor, he is directly vertical in March and Sepgard to the sun. The phenomena thence arising tember at the equinox. If a place is in 10° norih will be sufficiently understood from what is said latitude, the sun is vertical when he has 10° under the article ASTRONOMY.
north declination; and so of every other place. The space between the two tropics, called the 3. All who live between the tropics have the torrid zone, extends 47° of latitude all round sun at noon sometimes north and sometimes the globe; and throughout the whole of that south of them. Thus, they who live in a place space the sun is vertical to some of the inbabisituated in 20° north latitude, have the sun at tants twice a year, but to those who live directly woon to the northward when he has more than under the tropics only once. Throughout the 20° north declination, and to the southward whole torrid zone also there is little difference when he has less. 4. Such of the inhabitants between the length of days and nights. The of the earth as live without the tropics, if in the ancient geographers found themselves consider- northern hemisphere, have the sun at noon to ably embarrassed in their attempts to fix the the south of them; hut to the north, if in the northern tropic; for though they took a very southern hemisphere. 5. When the sun is in proper method, namely, to observe the most the zenith of any place, the shadow of a man or northerly place where objects had no shadow on any upright object, falls directly upon the place a certain day, yet they found that on the same where they stand, and consequently is invisible; day po shadow was cast for a space of no less whence the inhabitants of such places were than 300 stadia. The reason of this was, the called Ascii, or without shadows. . Those who apparent diaineter of the sun; which, being live between the tropics, and have the sun someabout half a degree, seemed to extend himself times to the north and sometimes to the south over as much of the surface of the earth, and to of them, have of consequence their shadows be vertical every where within that space. projecting north at some seasons of the year
The division of the earth into zones has arisen and south at others; whence they were called from the various appearances of the sun, and Amphiscii, or having two kinds of shadows. the effects of his light and heat upon different They who live without the tropics have their parts of it. These are five in number: 1. The noon shadows always the same way; and are torrid zone lying between the two tropics for a therefore called Heteroscii, that is, having only space of 470 of latitude. This is divided into one kind of shadow. If they are in north latitwo equal parts by the equator; and the inhabi- tude the shadows are always turned towards the tants have the sun vertical to them twice a year, north, and, if in the southern hemisphere, toexcepting only those who dwell under the tropics, wards the south. When a place is so far distant to whom he is vertical only once. 2. The two from the equator that the days are twenty-four temperute zones lie between the polar circles and hours long or longer, the inhabitants were called the tropics, containing a space of 43° of latitude. Periscii, because their shadows turn round And, 3, The two frigid zones lie between the them. polar circles and the poles. In these last the Names have likewise been given the inhabilongest day is never below twenty-four hours; tants of different parts of the earth, from the in the temperate zones it is never quite so much, parallels of latitude under which they live, and and in the torrid zone it is never above fourteen. their situation with regard to one another. The zones are named from the degree of heat Thus, when two places are so near each other that they were supposed to be subjected to. The the inhabitants have only one horizon, or at least torrid zone was supposed by the ancients to be that there is no perceptible difference between uninhabitable by reason of its heat; but this is them, the inhabitants were called Synoeci, that now found to be a mistake, and many parts of is, near neighbours ; the seasons, days, nights, the temperate zones are more intolerable in this &c., in both places being perfectly alike. Those respect than the torrid zone itself. Towards the who lived at distant places, but under the same polar circles, also, these zones are intolerably parallel, were called Periceci, that is, living in cold during winter. Only a small part of the the same circle. Those who are on the same northern frigid zone, and none of the southern, side of the equator have the seasons of the year is inhabited. Some geographers reckoned six at the same time; but, if on different sides, the zones, dividing the torrid zone into two by the summer season of the one is the winter of the equator.
other; as explained under Astronomy. Some From the difference in the length and positions writers, bowever, by the name of Peræici, disof the shadows of terrestrial substances, ancient tinguish those who live under opposite points of geographers have given different names to the the same parallel, where the noon of the one is inhabitants of certain places of the earth; the the midnight of the other. When two places reason of which will be easily understood from lie under parallels equally distant from the the following considerations : -1. As the sun in ecuator, but in opposite hemispheres, the inha
bitants were called Anteci. These have a si- Sect. IV.- THE METHOD OF FINDING THE milar increase of days and nights, and similar LENGTH OF THE DAY, AND THE BEGINNING AND seasons, but in opposite months. According to ENDING OF THE TWILIGHT. some, the Antæci were such as lived under the
This has been represented mathematically, same geographical meridian, and had day and
thus : night at the same time. If two places are in parallels equally distant from the equator, and in opposite meridians, the inhabitants are called Antichthones with respect to one another, that is, living on opposite sides of the earth; or Antipodes, having their feet opposite to one another. When two persons are Antipodes, the zenith of the one is the nadir of the other. They have a like elevation of the pole, but it is of different poles : they have also days and nights alike, and similar seasons of the year; but they have opposite hours of the day and night, as well as seasons of the year. Thus, when it is mid-day with us, it is midnight with our Antipodes; when it is summer, with us, it is winter with them, &c. From observing the diversity in the length of
Let PZES of the diagram represent the cethe days and niglits, the rising and setting of the lestial meridian of any place, P and S being the sun, with the other phenomena already mentioned, poles of the sphere; let E Q be the equator and ancient geographers divided the surface of thé H O R the horizon, stereographically projected earth into certain districts which they called Cli- upon the plane of the ineridian; let PCS reNATES (see our article of that title); and instead present the six o'clock hour circle, and m (On the of the method of describing the situation of parallel of declination described by the sun or a places by their latitude and longitude, as we do star at any given day of the year; the point O now, they contented themselves with mentioning being that in which it cuts the horizon; then no the climate in which they were situated. When represents half the arch described by the sun more accuracy was required they mentioned also when above the horizon, and 0 m the half of the the beginning, middle, and ending of the cli- arch described when below the horizon. Let P mates.
OS represent an hour circle passing through the This distinction, however, was vague and inac- sun or star when in the horizon, and meeting the curate: for the only method they had of deter- equator E Q in A; the arch E A of the equator mining the difference was by the length of the intercepted between the meridian and hour cirday; and a climate, according to them, was such cle, being found, and converted into time (allowa space as had the day in its most northerly part ing fifteen degrees to an hour), will evidently half an hour longer than iu the most southerly. give half the time that the sun or star remains For the beginning of their first climate they took above the horizon, as the arch A Q will give half that parallel under which the day is twelve hours the time it remains below the horizon. As the and three-quarters long: those parts of the world arch EC contains ninety degrees, and correwhich lie nearer the equator not heing supposed sponds to six hours, it is only necessary to find to be in any climate, either because in a loose the arch CA, which is called the sun's ascensense they may be considered as in a right sphere, sional difference, it being the difference between or because they were unknown, or thought to be his right ascension and his oblique ascension; uninhabitable by reason of the heat. The prin- and, having converted it into time, to add it to cipal ancient climates are mentioned in the article or subtract it from six hours, according as the above referred to.
latitude of the place and sun's declination are of A parallel was said to pass through the mid- the same or of contrary names, that is both north dle of a climate when the day under that parallel or both south, or the one north and the other is a quarter of an hour longer than that which south, and the sum or difference sball be half the passes through the most sontherly part. Hence length of the day as required. it does not divide the space into two equal parts, In the spherical triangle CAO, right angled but that part next the equator will always be the at A, we have A O the complement of the sun's larger of the two; because, the farther we recede declination, to be found from astronomical tables, from that circle, the less increase of latitude will and the angle A CO, the complement of the be sufficient to lengthen the day a quarter of an latitude of the place, in order to find AC hour. Thus, in every climate there are three pa- the right ascension. Hence, from the princirallels; one marking the beginning, the second ples of spherics, we have the following proporthe middle, and the third the ending of the cli- tion :mate; the ending of one being always the begin- As radius to the tangent of the latitude, so ning of another. Some of the ancients divided is the tangent of the sun's declination to the sire the earth by these parallels ; others by a of the sun's ascensional difference required. parallel did not mean a mere line, but a space When the sun is in the same hemisphere with of some breadth: and hence the parallel may any place, and his declination is equal to the he understood as the same with half a cli- complement of its latitude, which can only hapmate.
pen to plares in the polar circles, then mn, the parallel of declination, will not cut the horizon, straight, and the former equidistant from each and consequently the sun will not set in those other. The degrees of longitude in every paralplaces during the time his declination exceeds the lel are the same; while the degrees of latitude co-latitude; but when the sun and place are in are all unequal, being lengthened towards the opposite hemispheres, then he will never rise at poles. Charts drawn on this construction are that place so long as his declination exceeds the particularly of use to navigators, because the co-latitude; and hence it is easy to see how to rhumbs, which point out the bearings of places, find the time when the sun begins to shine con- and consequently the courses to be steered to art stantly upon any given place within the polar rive at them, are all straight lines. See our charcircle; and also the time when that place begins of the world on this projection. to be wholly in the dark for a considerable time In maps of particular countries the top is getogether.
nerally considered as the north, the bottom as the It has been observed, in our article Astrono- south; and the east is consequently on the right My, that the twilight commences in the morning hand, and the west on the left. Where this rule and ends in the evening when the sun is eighteen is not followed a fleur-de-lis is usually placed on degrees below the horizon. The time of its com- some part of the map, pointing towards the north, mencement, or ending, may be found by spheri- by which the other points may be easily known. cal trigonometry as follows :- Let 7 be the From the top to the bottom of the map are zenith, P the pole of the sphere, and T the place drawn meridians, or lines of longitude; and from of the sun, eighteen degrees below the horizon H side to side parallels of latitude. The outermost R. In the spherical triangle, PZT, we have P of the meridians and parallels are marked with Z the distance of the pole from the zenith, which degrees of latitude and longitude, by means of is equal to the co-latitude of the place, and PT which, and the scale of miles commonly placed the complement of the sun's declination; also in the corner of the map, the situation, distance, ZT the distance of the sun from the zenith, &c., of places may be found. Thus, to find the which, in this case, is always 90° +18° or 108° distance of two places, suppose London and From these we are to find the hour angle Z PT, Paris, by the map, we have only to measure the which may be had by the following proportion. space between them with the compasses, and to Let V=} the perimeter of the triangle. Then, apply this distance to the scale of miles, which as sine Z Px sine PT to the square of the radius, shows that London is 210 miles distant from so is sine (V–ZP) x sine (V-PT) to the Paris. If the places lie directly north or south, square of the sine of half ZPT. The angle ZPT east or west, from each other, we have only to being turned into time will give the time from observe the degrees on the meridians and paralnoon of the beginning or ending of the twilight. lels; and by turning these into miles we obtain
the distance without measuring. Rivers are deSect. V.-OF THE CONSTRUCTION AND USE or
scribed in maps by black lines, and are wider MAPS.
towards the mouth than towards the head or A map is a representation of the earth, or a' spring. Mountains are sketched on maps as on part of it. A map of the world is a delineation a picture. Forests and woods are represented by in perspective of the globe, as it would appear to a kind of shrub; bogs and morasses by shades; an eye placed in a particular point. The circles sands and shallows are described by small dots; bounding such a map represent the brass meri- and roads usually by double lines. Near harbours dian; and the curve lines running across, at every the depth of the water is expressed by figures deten degrees, show the latitude north or south of noting fathoms. the equator. The top and bottom are the north Having discovered by maps, or any other way, and south poles; and the curve lines uniting the true situation of the different places of the them are other meridians passing through every earth with regard to each other, we may easily tenth degree of the equator, and showing the know many other particulars relative to them; longitude east or west from the first meridian. as, their distance from us, the hour of the day, The straight line intersecting these meridians, the season of the year, &c., at any particular and passing through the centre, is the equator or place. As each of these problems, however, equinoctial; at proper distances from which, on would require a particular and sometimes troueach side, are curve lines representing the tropics blesome calculation, machines have been inand polar circles.
vented, hy which all the calculations may be Maps and charts, especially the latter, are saved, and every problem in geography may be sometimes drawn on what is called Mercator's solved mechanically, and in the most easy and projection, so called from the inventor, Gerard expeditious manner. These machines are the Mercator, an eminent geographer in Flanders, celestial and terrestrial globes, for the use of who, about the middle of the sixteenth century, which see GLOBE. published a map of the world on this construction. For the mechanical details of the construction In these maps the meridians and parallels are of maps, see MAPPING.
G E O L 0 G Y.
. GEOLOGY; from Gr. yn, the earth, and doyos of the earth; but, when we consider how small a discourse; is a modern science which under- portion of even the crust of the earth has as yet takes to explain the structure of the earth. Some been examined and arranged by it, we prefer the of its more decided votaries have dignified it less assuming as the more suitable appellation. with the title of 'geognosy,'- or the knowlcdge Those speculations and fictions which, under the title of theories of the earth, amused the pub- being founded on the supposed agency of water lic in the infancy, or rather before the birth of in the original modification of the earth : there this science, while they excited surprise by their is but another modern theory called the Plutoparadoxical boldness or ingenious contrivance, nian, from its supposing the agency of fire in a neither originated in observation, nor applied to similar manner, that is at all worth the stuexisting appearances; they were formed in the dent's notice. closet by men who had never probably ascended Since the system of Werner has been adopted, a mountain or explored a mine ; who had never and his disciples scattered all over Europe, examined Nature in her operations, and of course ardor has been excited, and the number of obwere not fitted to become her interpreters. The servers multiplied. Adventurous travellers, carenumeration and structure of the simple mine rying with them a precise nomenclature and rals, the arrangement of fossil bodies into moun- distinctive marks of discrimination, have visited, tain masses, or into the more level parts of the and are visiting, the most distant regions, or earth's crust, the order in which the individual serving the surface of the earth in situations the strata are placed with regard to each other, and most remote from each other, and diffusing the the relation they bear to those parts that are not results of their examination over the scientific stratified, had not with them become an object world. We need only mention as instances such of attention or enquiry. The theories or dreams names as those of baron Humboldt, and M. Von which they formed must therefore be viewed Buch. More minute surveys are daily making merely in the light of philosophical romances, or in the different parts of civilised Europe; facts ingenious works of fancy, and would apply to are thus perpetually accumulating, and collecany other planet as well as to ours. Nineteen tions increasing. In our country a taste for of them may be found in the introduction to mineralogy and geology has been created, and Mr. Accuin's Chemistry. They bear the same rapidly diffused, by the labors and fame of Mr. relation to the state and appearances of the earth Jameson, one of Werner's most eminent disciples; as the Oceana or the Utopia bear to actually ex- by those of professor Buckland of Oxford; hy the isting governments; and can no more account institution and labors of the Geological Society; for its phenomena than the fictions of enchant- and by the attractive nature of the science itself, ment can explain the events of history. and its important contributions to the arts.
This general character, which will apply to all In giving a brief notice of the present state of the attempts at forming geological systems ante- geology, the following natural division of the cedent to the last thirty or forty years, will save science will probably be found both comprehenus the trouble of enumerating those productions, sive and precise. or of pointing out their errors and defects. Within The first will contain a general description of this time the science of mineralogy has made very the surface of the earth, and an explanation of the rapid progress, and geological enquiry and clas- terms to be employed in discussions concerning it. sification have kept pace with it. The compre- The second will contain a description of its inhensive mind of Werner, upon an extensive ac- ternal structure, and a similar detail of its quaintance with the mineral kingdom, formed a nomenclature. new nomenclature and arrangement, hy which The third will embrace a geological survey of simple fossil bodies may be discriminated and our globe, with an account of the particular described.' Carrying the same inquisitive and structure, relative position, and geographical disgeneralising talents to the examination of the tribution of its various mincral masses. crust of the earth, he discovered and pointed! The fourth division will employ the knowledge out the structure of its compound masses, the thus collected, in support of some rational theory, relations of its different strata or beds to each with regard to its formation, or in overturning other, the method in which they succeed each the fabric of chimerical hypotheses. other, in ascending from the lowest level to In the present stage of the science, this last. the pinnacle of the highest mountain-group, part will contain a short view of the Huttonian and those great arrangements of them that pre- and Wernerian theories; the doctrines of which vail, with few interruptions, round the globe. respectively compose the creeds or articles of He has given names to those different formations, faith in geology, of the two sects into which the and detailed the characters by which they may mineralogists of this country are divided. be distinguished. The apparent irreg'ilarities, I. We shall first then begin with a description disorder, and contortions, which interrupt the of the earth's surface, and shall avail ourselves more general formations, have not escaped his of the phraseology of Werner, employed by Mr notice nor transcended his powers of arrangement. Jameson in his System of Mineralogy, as being These, with the veins occurring in the strata, the more precise, and more generally received, than metallic and other deposits that fill them, have any other that has hitherto been invented. all been remarked, named, and classified. As- The dry land of our globe may be, fo- the sake cending from the consideration of present ap- of precision and convenience of description, dipearances to their cause, and inferring from vided into four classes of inequalities, comprising, changes that are now in progress the past history 1. The high land and low land ; 2. Alpine land and revolutions of our globe, his disciples have and plain; 3. Mountain range and valley; 4. formed a system of geology, with regard to the Single mountain and ravine. From the soundearth's formation, more reconcileable with ob- ings taken in different seas, and in different parts served phenomena, explaining a greater body of of the ocean, the portion of the globe covered facts, and liable to fewer objections, than any with water would appear to be similar in its surprevious or contemporary theory. This theory face to that part of it which we inhabit, and to has been often calied the Neptunian, from its possess eminences and depressions as strongly