furnished by these operations, the length of the arc M m is determined in the following manner:-from M and D, draw the lines Ma perpendicular to D a, parallel with the meridian line, meeting each other in a; Db, A b, A c, B c, m d, B d, are also drawn so as to be respectively perpendicular to and parallel with the meridian. Then it is evident that the length of M m is equal to the sum of the lengths of a D, 6 A, c B, B d, which are found thus: the inclination of MD to the meridian having been already determined by an astronomical observation, the angle D Ma in the right-angled triangle D Ma is known from it, and the side M D is also known, so that D a (which is equal to M D x sin. D M a) may at once be computed by trigonometrical tables. In a similar manner the sides b A, c B, d B are computed, and the sum of the whole gives the length of the meridian arc M m, and the length of a degree is the length of the whole arc divided by the number of degrees contained in it. Picard was the first person who measured an arc of the meridian by this method. The operation was performed in the year 1670; the arc commenced near Paris, and extended northward; the result of the measurement gave, as the length of a degree in latitude 4940, 121,627 yards, which differs only 35 yards from what is now considered as the most exact length; an accuracy which is justly supposed to be quite accidental. Since this period arcs of meridian lines have been measured in various countries, as well in intermediate latitudes between the equator and the north pole, as near both the equator and the pole. The following table represents the length of a degree in different latitudes as determined by the five most approved measurements: The following particulars will show at once the accuracy which now distinguishes geodesical operations, and some of the means taken to ensure it : The first base in the English measurement, of which the result is given in the above table of degrees, was about five miles in length, and was measured upon Hounslow-heath with a steel chain of exquisite workmanship. The same base had been measured three years before by General Roy, with glass rods, and the two measurements (in a length of five miles) differed only 2 inches. The French base was measured with rods of platina, that in Lapland with rods of iron, and an allowance was made for the changes of temperature affecting the length of the rods in the course of the operation. In a previous measurement in Lapland, the French astronomers, in order to guard against the extreme contracting effect of cold upon metals, employed rods of deal; this was the more necessary in that measurement, as it was performed in the depth of winter, and the frozen surface of a river was selected for the base line, with a view to obtain as level a plain as possible. It is usual also, in order to prove the correctness of the geodesical process, to measure, towards the conclusion, what is called a We have already base of verification. stated that all the sides of the series of triangles (with the exception of the base line AB, which is a side in the first triangle) are not measured but computed: to verify all the previous steps in the process, the length of one of the sides of the triangles, as it has been deduced from computation, is compared with its length determined by actual measurement. The side of the triangle thus measured is called a base of verification, and is taken as far distant from the first base as circumstances will admit. In the French operations the base of verification was distant between four and 500 miles from the first base, and was 7 miles in length, and yet the difference between its computed length and that obtained from its actual measurement did not amount to 12 inches. From an inspection of the table before given, it appears that the length of a degree from the equator to the pole increases the curvature therefore diminishes, and the earth is not a sphere but is flattened at the poles, and the polar diameter is less than the equatorial; and although the various modern measurements may not, on a comparison one with another, agree in giving to the difference of the two diameters precisely the same value, yet they all ascertain the fact of the polar diameter being less than the equatorial, and that a degree increases towards the poles; and this establishes the oblate-spheroidal figure of the earth. The value of the compression or the fraction expressing the difference between the two diameters, as deduced from a comparison of the lengths of a meridional degree in different latitudes, determined by the most approved measurements, has been lately shown by Professor Airy, in a paper in the last volume of the Philosophical Transactions to be that is, the polar diameter is less than the equatorial by the 278.6th part of the whole diameter. 1 278.6 Operations are now being carried on, on the continent, which have for their object the more precise determination of the fraction of ellipticity, and of the compression of the earth. The measurement of an arc of the parallel of latitude 45°, of 15° or 16° in extent has been already accomplished. One extremity of this are is at Marennes, on the west coast of France, and a little to the north of the Garonne, and traversing France, Piedmont, and the northern parts of Italy, its other extremity is at Fiume, in the Austrian dominions, and on the eastern shores of the Adriatic. The value of the ellipticity as deduced from these operations is 239.8° We have already stated that the pendulum experiments give. This similarity in the results afforded by such very different kinds of investigation is a strong argument in favour of the general correctness of both. 1 The mean degree of a meridian or the degree the length of which is as much greater than that of a degree at the equator, as it is less than that of a degree at the poles, is in latitude 45°, which is the mean latitude between the equator and the poles. Its length, according to the French measurement, is 60759.4 fathoms, or 12158.8 yards. The circumference of the elliptic meridian is found by multiplying the mean degree by 360, and is equal to 24855.84 miles. The circumference of the equator is 24896.16 miles, and is not quite 41 miles longer than the elliptic meridian. The French measurement, in 1792, was undertaken with a view to obtain a standard measure of length, to serve as the basis of a new system of weights and measures. According to this new system, the unit, or first element of linear measure, is called a metre; and the metre was declared to be equal to ten millionth part of the quadrant of the meridian-which is a fixed and un alterable quantity in nature. The quadrant of the meridian was by this measurement found to be 5,130,740 toises, or 10,936,578 English yards: the French metre, or the ten millionth part of this quantity, would accordingly be 1,093,578 yards, or 39.37 inches, nearly. This method of obtaining a standard of 'measure is not, perhaps, so good as that which consists in observing the length of the pendulum, which, in a certain latitude, beats seconds of mean time. For the length of this pendulum is ultimately ascertained by a reference to the equable motion of the earth upon her axis, and is, therefore, ascertainable without the aid and use of any linear measure whatever; whereas, in the very act of determining the French standard, or the quadrant of the meridian, some linear measure already in use must be employed; and thus the very basis of their new system is expressed in terms of that in the place of which it is substituted. The importance of possessing the true length of a degree of the meridian, is not confined to investigations having for their object the determination of the figure of the earth. Upon the simple fact of the length of a degree, seemed to depend the overthrow or establishment of the theory of Universal Gravitation. The particulars connected with the discovery of a principle productive of such various effects in nature, is not the less interesting in that it illustrates the secret dependency of parts of science apparently the most distinct, and the assistance which each in its place is calculated to afford to the rest. The corner-stone of the whole system of Universal Gravitation is, that the force which causes a heavy body to descend to the surface of the earth, is the same that retains the moon in her orbit, and makes her deflect from a straight line, or bend towards the earth. All that was requisite to establish the identity of the forces by which these two effects were produced, was to prove, that the quantity of effect produced in a certain time upon the moon in thus deflecting from a straight line, (taking into consideration the law by which the force varied, and the distance of the moon,) was in due proportion to the effect produced by the force of gravity, in the same time, upon a falling body at the surface of the earth. It is evident, therefore, that the determination of this question depended upon, and would in its solution be affected by, the distance pleting it. Thus, by the aid of the true length of a degree, was finally established the grand theory of Universal Gravitation. CHAPTER XI. THE subjects embraced in the foregoing treatise, are dispersed throughout a great number of different books, and are to be met with only in detached parts. The proofs of the spherical figure of the earth, and the methods of finding the latitudes and longitudes of places, will be found in every Treatise of Astronomy; we shall, therefore, only refer to that part of Malte-Brun's work, which is devoted to Mathematical Geography; to the Nautical Almanack; Woodhouse's Astronomy, vol. i.. chapters 1, 5, 42 and 43; Brinkley s Elements of Astronomy, chapters 1, 3, 16 and 17; Playfair's Outlines of Natural Philosophy, vol. ii. part 1, chapters 1 and 4; and, as a popular work, to Bonnycastle's Astronomy, letters 2, 9 and 10. For fuller information, with respect to the true figure of the earth, and the lengths of pendulums vibrating seconds in different latitudes, and measurement and lengths of degrees, we may refer to Malte-Brun; Brinkley, chap. 17; Playfair, chap. 3, of part 1, and chap. 6, of part 2; Bonnycastle, letters 15 and 16; Newton's Principia, book 3, props. 18, 19 and 20; Maclaurin's Account of Sir I. Newton's Discoveries, book 4, chap. 6; Pemberton's View of Sir I. Newton's Philosophy, book 2, chap. 6; Earth, Rees' Cyclopædia, articles, and Degree; various Papers in the Philosophical Transactions on the Measurement of Degrees, and on Experiments upon the Pendulum; Clairaut Figure de la Terre; Quarterly Journal of Science, for March 1827, p. 177. PHYSICAL GEOGRAPHY. PHYSICAL or natural geography might, if we regarded merely the strict meaning of the words, be limited to signify no more than a description of the principal features of the earth's surface; but it is usual, in treatises upon this branch of geography, to touch also upon the subject of climate and temperature, to show how these, together with other natural causes, affect the condition of the human race and to advert, in a general manner, to the animals and productions of the globe. Geographical terms explained. In looking over a map of the world, it is seen at once that the surface consists of various spaces of land, surrounded by an extensive field of water called the sea or ocean. Of these spaces of land, two are of vast extent, and on this account are termed continents*, (derived from a Latin word signifying, holding together or connexion). The larger of these continents includes the three divisions of Europe, Asia, and Africa, and is distinguished by the title of the old continent, from its having, till the discovery of America, by Columbus in the year 1492, been the only one with the existence of which Europeans were acquainted. The other, which includes North and South America, is named the new continent. The smaller portions of land which are scattered over the ocean are denominated islands. A • New Holland, by some geographers, is regarded as a third continent; but if we consider how much smaller it is than either of the two vast tracts above mentioned, it will appear correct rather to assign it the first station among the islands of the globe. New Holland and the islands around it are, however, not unworthy of being classed as the fifth grand division of the world. English geographers have named them Australia (that is, Southern lands.) great many islands lying together are called an archipelago. In many places the land and the ocean run one into the other. When the ocean penetrates into a continent by a narrow passage, and then spreads again into a large expanse, this inland portion of the ocean is usually termed a sea. If the extent of such an inland sea be less, or the passage by which it communicates with the main ocean larger, it is called a gulf or bay. An inland body of water not connected with the ocean or any of its branches, is called a lake. A narrow passage of water leading from one sea to another is called a strait; a narrow neck of land lying between two seas, and connecting two masses of land greater than itself, is called an isthmus. When, on the out into the sea, and is joined to the other hand, a part of a continent runs main land by only a small portion of its circumference, it is named a peninsula, (that is, an almost island). If the projections of land reach but a little way into the sea, they are called headlands, or promontories. capes, General View of the Globe as consist ing of Land and Sea. There is, in fact, only one continuous fluid surrounding the land, all the gulfs and inland seas being branches of this universal ocean; but for the sake of convenience different parts of it have distinct names given to them. The following table, exhibiting the principal seas into which the ocean has been divided, will be clearly understood upon referring to the map of the world on Mercator's projection : + The Caspian Sea, as it is generally termed, forms no exception to this remark, because it is in fact only an immense lake. B I. The great South Eastern basin, the waters of which cover near ly half the globe. It includes II. The Western basin, forming a channel between the old and new continents. 1. The Antarctic Ocean, which is comprised within the Antarctic circle, that is, between the parallel of 66° 32′ of southern latitude and the South Pole. 2. The Southern Ocean, the boundary of which on one side is the Antarctic circle, on the other a line drawn from Cape Horn to the Cape of Good Hope, thence to Van Diemen's Land, and again by the south of New Zealand to Cape Horn. This line forms the southern boundary of Nos. 3 and 4. 3. The Indian Ocean, lying between Africa on the west, and the peninsula of Malaya with the islands of Sumatra, Java, &c., and New Holland, on the east, and bounded by Persia, and Hindustan on the north. The Red Sea, or Arabian Gulf, the Persian Gulf, and the Bay of Bengal are all parts of this ocean. 4. The Pacific Ocean, divided by the equator into North and South, and inclosed between America on the east, and New Holland, the islands of Java and Sumatra, and the continent of Asia, on the west. On the north it terminates at Behring's strait. The seas of China, Japan, Okhotsk, &c. form parts of this ocean. 1. The Atlantic Ocean, commencing in the south from a line drawn from Cape Horn to the Cape of Good Hope, and terminated on the north by the Arctic circle. It is divided into North and South by the equator, and its branches are the Mediterranean, the North Sea or German Ocean, the Baltic, Baffin's Bay, Hudson's Bay, the Gulf of Mexico and the Caribbean Sea. 2. The Arctic Ocean, surrounding the North Pole, and bounded by the Arctic circle and the northern shores of the two continents. The White Sea, the sea of Kara, and the Gulf of Obe are parts of it. The Ocean is spread over nearly seven-tenths of the globe; but it is remarkable how unequally the land and water are distributed. If we look at a map of the world projected upon the horizon of London, in which map, consequently, London forins the centre of the one hemisphere and the antipodes * to London, the centre of the other; the first hemisphere, it will be seen, contains a very large proportion of the whole of the land, while the second, if we except New Holland and the extremity of South America, from the twenty-ninth degree of south latitude, consists almost entirely of water. The distribution of water and land is still very unequal, if we compare only the northern and southern hemispheres, that is, the two equal parts into which the globe is divided by the equator. The following calculation will plainly exhibit this fact: Considering the whole space included in the northern part of the torrid zone, as equal to 1, the proportion of land is On the same supposition, the proportion of land in the northern temperate zone is And in the northern icy zone In the southern part of the torrid zone, the portion of land is In the southern icy zone (supposed) 0.297 0.559 • none. In other words, if the quantity of land in the northern hemisphere be represented by 16, the quantity in the southern will be scarcely equal to 5. About the middle of the last century it was sserted that a great continent must exist towards the south pole, in order to counterbalance the mass of land in the northern hemisphere; but by the voyages of Cook and others, it has been proved that the high southern latitudes contain only a few islands.The absence of a continent near the south pole does not of itself prove that there is less land there than in the north, since it is possible that the land in general may be only rather more depressed in the south, the necessary result of which would be, that the ocean would spread itself more extensively over the surface of the earth in that quarter. • A small island lying to the south-east of New Zealand, and called Antipodes island, is very nearly the antipodes to London. |