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fron, for instance, at New York, is not a pound at Quebec or Panama, but this variation in its weight leno "freak of gravity" as some people have imagined, but in strict agreement with the law of gravitation and the physical condition of the earth. It would, too, weigh a trifle less on the top of a high mountain or in a deep mine or pit, than at the surface. Gravity retains the atmosphere around the earth. Marine animals live at the bottom of the ocean at the depth of a mile or two, and, of course, are subject to great pressure from the superincumbent water, so likewise we ourselves crawl around on the surface of the earth at the bottom of an aerial ocean not less than 50 miles in depth and also under great pressure. Some people can hardly realize that gases can have weight, but a column of air one inch square at the base and extending to the top of the atmosphere, weighs about fifteen pounds and a square mile of the earth's surface sustains an atmospheric pressure of 30,108,672 tons. If our pound of iron could be taken to the centre of the earth it would have no weight because. It would be equally attracted in all directions. Again, there is a point on the line joining the centres of the earth and moon, at which it would have no weight, being there equally attracted in opposite directions. There is also another point on the same line produced beyond the moon at which it would be equally attracted, but it would have weight because the attractions now act in the same direction and it would fall toward the moon. In all these instances it must be remembered that the mass or quantity of matter in the body is constant in all places, but the weight varies from place to place, by reason of causes which will now be explained. Astronomy and geology furnish abundant evidence that the earth was once in not only a gaseous but also subsequently in a plastic condition, having been detached or set free from the parent mass which ultimately became the sun. At its birth it would, by the laws of motion, receive not only a motion of translation but also a motion of rotation around an axis passing through its centre of gravity, and by virtue of this latter motion it would necessarily take the form of an oblate spheroid, or the shape of the body generated, by revolving an ellipse about its minor axis, that is to say, a globular body flattened at the poles, and protuberant along the equator like a Satsuma orange. This form or shape, first derived from theoretical considerations, was subsequently verified by actual measurements of ares of meridians in Lapland, Peru, India, South Africa, England, France, and the United States. The boundary ne between Maryland and Delaware, having a bearing of N. 3° 43' 30" W. was used for this purpose by the English astronomers Mason and Dixon, who also in 1763-8 established the boundary line between Maryland and Pennsylvania, the real Mason and Dixon line of ancient American politics. The dimensions of the earth deduced from these measurements as well as from the geodetic operations In this country, England, France, India, and Russia are as follows: Equatorial radius 3963.23 miles, polar radius 3949.79 miles, difference 13.44 miles, from which it appears that the poles are very nearly 13% miles nearer the centre of the earth than any point on the equator is, and since the attraction of the whole spheroid is the same as if its entire mass were collected at the centre, the force of attraction at the poles is greater than at any other point on the earth's surface. For this reason alone the weight of a body increases as we approach the poles.
The earth makes one revolution on its axis in 24 hours and its circumference at the equator Is 24,900 miles, which gives there a velocity of more than 1,000 miles per hour. This great velocity generates a centrifugal force which tends to lift bodies from the surface and therefore to oppose gravity. This force has already been referred to in the case of the pressure of the flange of the outer wheels of a railway car against the inner edge of the outer rall while the car is running on a circular track. It may also be easily demonstrated by tying a body to the end of a string and holding the other end in the hand, whirl it around; the body will tend to fly off, and if whirled fast enough it will break the string. This tendency of revolving bodies to depart from the centre of motion is called the centrifugal force and in the case of the earth is manifestly greatest at the equator and decreases as we approach the poles, where it vanishes.
Therefore, for this reason also, the weight of a body will increase as we travel north or south from the equator. These two causes, depending on the figure and motion of the earth, conspire to Increase the weight of a body as its latitude increases.
Notwithstanding the great velocity at the equator, gravity is there 289 times greater than the centrifugal force, but if the earth revolved 17 times faster (the square root of 289) than at present. bodies at the equator would lose their weight and remain suspended without any visible support. The centrifugal force varles directly as the square of the velocity and inversely as the radius of the circle in which it moves.
In ascertaining the difference in weight of a body in different latitudes, common scales with weights cannot be employed, because the weights would be affected in the same way as the body to be weighed. A delicately and accurately constructed spiral steel spring is used, to which is attached a carefully graduated scale with a microscope or vernier for accurate reading. The elasticity of such a spring is not affected by gravity or the centrifugal force.
For all accessible heights above the earth gravity is regarded as a constant force, and is such that it will cause a body to fall from rest, 16 feet in the first second; 48 feet in the next second; 80 feet In the third second; 112 feet in the fourth second and so on, increasing as the numbers, 1, 3, 5, 7, etc. At the end of the first second its velocity is 32 feet, twice the distance it just fell; at the end of the next second the velocity is 64 feet, at end of the third second 96 feet and so on, increasing by 32 feet at the end of every second. This number, 32 feet per second, is constant and is taken as the unit or measure of gravity and denoted by the letter g. in all works on physics.
In all these illustrations, fractions of a foot have not been taken into account, nor the resistance and buoyancy of the atmosphere. Strictly speaking, the exact value at the equator is 32.0902 feet, at the poles 32.2549 feet, and at New York 32.1071 feet.
Most people believe that the pressure below the surface of the earth--say 100, 400 or 1,000 miles below the surface-must be very great, but such is not the case. Below the surface, gravity varies not inversely as the square of the distance, but directly as the distance from the centre, that is to say, taking 4,000 miles for the radius, at the depth of 1,000 miles, gravity would be three-fourths of what it is at the surface, 2,000 miles down it would be only one-half, at 3,000 miles, one-fourth and at the centre it would be zero.
The law apparently changes below the surface, but this is no "freak of gravity," but in strict accord with the law of inverse square as before stated. The weight or pressure of the material composing the earth's crust becomes less and less as we descend below the surface.
Suppose we could go down 1,000 miles, there would then be a sphere of 3,000 miles radius below us, imagine this sphere to be removed leaving behind a hollow spherical shell of 1,000 miles in thickness and we will assume of uniform density, now If a body be placed anywhere within the shell, It will have no weight and remain at rest, that is to say, the attraction of the nearer portions of the shell will be exactly neutralized by the opposite portions, but the attraction of the shell on a particle exterior to it varies inversely as the square of the distance from Its centre.
There are some other interesting facts deduced by the aid of the higher mathematics that serve to impress us with this wonderful force; for instance, if a hole were cut through the centre of the earth from surface to surface, and if a number of bodies were placed at different points in this hole say at the surface, 100, 500, 2,000 and 3,000 miles or only a few feet from the centre, and If all these bodies were dropped at the same instant they would all reach the centre at the same time, but with different velocities; the velocity of each, however, would carry it as far beyond the centre as it just fell: It would then stop and return, vibrating to and fro forever. This would be a genuine case of perpetual motion. The time of falling to the centre from the surface or from any other point in the
Elements of the Solar System.
hole is 21 minutes and 7 seconds and the velocity acquired by falling from the surface is 4.914 miles per second.
Again, the mass of the earth is such as to give to a body falling, say from the region of the fixed stars, a velocity of 6.9505 miles per second when it reaches the surface, or in other words If a body could be projected with a velocity greater than this, say seven miles per second, it would never return to the earth; gravity would not be strong enough to draw it back.
We now come to the consideration of a circumstance where it has been assumed that the centrifugal force, resulting from the diurnal motion of the earth, was suspended or rather reversed for a while. It is recorded in two different places in the old Testament, viz., II. Kings, chap. XX., also Isalah, chap. XXXVIII., that at a certain time the shadow cast by the style on the sun dial of Ahaz, went back ten degrees. Theologians and philosophers in all the churches of Christendom have been wrestling with this phenomenon, all down through the centuries, but with negative results, regarding it finally as one of the most stupendous miracles ever exhibited, surpassing even the performance of Joshua. He only made the sun stand still, but in this case, the sun not only stopped but went back in its apparent course ten degrees, which of course implies that the earth stopped in its diurnal motion, and then rotated back from east to west ten degrees-a movement which would have wrecked our globe and torn it into fragments-but notwithstanding its apparent Impossibility, there is not the shadow of a doubt but that the phenomenon actually happened as recorded.
When the higher criticism brings the power of the higher mathematics to bear on the subject. It tells its own story. It tells us that it is simply an astronomical phenomenon which must occur under certain conditions and is no more miraculous than the dally rising and setting of the sun, moon and stars. The phenomenon can be demonstrated here in New York as well as in Jerusalem or any other part of Palestine, nor is it necessary to erect a sun dial to show that the shadow of its style can go back ten or even twenty degrees while at the same time the sun moves on in its apparent course through the sky. An ocular demonstration of the phenomenon can be shown by simply placing a straight stick in a certain position into the level ground, when the reason or cause will be apparent to the spectator. The mathematical discussion of the subject is too abstruse to be given here. Joshua's "Sun standing still" was a meteorological phenomenon whose explanation does not fall within the domain of mathematics.
Elements of the Solar System.
Facts About the Earth.
ACCORDING to Clark, the equatorial semi-diameter is 20, 926, 202 feet-3963. 296 miles, and the polar semi-diameter is 20,854,895 feet-3950, 738 miles. One degree of latitude at the pole 69.407 miles. One degree of latitude at the equator-68. 704 miles.
The estimate in areas in the above table is the newest made by a competent geographer and is by Professor Supan of Gotha in his work entitled "Bevolkerung der Erde." It varies considerably from previous estimates. Central America and the West Indies are included in the estimate for North America. The estimate of population is from Whitaker's (London) Almanack for 1912, excepting that of the Polar Region.
Ravenstein's estimate of the earth's fertile region, in square miles, is 28, 269, 200; steppe, 13,901,000; desert, 4, 180,000; polar regions, 4, 888, 800.
The population of the earth at the death of the Emperor Augustus, estimated by Bodio, was 54,000,000. The population of Europe hardly exceeded 50,000,000 before the fifteenth century.-Mulhall
The proportion of females to 1,000 males in 1901 was: Great Britain, 1,062; France, 1,033; Ger- . many, 1,032; United States, 959.- Webb-Mulhall
The area and cubic contents of the earth, according to the data of Clark, given above, are: Surface, 196, 971, 984 square miles; cubic contents, 259, 944, 035, 515 cubic miles.
Murray (Challenger expedition) states the greatest depth of the Atlantic Ocean at 27,366 feet; Pacific Ocean, 30,000 feet; Indian Ocean, 18, 582 feet; Southern Ocean, 25, 200 feet; Arctic Ocean, 9,000 feet. The United States Collier Nero obtained a depth of 31,614 feet in the Pacific Ocean, near Guam, November 14, 1899. The Atlantic Ocean has an area, in square miles, of 24, 536,000; Pacific Ocean, 50,309,000; Indian Ocean, 17,084,000; Arctic Ocean, 4,781,000; Southern Ocean, 30, 592, 000. The highest mountain is believed to be Deodhunga or Everest, one of the Himalayas, 29, 002 feet.
For population of the earth according to creed, see RELIGIOUS STATISTICS.
POPULATION OF THE EARTH ACCORDING TO RACE.
(Based on latest estimate by John Bartholomew, F. R. G. S., Edinburgh.)
The human family is subject to fifty principal governments. As to their form they may be classified as follows: Absolute monarchies, Abyssinia, Afghanistan, Morocco, Siam; Limited monarchies, Austria- Hungary, Belgium, British Empire, Bulgaria, Denmark, Germany, Greece, Italy, Japan, Montenegro, Netherlands, Norway, Persia. Roumania, Russia, Servia, Sweden, Spain, Turkey; Republics, Argentina, Bolivia, Brazil, Chile, China, Colombia, Costa Rica, Cuba, Dominican Republic, Ecuador, France, Guatemala, Hayti, Honduras, Liberia, Mexico, Nicaragua, Panama, Paraguay, Peru, Portugal, Salvador, Switzerland, United States of America, Uruguay, Venezuela. Besides these are the undefined despotisms of Central Africa, and a few insignificant independent States.
The average duration of human life is about 33 years. One-quarter of the people on the earth die before age 6, one-half before age 16, and only about 1 person of each 100 born lives to 65. EUROPEAN LANGUAGES SPOKEN.
These estimates (that for 1801 being by Mulhall) exhibit the superior growth of the English language.
MEAN RELATIVE HUMIDITY, IN PERCENTAGES.
From a table prepared by the United States Weather Bureau, showing the monthly and annual values of relative humidity at regular Weather Bureau stations in the United States, based upon observations made at 8 A.M. and 8 P.M. respectively, 75th meridian time and covering a period of about 14 years of record. Jan. Feb. | Mar | Apr. May June July Aug. Sept. Oct. (Nov. Dec. Ann'
Albany, N. Y
Atlantic City, N. J....
Bismarck, N. D
Carson City, Nev
Des Moines, Iow
Dodge City, Kan..
El Paso, Tex
Fort Smith, Ark.
Grand Haven, Mich
Hatteras, N. C.
Pierre, S. Dak..
Rapid City, S. Dak.
Roc N. Y
St. Paul, Minn..
Salt Lake City, Utah.
The Geological Strata.
THE strata composing the earth's crust is divided by most geologists into two great' classes: 1. Those generally attributed to the agency of water. 2. To the action of fire: which may be subdivided as follows: (a) Aqueous formations, stratified, rarely crystalline (sedimentary or fossiliferous rocks: metamorphic or unfossiliferous). (b) Igneous formations, unstratified, crystalline (volcanic, as basalt: plutonic, as granite).
The geological record is classified into five main divisions or periods: 1. The Archæan, lifeless and dawn of life. 2. The Paleozoic (ancient life). 3. The Mesozoic (middle life). 4. The Cenozoic (recent life). 5. Quaternary, the age in which man's first appearance is indicated.