578 



PLANETS PLASTIC. 



sity being less, the quantity of matter is only one- 

 seventh. Spots and belts are often observed on 

 Mars ; from which it is conjectured that it has a 

 dense atmosphere. 



Between Mars and Jupiter there is n great dis- 

 tance, which led to the supposition that there was 

 some body between them; and this conjecture was 

 verified, in the beginning of this century, by the dis- 

 covery of four new planets. January 1, 1801, 

 1'iazzi, at Palermo, discovered Ceres, which, at a 

 distance of 263,000,000 miles from the sun, com- 

 pletes its revolution in four years seven months, 

 moving with a mean velocity of 11^ miles a second. 

 On account of its small size, it is not visible to the 

 naked eye, and, viewed through a telescope, has the 

 appearance of a star of the seventh magnitude. 



This discovery was followed, March 28, 1802, by 

 that of Pallas by Olbers, at Bremen. It is about 

 the same distance from the sun, and accomplishes 

 its revolution in about the same time as Ceres. It 

 is supposed to be rather larger than either Vesta, 

 Juno, or Ceres. This planet is distinguished from 

 every other by the great inclination of its orbit to 

 the ecliptic. 



Juno, which revolves around the sun in four years 

 and four months, commonly appears like a star of 

 the eighth magnitude, and was discovered Sept. 1, 

 1804, by Harding, at Lilienthal. 



Finally, March 29, 1807, Olbers discovered Vesta, 

 which appears of the fifth to the seventh magnitude, 

 is 225,000,000 miles from the sun, and completes 

 its revolution around the sun in three years and 

 eight months. 



Jupiter, the largest of the known planets, at a 

 distance of 490,000,000 miles from the sun, ac- 

 complishes its revolution, at the rate of seven miles 

 a second, in eleven years and 314 days, and is at- 

 tended by four moons, which were discovered by 

 Galilei at Florence, January 7, 1610, and the largest 

 of which has a diameter nearly equal to the semi- 

 diameter of the earth. The diameter of Jupiter 

 itself is Hi times greater than the diameter of the 

 earth; its surface is 118 times, and its bulk 1281 

 times greater than that of the earth. In nine hours 

 fifty-six minutes it revolves on its axis, which is in- 

 clined at an angle of eighty-seven degrees to its 

 orbit, and at the poles it is flattened one-fourteenth 

 of its diameter. On the surface of this planet belts 

 parallel to the equator are usually observed. 



At nearly twice the distance of Jupiter, or 900 

 million miles from the sun, Saturn passes through 

 its orbit, 5760 million miles in length, in twenty- 

 nine years and 169 days, accompanied by seven 

 moons (of which five were discovered in the seven- 

 teenth century by Huygens and Cassini, two in 1789 

 by Herschel), and by a very remarkable double ring, 

 which is 2 1 ,000 miles from the surface of the planet, 

 and 27,000 miles in breadth ; and the interval be- 

 tween them is about 3000 miles. According to 

 Herschel, this ring completes its rotation in ten hours 

 thirty minutes, while that of the planet itself is ten 

 hours eighteen minutes. 



Finally, the knowledge of our solar system was 

 enlarged, March 13, 1781, by Herschel's discovery 

 of the Georgium Sidus (Herschel, Uranus), which is 

 toUO millions of miles distant from the sun, and, ac- 

 companied by six satellites, accomplishes its revolu- 

 tion in fighty-four years nine days, at the rate of 

 about four miles a second. Its surface is nineteen 

 times larger than the earth's, but so much less solid, 

 that its quantity of matter is only 17 times larger. 



To render the vast distances from the planets to 

 the sun more comprehensible, an illustration, ad- 

 dressed to the senses, is often drawn from the velo- 

 city of a cannon-ball, moving at the rate of eight 



miles a minute. With this velocity a cannon-ball 

 would go from the sun to Mercury in nine and a 

 half years, to Venus in eighteen, to the earth in 

 twenty-five, to Mars in thirty-eight, to Vesta in 

 sixty, to Juno in sixty-six, to Ceres and Pallas in 

 sixty-nine, to Jupiter in 130, to Saturn in 238, and 

 to Uranus or Herschel in 479, while it would go from 

 the earth to the moon in twenty-three days. 



PLANIMETRY; that part of geometry which 

 considers lines and plane figures, without any regard 

 to heights or depths. Planimetry is particularly 

 restricted to the mensuration of planes and other 

 surfaces, as contradistinguished from stereometry, 

 or the mensuration of solids, or capacities of length, 

 breadth, and depth. 



PLANISPHERE; a projection of the sphere, and 

 its various circles on a plane, as upon paper, or the 

 like. In this sense, maps of the heavens and the 

 earth, exhibiting the meridians and other circles of 

 the sphere, may be called planispheres. 



PLANT. See Botany. 



PLANTAGENET, FAMILY OF. See the article 

 Britain. 



PLANTAIN (plantago major); & common weed, 

 the leaves of which are all radical, oval, and petio- 

 late, and from amongst them arise several long cylin- 

 drical spikes of greenish inconspicuous flowers. 



PLANTAIN-TREE. This name is frequently 

 applied to a species of banana (musa paradisiaca) 

 now cultivated in all tropical climates. The stein 

 of this plant is soft, herbaceous, fifteen or twenty 

 feet high, with leaves often more than six feet long, 

 and nearly two broad. The spike of flowers is 

 nearly four feet long, and nodding. The fruit, which 

 succeeds the fertile flowers on the lower part of the 

 spike, is eight or nine inches long, and above an 

 inch in diameter, at first green, but when ripe, of a 

 pale-yellow colour, and has a luscious, sweet pulp. 

 It is one of the most useful fruits in the vegetable 

 creation, and, as some of the plants are in bearing 

 most of the year, forms the entire sustenance of 

 many of the inhabitants of tropical climates. When 

 used as bread, it is roasted or boiled when just full 

 grown ; and when ripe, it is made into tarts, sliced, 

 and fried with butter, or dried and preserved as a 

 sweetmeat. Three dozen plantains are esteemed 

 sufficient to serve one man for a week, instead of 

 bread, and will support him much better. 



PLASTER OF PARIS. See Gypsum. 



PLASTIC, in the English language used as a 

 adjective only (from the Greek <jr\,anx.<i;, from 

 it\<t.irgt , I form or shape) ; but in some other lan- 

 guages a word exists, to which, in English, plastics 

 would correspond (Greek (rx<rr<*j). The term is of 

 much importance in the theory of the arts, and in 

 criticism. With the Greeks, Germans, c., it com- 

 prises the whole art of shaping figures from hard or 

 soft masses. Three species are distinguished : I. 

 The art of shaping forms from soft masses, as clay, 

 wax, gypsum, wheat-flour the ars plastica proper, 

 according to the original meaning of wA.a<rr<a : it 

 precedes sculpture : 2. Sculpture, or the art of mak- 

 ing statues of harder masses (e. g. marble, alabas- 

 ter, sandstone) the ars statuaria : 3. The sculptura 

 of the ancients, comprising works cut in wood and 

 ivory.- The materials which the ancient artists used 

 chiefly were, I. Clay. Dibutades of Sicyon invented 

 among the Greeks figures of clay. There are very 

 ancient figures of this kind, of Greek and Egyptian 

 origin. 2. Gypsum, used for stucco-work, and still 

 found in antique buildings. The art of casting in 

 gypsum was not known to the Greeks till late. 

 Lysistratus, brother to Lysippus, who lived in the 

 time of Alexander, invented it. Mengs, among the 

 moderns, has devoted great attention to this art. In 



