Page:The New International Encyclopædia 1st ed. v. 16.djvu/120

* PLANETS. 92 PLANKTON. from the sun, yet Mars, which is farther from the sun, is much smaller than either the earth or Venus; and the planetoids, which are still farther ofT, are the smallest of all. Jupiter, which is next in order, is by far the largest, being about one and one-half times as large as all the others together; and as we proceed farther out- ward the planets become smaller and smaller. With reference to their distance from the sun, as compared with that of the earth, the planets are divided into siijieriur and inferior ; !Iercury and Venus are consequently the only inferior planets, all the others being superior. The in- ferior planets must alwajs be on the same side of the earth as the sun is. and can never be above the horizon of any place (not in a very high latitude) at midnight; they are always invisible at their superior and inferior conjunctions, ex- cept when, at the latter, a transit {see Tkansit OF Vexis) takes place. The superior planets are likewise invisible at conjunction, but when in opposition they are .seen with the greatest dis- tinctness, being then due south at midnight. A trans-Xeptunian planet, owing to disturb- ances produced on the outer planets, is suspected to revolve outside the orbit of Xcptune. Even before the discovery of Xeptune it was suggested that the Uranian disturbances were not due to a single exterior planet alone. The matter was in- vestigated by Todd (q.v.) in 1877. He computed the probable elements and searched the heavens on "30 clear, moonless nights, between Xoveniber 3, 1877, and March 5, 1878." Every effort to find the hypothetical planet has, however, so far proved fruitless. Consult Todd's account in the Amrrican Journnl of Hcience (September, 1880). In astronomical tables, almanacs, etc., the planets are for convenience denoted by symbols instead of their naines, as follows: Mercury, g ; Venus, J ; the earth, ^ ; Mars, ■' ; the plane- toids, in the order of their discovery, Q, @, ©, etc.; Jui>iter, U ; Saturn, ^ ; Uranus, ]Jl ; Neptune, ^ ; the sun, o ; the moon, d. For a table of the periods, distances, size, density, etc., of the planets, see Solar System. See also the several planets under their respective names. For the hypothetical planet Vulcan, see Vulcan ; for the elements necessary to determine the orbit of a planet, see Orbit. For theories regarding the origin of the system, see CosjloG- ONY; XebULJE. PLANIM'ETER (from Lat. planum, level ground, plain -f- iiirpoii, mctrnn, measure). An instrument for automatically measuring the areas of maps and i)lans. The first usable plan- imeter seems to have been constructed in 1814 by a Bavarian engineer named Hermann (1785- 1841). This was followed by an ingenious device invented by a Swiss engineer, Opikoffer, in 1827, and perfected ten years later by Ernst of Paris. The next notable type was the polar plan- imeter made by the firm of Amsler-Laffon of Schaffhausen (1854) and known as Amsler's planimeter. This well-known instrument con- sists of two arms hinged at one end. the outer end of one being a pointed support and that of the other a tracing pencil. The latter arm also bears a graduated cylinder which rolls on the paper, its axis being parallel to the arm. To obtain the area of a figure the instrument is moved bodily about the pointed support on the first arm, while the tracer of the other arm de- scribes the perimeter of the area, and the gradu- ated cylinder records the required area. This and the 'Wetlisclie Linearplanimeter,' manufac- tured in Vienna, are the best ])laniineters now made. The mathematical principles involved may be found in works on surveying. The best description of the Amsler planimeter is "given in Bauernfeind's Elemente der ermessutigskunde (7th ed., Stuttgart, 1890). Consult, also, Ams- ler, Ueber die mcch<i)iische licstiiiuiiung des Fliicheninhalts (Schaflhausen, 185C). The latest improvements on the polar ])lanimcter have been made by Hohmann and Coradi in their 'Priizi- sions-Polarplauimeter,' and by Coradi in -his 'Kugcl-RoUplanimeter,' on which consult Lorber, Zciischrift fiir Vermessungsicesen (vols. viii. and xvii., Stuttgart, 1884, 1888), and Amsler- Laffon, "Planimeterkonstruktionen,' in the Zeit- schrift fiir Inslrumentcnkunde (4. .Jalirg., 1884), and Das Momentenplanimeter (Ziirich, 1875). For the histoi-y of the subject, consult: Trunck, Die Planimeter ; deren Theorie, ]axis und Ge- schiehte (Halle, 1865) ; Bauernfeind, in Dingler's I'olyteehnisches Journtil (vol. cx.xvii.) ; and Fa- varo, in the Allgemeine Bauxeitung (1873). PLANIM'ETRY (from Lat. planum, level ground, plain -f Gk. -lierpla, -metria, measure- ment, from nirpov, inetron, measure). That por- tion of geometry which treats of the measurement of plane areas. See Planimeter ; Mensuration. PLANING MACHXNE, or PLANER. See !Metai.- Working ^Machinery ; Wood-Working Machinery. PLANKTON (Neo-Lat., from Gk. irayKTb%, phinktos, wandering, from iri^£iv,plazcin, to wan- der). A collective term for the free floating or swimming organisms in oceans, lakes, or rivers. The oceanic plankton is more interesting and im- portant than that of lakes and of rivers, inasmuch as it forms the basis for all oceanic life. Among the more important forms which are found in the plant plankton are blue-green algie, bacteria, and diatoms. The colors of certain waters are due to plankton organisms. For example, the Red Sea is colored by a red alga. Diatoms are found in especial abundance in the colder .seas. Among the more interesting structures and life habits of the plankton plants there may be noted a high devel- opment of adaptations for active movements through the water, a type of structure which is fotmd in plants only among the lower forms. An- other very striking feature of the plankton is its periodicity; at various seasons its constitution varies widely and the plankton organisms are known to rise and sink more or less periodically. The mechanism involved in these movements is not known. In zoology the term is restricted to the pelagic life which drifts, the actively swimming surface forms constituting a separate assemblage — the 'nekton.' It consists mainly of 'jellyfishes' (Medusan, Siphonophora, and Ctenophora), as- eidian, especially the salpae (qv. ), and a great variety of pelagic larva> and minute Crustacea with feeble powers of locomotion that are carried along almost passively by the oceanic currents. Much of this occurs in innumerable flocks, and sometimes tinges large areas of the sea by its color, OT causes them to glow with phoshores- cenee ; and it furnishes food to many surface-feed- ing pelagic animals.