PLANETS 



PLANTAIN 



219 



ful, for Olbers (q.v.) discovered two (Pallas and 

 Vesta) in 1802 and 1807, and Harding one (Juno) 

 in 1804 ; but, as all researches for some time sub- 

 sequent to 1807 were unavailing, astronomers 

 gradually allowed themselves to settle down into 

 the belief that no more planetoids remained to 

 be discovered. But the detection of a fifth 

 (Astnea) by Hencke in 1845 revived the hope of 

 fresh discoveries, and from this period no year 

 (excepting 1846) has passed without adding to the 

 list. The number known at the beginning of 1851 

 was 13, of 1861 was 62, of 1871 was 112, of 1881 

 was 219; in 1898 there were 430. This great 

 success of the astronomers of our time is due to the 

 systematic manner in which the zodiacal belt has 

 been explored, and the place and apparent size of 

 every star of this region distinctly determined ; so 

 that the presence of a wandering oody can at onco 

 be detected. Among the most successful of the dis- 

 coverers of planetoids have been Palisa of Vienna, 

 and C. H. F. l'eters( 1813-90) of Hamilton College, 

 U.S. The former, since 1872, has discovered more 

 than 80, on one occasion as many as 5 in a week ; 

 while the latter, after 1861, discovered 48. 



The magnitudes of these celestial bodies have 

 not been accurately ascertained, but it is certain 

 that they are exceedingly small as compared even 

 with Mercury, the least of the other planets ; the 

 <liameter of the largest among them ueinj* gener- 

 ally believed not to exceed 450 miles, while most 

 of the others are very much smaller than this. 

 They also differ, generally speaking, from the 

 rest of the planets in other respects ; their orbits 

 are of greater eccentricity, are inclined to the 

 ecliptic at a greater angle, and are interlaced in 

 a most intricate manner, crossing each other so 

 frequently as to form, when viewed perpendicularly, 

 a kind of network. The consequence of this is 

 that a planetoid which is nearest the sun at one 

 part of its orbit is, when at another part of its 

 orbit, farther from it than are several of the 

 others, and a mutual eclipsing of the sun at 

 different periods by two planetoids must lj of 

 very frequent occurrence. The mean orbit of the 

 first 251 planetoids coincides, however, within 30' 

 with that of Jupiter. Of the planetoids of 

 which the elements had by 1891 been satisfactorily 

 calculated, Medusa (No. 149) has the shortest 

 period of revolution, 1137 '69 days, and Hilda 

 (No. 153) the longest, 2869 '92 days. The corre- 

 sponding mean distances from the sun, expressed 

 in parts of the earth's mean distance, are respect- 

 ively 2-13275 and 3-95228. Till 1876 the extremes 

 known in this respect were Flora and Sylvia 

 respectively. The nearest approach to the sun is 

 made by Phocea (perihelion distance, 1'787). 

 Freia recedes farthest from him (aphelion dis- 

 tance, 4-002). Polyhymnia's orbit has the greatest 

 eccentricity, amounting to 0-33998 ; Lonua's the 

 least, 0-2176. Massalia's orbit makes a smaller 

 angle only 41' 7" with the ecliptic than that of 

 any other planet in the solar system, while the 

 inclination of the orbit of Pallas is no less than 

 34" 42' 41". After the first two or three of these 

 bodies had been discovered the opinion was pro- 

 pounded by Olbers that they were but the frag- 

 ment* of some large planet ; and this hypothesis 

 received corroboration from the intimate con- 

 nection which was shown to subsist among them ; 

 but of late years it has fallen out of favour with 

 astronomers. Some infer that the planetoids are 

 best accounted for by the nebular hypothesis. It 

 has been calculated that the combined mass of all 

 the planetoids cannot exceed one-fourth of the 

 earth's mass. 



For a Table of the periods, distances, size, den- 

 sity, &c. of the planets, see SOLAR SYSTEM. See 

 also PHOTOGRAPHY, and SPECTRUM. 



Plani'meter, a machine for measuring areas on 

 a plane. The best-known form is that of Pro- 

 fessor J. Amsler-Laffon of Schaffhausen, the theory 

 of which will be found discussed in Williamson s 

 Integral Calculus, and in Minchin's Uniplanar 

 Kinematics. It consists of two rods hinged together. 

 The extremity of the one rod is fixed, so that the 

 free extremity of the other is able to trace out any 

 form of curve limited only in size by the dimensions 

 of the apparatus. The theory of the instrument 

 depends upon the fact that, as the free end is made 

 to trace out the boundary of any closed area, the 

 hinged end oscillates to and fro along a curve, but 

 traces out no area. Rigidly attached to the rod 

 whose one end traces out the plane area is a 

 graduated roller fixed with its axis of rotation 

 parallel to the line joining the hinge and the trac- 

 ing point. As the closed curve is being described 

 the roller rotates because of the lateral movement 

 of the rod ; and the difference of the readings before 

 and after the tracing has been accomplished gives 

 a number proportional to the area that has been 

 gone round.. To Amsler-Laffon also is due a more 

 elaborate form of integrator capable of measuring 

 moments of inertia of areas. It greatly facilitates 

 the calculation of displacements, metacentres, and 

 curves of stability in naval designing. 



the surname of an Angevin 

 family which in 1154 succeeded in the person of 

 Henry II. to the throne of England on the ex- 

 tinction of the Norman dynasty in the male line, 

 and reigned till 1485, when the battle of Boswortli 

 gave the crown to the family of Tudor. The name 

 was first adopted by Geoffrey, Count of Anjou, 

 husband of Matilda, the daughter of Henry I., from 

 the l>adge of a sprig of broom (planta genista) 

 which he wore in his bonnet ; and Henry I. is the 

 only king to whom Mr Freeman would allow the 

 name. The Plantagenet kings were Henry II., 

 Kichard I., John, Henry III., Edward I.-IIL, 

 Richard II., Henry IV. -VI., Edward IV. -V., and 

 Richard III. See the separate articles on these 

 names ; also, for 

 the great struggle 

 between its two 

 rival branches, the 

 article ROSES 



(WARS OF THE). 



MUs Norgate's 

 England under the 

 Angevin Kings (2 

 veils. 1887) is an 

 altogether admir- 

 able history of the 

 Plantagenet period 

 as far as the reign 

 of John. 



Plantain. For 



the tropical plan- 

 tain (Musa), see 

 BANANA. The 

 English plant so 

 called belongs to 

 the Plantaginefe or 

 Plantaginacese, a 

 natural order of 

 exogenous plants, 

 mostly herbaceous 

 and without stems ; 

 the leaves forming 

 rosettes, flat and 

 ribbed , or taper and Greater Plantain (Plantago major ). 

 fleshy ; the flowers 



usually in spikes, and generally hermaphrodite; 

 the calyx 4-parted, persistent; the corolla hypo- 

 gynons, membranous, persistent, its limb 4-parted : 

 the stamens four, inserted into the corolla, with 



