SEDUCTIOH. 



SEISMOLOGY. 



Th toman sens* of ^^ <ri or ar. and toX a (joing apart, a sepa- 

 ration) U properly disunion among the dtlaem, a riot, or turbulent 

 stnsniblage of |*~.|Je (or th purpow of Moomplubing tome object by 

 riolvncn or causing fear. It vrw included among other forbidden act* 

 in th.- Ux Julia de Mujwtate. (' Dig..' 48, tit. 4.) It ii often tued in 

 eoontction with "tuumltus" and "turba," and the three terms seem 

 to hare the aame signification. 



tKrtii. JUmitettt Criwi'no/m-A/, p. Stt.) 



SKIMVTIOX. [PARK-XT AKD CHILD.] 



8KOMKNT (part out off), a term which, in ita general senae, needs 

 no explanation. It is. in mensuration, most frequently applied to the 

 part out off from a circle by a chord, and the measurement of this 

 segment of a circle is the only point for which reference is likely to be 

 made to the word Let A B be the segment of a circle, and c ami n 

 the middle point* of iU are and chord. The segment AC B is easily 



C 



expressed by the angle which the arc subtends at the centre : if this 

 angle (measured in the theoretical uniU [ANGLE]) be t, and the radius 

 r, the number of square units in the segment is 



But when a segment is actually to be measured in practice, it usually 

 happens that the radius is not given, and the circle is too large to 

 measure it conveniently. In that case the middle point o must be 

 fouud, and A B and A c must be measured, as also c D. This being 

 duue, the length of the arc A B can be found with great exactness 

 from the formula " one third of the excess of eight times A c over A B, 

 or 1 (8 AC A B)." This formula errs only about one foot out of 80 

 (always giving the arc a little too small) in a whole semicircle, and the 

 error diminishes nearly as the fourth power of the arc ; thus at half 

 a semicircle the error is about one foot out of 2 4 x 80, or 1280 feet ; 

 at one-third of a semicircle it U only about one foot out of 3 4 x 80, or 

 6480 feet, and so on. Another formula of the same sort, but so close 

 to the truth that no measurements could ever be taken in practice 

 sufficiently exact to make its inaccuracy appreciable, is the following : 

 find E the middle point of A c, and measure A E ; then the arc in very 

 nearly (but a little less than) 



AB + 256 AE 40 AC. 



45 



The error here is less than one foot out of 390 on the whole circle, 

 and diminishes with the sixth power of the arc. 



Taking the arc from one or other of these methods, the area of the 

 segment is then to be found as follows : Determine R, the radius, 

 from A c*-r 2 c D, and compute 



RX Arc AB (B OD) t 



2 

 which gives the area required. This formula may be reduced to 



C + A P) A P*. 



Thus (to take an instance of Bonuycastle's) if A D = 12, A c=13> 

 whence c D- 5 (all feat), we have for the area of the segment 



Another approximate rule is (giving somewhat too little) 

 D (12 A D + 8 A c) 

 ~IS~ ~ ' 



which is more exact than the preceding. The error is only one per 

 cent, when the segment is a semicircle, and it diminishes with the 

 seventh power of the subtended angle nearly. 



It answers well enough for rough purposes, and particularly when 

 tha segment is small, to consider the arc of the circle as being part of 

 a parabola, and to take two-thirds of the rectangle under A B and c D 

 for the area. 



SKIUNORAOE. [CuRBEKcr.] 



>KI(iNOKY. [TKSUBE.] 



SKISIX is a term properly applied to estates of freehold only, so 

 that a man is said to be Kited of an estate of inheritance or for life, 

 and to be fanated of a chattel interest, such as a term of years. This 

 distinction does not appear to have existed in the time of Bracton ; at 

 leant he urns* the two words aa identical in meaning (" possessio sive 

 iKiMni multiplex est,' lib. ii., foL 88). 



The seinin of the tenant of a freehold is the legal possession of the 

 land. It is actual seisin, called seisin in deed, when he has corporeal 

 possession of the land, or, as Bracton expresses it, " corporalis rei 

 detentio : oorporis et anima cum iuris adminiculo concurrente." It is 

 scisia in law when lands have descended to a person, but he has not yet 



actually entered into possession of them, and no person has usurped the 

 possession. When an estate of inheritance is divided into several estates, 

 M fur instance an estate for life, and a remainder or reversion in fee, the 

 tenant in possession has the actual seisin of the lands; but the 

 persons in remainder or reversion have also seisin of their respective 

 MM, 



In the conveyance of land by feoffment, which is now, however, almost 

 unknown, the delivery of the possession, or livery of seisin, as it is 

 termed, is the efficient part of the conveyance. [FEOFMIKNT.] 



Seisin in deed is obtained by actually entering into lands, and an 

 entry into part in the name of the whole is sufficient ; by the receipt 

 of rents or profits ; and by the actual entry of a lessee to whom the 

 lands are demised by a person who is entitled to but has not obtained 

 actual possession. 



Seisin may also be acquired under the Statute of Uses, 2 7 Hen. V 1 1 1 ., 

 which enacts that when any person shall be seised of any lands In thi- 

 toe, 4c., of another, by reason of any bargain, sale, feoffment, Ac., the 

 person having the use, Ac., shall thenceforth have the lawful seisin, Ac., 

 of the lands in the same quality, manner, and form as he had before 

 in the use. 



A disseisin supposes a prior seisin in another, and a seisin by the 

 disseisor which terminates such prior seisin. To constitute a disseisin, 

 it was necessary that the disseisor should not have a right of entry ; 

 the disseisee should not voluntarily give up his seisin ; and that the 

 disseisor should make himself the tenant of the land, or in other 

 words, should put himself, with respect to the lord, in the same 

 situation as the person disseised. " But," it is well remarked (Co. Litt., 

 266 b, Butler's note), " how this substitution was effected, it is difficult, 

 perhaps impossible, now to discover. From what we know of the 

 feudal law, it does not appear how a disseisin could be effected with- 

 out the consent or connivance of the lord ; yet we find that the 

 relationship of lord and tenant remained after the disseisin. Thus 

 after the disseisin the lord might release the rent and services to the 

 disseisee ; might avow upon him ; and if he died, his heir within age, 

 the lord was entitled to the wardship of the heir." But the doctrine 

 of disseisin is in many respects very obscure, and at present of very 

 little practical importance. 



SEISMOLOGY, from a( KT/UOS, an earthquake (that is, a movement liku 

 the shaking of a sieve), and \iyos, a discourse, the science of earthquakes ; 

 and SEISMOMETKY.from the same.and nfTpeu,to measure.the science 

 of the mensuration of all the phenomena of earthquakes which can be 

 " in numbers, or by their relation to the co-ordinates of space. 



The origin of these new departments of science has been stated in the 

 article EARTHQUAKES, in which the physical definition of an earthquake 

 was also given, agreeably to the researches of Mr. Robert Mallet. We 

 now proceed to state some of the elements of seismology, and the 

 nomenclature applied to them, premising that the basis of this is 

 formed by the adjectives scismal and seitmtc, both signifying what has 

 relation to an earthquake. An earthquake being the transit of a wave 

 of elastic compression through the earth, there are three elements of 

 this earthquake-wave, namely, the velocity of transit, the velocity of 

 the wave-particle (or wave itself [ WAVE]), and the direction of motion 

 at each point of the teumlc area, or tract on the earth's surface within 

 which the earthquake is felt. From the last element may be deter- 

 mined, 1st, the point upon the earth's surface vertically over the centre 

 of effort, or focal point, whence the earthquake impulse was delivered : 

 and 2nd, the depth below the surface (or rather sea-level) of the focal 

 point itself. The line passing through both these points is the teitmic 

 vertical. The wave starts from the focal point with one normal and 

 two transversal vibrations, and may be imagined transferred outwards, 

 in all directions, in concentric spherical shells, whose volume at the 

 same phase of the wave is constant. The shock reaches the surface 

 directly above the focal point vertically; but, for all points around 

 that, it emerges with angles becoming more and more nearly horizontal 

 as the distance measured on the surface increases. The intersecting 

 circle of any one shell with the surface, which is that of simultaneous 

 shock, is the coseismal line, or crest of the earth-wave : circular 

 if in a homogeneous medium, more or legs distorted if in a hetero- 

 geneous one (such as constitutes the various geological formations 

 of the earth's crust), but always a closed curve. The mazotasmul area, 

 is that in which the shock is experienced in the greatest degree, and 

 isoseismat areas are those in which it is felt at the same time. A 

 riiji'in is a tract of the earth's surface, the earthquakes of which have 

 some community of origin, manifested by their simultaneous occurrence. 

 Seitmic energy is the total amount of force exerted in earthquakes at a 

 given period, or in a given space, or with relation to any other element 

 of the subject. A cjirvno-seimic curve, laid down from the records of 

 earthquakes, the ordinate of which is that of epoch, and the abscissa that 

 of seismic intciuitg (in this case the force of a given earthquake), 

 expresses the progression of seismic energy in time. 



Every object displaced by an earthquake shock is in fact a seismo- 

 meter, and in the selection and arrangement of suitable objects for such 

 displacement, so as to give the required evidence of the velocity and 

 directions of the motion, consists the practical science of seismometry. 

 For particulars of the construction and principles of complete seft- 

 regintering seismometers, we must refer to Mr. Mallet's Fourth Report to 

 the British Association ; but we proceed to notice some extemporaneous 

 seismometers with which important observations may be made, under 



