KI.I-. 



I.I.I.I1TIC COMPASSES. 



(40 



building Mt-iuing at a distance to be of tone, and when approached 

 dinorerad to bo of an inferior material. The brickwork itaelf u often 



tangent r T) the conjugate temi-diameter or cmi-tojwjatt of c p. Also 

 he fraction which c s i of c A u called the eccentricity of the ellipe. 



[SUircut lit Clarerton, Somersetshire, from Richardson.] 



variegated by the intermixture of darker bricks, so disposed 03 to form 

 a regular pattern on the walls, generally consisting of intersecting 

 diagonal lines. There are also many instances of brick alone being 

 employed, the ornamental members being formed of moulded bricks ; 

 and though the effect is comparatively sombre, it is by no means 



(Richardson, Architectural Remaini of the Keiynt of Elizabeth and 

 Jama; Sb&w, Elizabethan Architecture; Nash, Old EnyliihMawians,lic.) 

 ELL (Ulna), a measure of length now almost disused. " It is 

 properly," says Ducange, " the length between the ends of both the 

 extended hands, though Suetonius makes it to be only one cubit." 

 It is not worth while to attempt to follow a measure of secondary im- 

 portance through its various changes, and this measure in particular 

 has denoted very different lengths in different countries. The three 

 ells which have preserved a place in our arithmetical works, namely, 

 the Flemish, English, and French ells, are respectively three, five, and 

 six quarters of a yard. 



ELLAGIC ACID (C,,H,0,,HO). This acid exists in the gall-nut 

 along with gallic acid ; and they are deposited from the aqueous 

 infusion in the state of a yellowish crystalline mass. The two acids 

 are separated by boiling water, which dissolves the gallic acid, anc 

 leave* the ellagic unacted upon, but mixed with a little gallate of lime 

 By treatment with a weak solution of potash the ellagic acid is dis 

 solved, and the gallate of lime remains insoluble; the ellagate ol 

 potash is then treated with hydrochloric acid, which uniting with 

 the potash, precipitates the ellagic acid in a pulverulent state. Ellagic 

 acid U also a constituent of certain intestinal concretions which occur 

 in the wild goat, deer, 4c., of central Asia, and termed bezoars. Hence 

 this acid U sometimes called bezoaric acid. 



The properties of this acid are the following. It is of a light fawn 

 colour. It is insipid, sparingly soluble in boiling water, and reddens 

 litmus paper slightly. When heated in close vessels it decomposes 

 yielding a yellow vapour, which condenses in crystals of the same 

 colour. Thi acid becomes of a blood-red colour by digestion in nitric 

 acid, and is converted afterwards into oxalic acid. It unites witl 

 potash, soda, and ammonia, to form neutral salts ; the two first ar 

 insoluble in water, except when an excess of base is present; and th 

 ellagate of ammonia does not dissolve under any circumstances. Its 

 acid" powers are weak, for it is incapable of decomposing the ulkalin 

 carbonate*. [BEZOABS.] 



ELLIPSE (fXA4). This curve, which is one of the CONIC SEC 

 TIOK8, ranks next in importance to the circle (which is in itself an 

 extreme form of the ellipse) and the straight line. We shall here con 

 rider the ellipse independently of the other conic sections, and simpl 

 state tome of the most remarkable properties which can be exhibitec 

 without algebraical symbol*. 



1. Let two any point* 8 and u be taken, and their distance bisectec 

 in c. Set off c A and c x, equal lines, each greater than c a, and let 

 point r move in such a manner that H p and p 8 together are alway 

 equal to A x. The curve described by the point p is an ellipse. 



2. c A is called the temi-oxit major, c B the temi-a.at minor, c th 

 centre, s and H the/oci, 8 p and H p the focal diitancet of the point P 

 c P the >emi-diam<tcr of the point P, and c D (drawn parallel to th 



i. Let SAbetoAKascstoCA. Then K R is called the directrix 

 of the ellipse, and spistoPRasSAtOAK. 



4. The tangent PT bisects the angle made by s r and the continuation 

 of HP. 



5. c A is a mean proportional between c N and o T. 



6. If, A and M remaining the name, the figure of the ellipse be altered 

 >y varying s and H, the tangents drawn through the several points iu 



which the ellipses cut N Q will all pass through the same point T of the 

 axis. The circle A <J M is the extreme form of the ellipse, when s and 

 H meet in c : and the tangent at Q passes through T. 



7. Wherever the point p may be token, N P bears to N Q the same 

 proportion as c R to c A, and so does the area A N P to the area A N Q. 



8. The perpendicular let fall from either s or H upon p T must cut it 

 in a point of the circle A Q M. 



y. If c D be parallel to the tangent at P, then c p is parallel to the 

 langeiit at D. 



10. The parallelogram PCDK is equal to the rectangle of Bcand 

 c A, and the sum of the squares on P c and c D is equal to the sum of 

 the squares on A c and c u. 



11. The square on PN is lea than the rectangle contained by AN 

 and N M in the proportion of the square on c B to the square on c A. 



12. p o bisects every line parallel to c D which is bounded at both 

 ends by the ellipse, and the square on x Y is to the rectangle contained 

 by a Y and Y p in the proportion of the square c D to that on c P. 



1 3. The square on c D is equal to the rectangle contained by s p and p H. 



14. The double ordinate parallel to B c through either focus is 

 called the lattu rectum. The square on P N is always less than the rect- 

 angle under A N and the latus rectum by the square on a line whioli is 

 to A .N as B c to c A. From this deficiency the ellipse derives its n:imi'. 

 as does the HYPERBOLA (intepPo\^i) from a corresponding ej-cea. 



Such are a few of the countless properties which might be exhibited. 

 But it is to be noticed that the most common and elegant theorems 

 are not those which are found most useful. The striking use of this 

 curve lies in its being the nearest representative of a planetary orbit 

 which can be given in a simple manner. If the planets did not attract 

 each other, but were only attracted by the sun, they would describe 

 absolute ellipses. Their mutual actions being small compared with 

 that which the sun exerts, they consequently move in ellipses very 

 nearly. Hence the utility of the ellipse in astronomy ; but at the 

 same time the properties of the curve which facilitate the investigation 

 of the heavenly motions present nothing so striking as those which we 

 have given. 



The reader who is not versed in geometry must remember that, 

 though an ellipse be an m'al, yet an oval is not necessarily an ellipse. 

 A figure may be formed by arcs of circles which shall have the appear- 

 ance of an ellipse without possessing any of its properties. 



ELLIPSOID. [SURFACES OF THE SECOND DEOKI:E ; SPHEROID.] 



ELLIPTIC COMPASSES, the name given to any machine for 



describing an ellipse. We shall only mention two contrivances of the 



