Ill 





':' 





although iu tms become somewhat thicker. The free access of the 

 rayn of the *un through the water appear* to be essential to the per- 

 htttia* of the coral, which, we are told. always grows nearly perpen- 

 dioular to the .urtuje of the rock to which it U attached, what. v. r ilu< 

 position of the rock Itself may be. It is of various ooloura, from black 

 or brilliant red to a dull white, and varies exceedingly in value, some 

 wiling for aa much aa eight or ton guinea* per ounce, while other 

 aoru are not worth a shilling per pound. The mode of conducting 

 the fishery in varioua places u much the same. In that described by 

 Bpalhuuani. the instrument used consist* of two heavy beams of wood, 

 soured together at right angles in the form of a cross, and loaded at 

 their intersection with stones or other weights sufficient to sink them. 

 A quantity of loosely twisted hemp, intermixed with coarse netting, is 

 attached to the under side of the beams, at the ends of the cross, to 

 the mioMlc of which u secured one end of a strong rope by whioh the 

 apparatus is let down from a boat, and guided to the spots where the 

 coral is supposed to be most abundant The branching form of the 

 coral causes it to become entangled in the hemp and network attached 

 to the cross, by which means it is broken off from the rock, and drawn 

 up with the apparatus to the surface of the water. Owing to the great 

 depth at which the fishery U carried on, the submarine movement and 

 drawing up of the cross is a laborious operation, sometimes requiring 

 the united efforts of half a dozen boats ; and the men employed are 

 exposed to considerable danger in case of the rope breaking. Other 

 machines are used by the coral-fishers of Marseille, in the fishing- 

 groiindit of what is called the " bastion of France ; " onu of these, called 

 in the Provencal language the talakre, ia intended for situations in which 

 the bottom of the sea is rough and uneven. With a few trilling 

 variations, however, all the instruments of which we find any account, 

 consist of weighted beams supplied with network, or some substitute 

 fur it, to entangle and break off the coral. In some cases a diver 

 descends to guide the ends of the crow into caverns which they \voulil 

 nt otherwise enter; and on iron hoop fixed to the beam is occasionally 

 used to break coral from the roof or sides of caverns, a net being 

 attached beneath it to secure what is thus broken off. 



Some of the ornamental articles made of coral are very costly. 

 Besides necklaces, and the trinkets called childrcns' corals, the 

 material is wrought into bracelets, ear-ring drops, caiueoa, carvings, and 

 Rtatu ties. The material is very hard, and requires sharp instruments 

 to work it. 



CORANTO. [COUBASTE-] 



CORBEL, a term used chiefly in Gothic architecture to signify a 

 projecting piece of stone, wood, or iron, placed so as to support a 

 weight of materials. Corbels are sometimes in the form of the 

 modillion or mutule employed in entablatures, and also like the con- 

 sole of a window. Small semicircular towers projecting at the angles 

 on perpendicular surfaces of large towers or other edifices are supported 

 on a series of plain or moulded corbel stones. The weather-mouldings 

 of semicircular windows are often supported in the same manner. 

 The machicolations of towers are almost always supported on corbel 

 stones, as may be observed in the old gates of Southampton, Canterbury, 

 and York. This projecting of one stone beyond another ia technically 

 called " corbelling out." This is done in brickwork as well as in 

 masonry. In the ulterior of some churches the construction of the 

 roof appears to be supported on corbels, the ends of which ore often 

 carved, and represent on angel holding a shield. In Norman and 

 Early English architecture the cornice is supported by a row of corbel 

 stones, known as the corbel-table, the ends of which are also carved. 

 In old English castles the main beams of the floors were frequently 

 carried on large corbel stones, as at Porchester Castle. The term 

 bracket is sometimes used for a corbel. Bracket, however, is better 

 applied as synonymous with cantilever. Small wooden brackets often 

 differ from corbels and cantilevers in being merely nailed to a per- 

 pendicular piece of wood and not having a bearing on a wall, as in 

 always the case with corbels and cantilevers. 



CORD, frequently spelt chord, means an elastic string fixed at the 

 two extremities and stretched with force sufficient to enable it to yield 

 a musical note. [ACOUSTICS.] The close analogy which exists b 

 string and a column of air in a state of vibration \voul<i require more 

 Space to elucidate properly than this subject will allow us to givr : wi- 

 shall therefore assume some results of mathematical reasoning, point out 

 the probability of those results, and consider the theory of the vibrating 

 (-'.I independently. 



Firstly, we suppose our cord to be of uniform thickness ami . i 

 so that any given length is precisely of the same quality, from whit 

 part soever of the string it may IMS cut. The want of thU condition 

 being practically fulfilled is sometimes sensibly felt iu violin and 

 violoncello strings, which are then called fnl . A player whoso in- 

 tonation is perfect upon a perfect string, has to learn a new instrument 

 win n he attempts to play with a false string; to say nothing of tliu 

 harmonies which must be heard more or less becoming uncertain, if 

 not discordant 



Let us first suppose a string of indefinite length, and not so acted 

 upon by gravity as to bend downwards, stretched at the two ends by 

 equal weights. The string is a cylinder of uniform diameter and 

 density. Let us next suppose that a part of the string U placed on a 

 mould which catches two points and holds them fast, and stretches 

 the intermediate part into a curve which differs very little from a 



straight line. Let the mould be suddenly removed, and let us furth 



-O* i J^~ *^"w**^ in iu> ivfuuvt mi; 



diagram, where A (dotted) represents the part in question bef 



mould was applied, and it U A c B the instant the mould is taken away , all 

 the points between A and B being hi a state of n. rd or down- 



wards. The string will not remain an instant in its present state : the 

 first presumption is that the moment the points A and B are set free a dis- 

 turbance will take place in the parts of the string both between i- 

 and between q and B, the disturbance travelling from A towards p as well 

 as from B towards q. The first point in which we are interested U this : 

 with wli will thu disturbance be propagate*! ? and the answer 



is, that the disturbance travels throughout the string with the same 

 velocity, depending upon its material and the weight with which it 

 is stretched, according to the following law : Let each of the equal 

 weights i> and q be as heavy as e feet of the string; then 



, iiini.-at.il .n is so many feet per second as a bullet would . 

 if it were allowed to fall in vacua down a perpendicular of r i 

 V2jrc, where jr is the velocity which gravity communicates iu one 

 second, or 82J feet. For instance, let the string be iron win, 

 cubic foot of which weighs 7200 ounces avoirdupois. Let the diameter 

 be one-twentieth of on inch, and let the weights p and y I < < -;i. -ii lin 

 pounds or 320 ounces. Then the weight of x feet of the string is 

 -a inch being jj,, of a foot) 



3-14159 x (jj,,) 1 xxx 7200 ounces 

 whieh made equal to 320 gives x = 3259-5 feet and 



S-2 x 32J x 325U-5 = 457'93 = 458 nearly, 



or 458 feet per second is the rate at which the disturbance is com- 

 municated. 



We now ask what is the nature of the disturbance commun 

 It suggests iteelf as possible, that there may be some species of 

 disturbances which travel only in one direction : for 

 might so proportion the velocities of the disturbed point- t.. their 



is, that A I' should remain undisturbed, that the points i 



the right i-ho-.ild drop on > -ther into i' 



while those from B to o. should be fir.. -':c ivily di.-inrbl. The n 

 again derived from mathematics, i*. that sucli - 



and that the conditions under whieh it will tdto place are : 1. That O 

 being the highest of the disturb, d p..in. .all the joints froii, 

 must be moving.' ' the lir.-t id all thos. 



B to Q npicanlt. 2. That the velocity of the ]uints must be as f . 

 At any {mint K draw a tangent BT to the curve of disturb. 



city of the point n must be such as would, i; 

 uniformly, carry the point R from B to N or from x to B in th. 

 time as the whole disturbance is propagated from T to N. The 

 ditions being fulfilled, we may represent the successive states . 

 string by cutting out a pkiv of paper of the form ACB, and carrying 

 it along the string r q at tho uniform rai d which v, 



found for the propagation of the disturbance. If the dire, 

 velocities be all n-ver. ed, then the disturb: 1 lY.nu n tov 



Lot us call such disturb' .- and with respect to the dii 



iu v.hich they travel, tlircrt islur learn from 



mathematical analysis, that any disturbai ; 

 upon the same points at the same instant, produce a co:i 

 turbaiice of which the values of n s and tin- velocities at the point u 

 in the compound disturbances are the :. 



of K x, Ac., in tho component disturbances. And moreover, that any 

 disturb, ,-ver. be the law of its form and velocities w] 



in be compounded of two simple disturbances, on. 

 one retrograde. So that the moment tho mould is remo-. 

 simple disturbances will begin to travel in different ways, 

 they have not completely separated, there will bo points rein 

 under the ell', ei- "f both: but when they have had time to s. 

 completely, we . iiould. were the rapidity of transmission n 

 1 travelling by it-elf in one iiin.-ci 



le in tin- other. 



To illustrate this, suppose v ascertain 



disturbance ^ c B, as drawn, tl. thing 



at oil at the instant the mould ia withdrawn. Tl. 



turbanccs whi.'li would prod.;. Ihiii are two 



that at any point n the v.-Ii.i ity would bring the point throi 



while the disturbance is communicated through A N. U we i npposs 



