CilAPTKIJ r. 



TIIROKl-'/l'ICAl. ('(iNSIhKIIAIlHNS I; IK ; A I,' 1 1] NU 

 KAKTHi.il'AKK MuTIttX. 



ü I. ]'ilinitliiii.< pnn-iriliiii/ ffii)ii (I I'ruirr irllliiii iiii rh(<llr .iiih.-.linir,\ 



Wlif'ii a sinL:!»' smlilcn clislnrliancr oi' tln' iiiosi nciicinl cliaractcr nccins at 

 anv |>1;U'(> in an iiulcliniti'lv cxtfiuli'«! iKunnnciiciHis isi(tin|iic clastic solid, two 

 waves or states nf vilnaforv iiinliDii will iiruccci) oiilwarils mIhiil:' sll■ai^llt lines 

 friim tlie orisfin, willi twn (lill'erciit velocities. II' (lie space in wliicli the tiis- 

 tiirliaiU'c ovi^-iiiati-s is very siiiall <'i.iii|i:ir<'il willi ihal in(<i wliieji (lie \\aves 

 siiliseiiiientiv (livcrLCC. we ina\- witlioiil sensililc error consider tile wave-tVonts as 

 splierii'al surfaces liavinLf tlie origin for centre. I'v wavc-fVont is nu'ant anv 

 surface f'orniinii' tlic /m-ns ni' | articles wiiicii wvr siniultiincoiisl\- nuvxiuL; in lln' 

 same |ili:is<>. 'I'lie line of tr-an-it, oi- direction alouij; wliicli each wave travels, is 

 normal to ihc wave-front. ()ni' of the two \va\cs consists of <iim|iressioii and 

 dilatation of the niatcriai in the liirci'lioii of thi' line of transit. This, which is 

 |tro|iai;atcd with the hii^her vclocitv of the two, is called the Normal wave: its 

 |ias.sa>;c causes each [larticlc to mii\e in the direelion ni' (he line of transit of the 

 wave. Till' other wave, which tra\cls more slowly, consists of distoition of the 

 niatcri;il nnaccom|>anicd ii\' anv chani;-e of volume, ft is called the 'I'ransverse 

 wave, and its passiiifc causes each parlicle to mo\e :it ri;;hl anales to the line of 

 tr.msil of the wave. 'I'he path in w iiicli a part idc niovi-s must I«' rectilinear so 

 far as the normal Avavi- is eoncernul ; lull the transverse wave mav u'ix'c ri~e |o 



motion in any curve lyiuj^ in the jilane tangent to the v.a\e-ti I. In iiiither 



eas<' is it nei-css.arv that the p.atli slienhl lie closed. Thi^ uill happen ouK' when 

 the oriirin.atini; disturli.'iuce is cvclie ; niore jjenerally a |iarlic!e will he f.nncl 

 |M'rnianr'ntly displaced from its initial |iosition aft'^r the wave has passi'd. 



We can o.asilv iinas^iiw .an oriiri'iatiui;- impulse of such a character as would 

 ^ive rise to either wave alone, Imt in «rcucral hotli w.aves will he picMlnced. 

 Ay.iin. the pro|M'rties of the disturhed medium may lie ••uch that one or otiicr 

 wave is im|MissiMe: thus a Iniusvcive wavi- cannot he fjenei-ated in or transmitted 

 hy lliiirls nwinjj to their want of ehastieify of form ; on the othei- hand an iucom- 

 pre-.~ilih' medium c.muot hi' llio scat rif normal waves. 'I'he Iraiismi.ssion of.souud 

 liv the air illustrates the existence of normal waves in a sniislunee eapahle of 

 tlipni oidy ; while thi' tiimsmission of radiant enerfry l>v transverse waves in tlu- 

 liiminiArroim ether exi-mplifics the otiier ca«c. 

 1 



