82 Prof. C. G. Knott on Reflexion and Refraction of 



rock is hard and solid. Very frequently earthquake-sounds 

 die away before the earth-swayings have ceased — a fact which 

 is probably connected with the short-lived character of the 

 vertical motion as compared with the horizontal motions traced 

 out by our seismographs. 



With the air pulsations, which if rapid enough constitute 

 audible sounds, the following curious effect of earthquakes 

 may have some connexion. 1 am indebted to Professor Sekiya 

 for the information. It seems that at the time an earthquake- 

 shock passes, or it may be a little sooner, birds flying in the 

 air have been seen to drop suddenly, as if for an instant 

 paralysed, and then to recover themselves. This effect 

 might be sufficiently explained as due to a momentary mental 

 paralysis produced by fear. Perhaps, however, we have a 

 sufficient physical cause in the air pulsations, a slight change 

 of density being enough to disturb the delicate poise of the 

 hovering bird. 



Part II. 



Additional Examples of Reflexion and Refraction of 

 Elastic Waves in the Earth's Substance. 



To the detailed numerical results formerly published for 

 the case of rock and water, I now add the corresponding 

 results for rock and rock, for rock and air, for solid rock and 

 fluid rock, assuming certain relations among the densities 

 and rigidities. The hypothetical case of rigidities equal and 

 densities as 1 to 2 was worked out several years ago ; the 

 others have been worked out quite recently. In calculating 

 these I have taken as the angles of incidence the angles for 

 which the cotangent has values go, 4, 2, 1, 0'6, 03, 0*1. 

 When expedient I have introduced other angles, especially 

 when there were critical angles corresponding to cases of 

 total reflexion. The angles of reflexion and refraction corre- 

 sponding to these are given in the tables only to the nearest 

 degree. The angle of incidence or reflexion of the condensa- 

 tional wave is represented by 6, and &' is the angle of 

 refraction of the wave of the same type. The symbols (j) and 

 <ft represent the angles of incidence (or reflexion) and 

 refraction of the waves of distortional type. The letters A 1? 

 A', represent the derivative condensational waves ; and B 1? B', 

 the derivative distortional. The tabulated numbers give the 

 energies, the energy of the incident wave, A or B as the case 

 may be, being taken as unity. 



