T. Mellard Reach — Subsoil Denudation. 



265 



a map, such as Mr. Mallet's, in which the whole area disturbed 

 during each earthquake is coloui-ed. 



(2.) Having given the position of the place where a vorticose or 

 vertical shock is observed, and also the times at which the earth- 

 quake is felt at three places at different distances from the seismic 

 vertex, and assuming that the velocity is the same along the radii 

 drawn to these places, we can determine the depth of the seismic 

 focus and the velocity of the earthquake- wave. 



Thus, let be the seismic focus, P the place where the vorticose 

 shock is felt, so that OP is nearly 

 or quite vertical, and A, B, C, three 

 places where the time of the shock 

 is observed, and which we may 

 (for simplicity of figure, and with- 

 out loss of generality) suppose to 

 be in a straight line. Let PA = a, 

 PB = b, PGz=c: OP = z, the 

 depth of the seismic focus ; t the 

 number of seconds between the 

 occurrence of the shock at C and 

 at A, t' between that at C and at B 

 earth-wave. 



Ft = OA-OC = Vz^ + «^ — Vz" + 



and V the velocity of the 



Then, 



Vt' = OB— OC = Vz^-{-b^ — Vz" + c\ 



Transposing and squaring each of these equations, we obtain — 



2 nVz"" + c^ = P — c^— VH'^ . . . . 

 ^' («2 _ ^2 _ YH') = t{b^ — c''— VH'^) 



V' = 



_ t' {a" — c"') — t {¥ — c") 



(1) 

 (2) 



(3) 



tt' {t—t') 



From equation (3) we can find the value of V, which, substituted in 

 (1) or (2), will give the value of z, i.e. the depth of the seismic focus. 

 If one of the places where the time is observed be that at which 

 the vorticose shock is felt, c =r 0, and the equations for determining 

 V and z become 



t'a? — tV- , a" — VH'' 



V = 



tt' {t—ty 



and z =: 



2 Ft 



IV. — Subsoil Denudation of Boulder-Clat. 

 By T. Mellard Reade, F.G.S. 



ON removing the superficial sand and soil to prepare for getting 

 clay for brick- making, it is not unusual to find the surface of 

 the Boulder-clay in this neighbourhood corrugated with tortuous 

 channels. These channels are commonly filled with slutchy sand 

 and gravel, which must be removed before the clay can be obtained. ' 



