21(> On Structures in an Earthquake Country. 



Square house with no roof. 

 Let 7i=30 feet, a=30 feet, c=2G feet; then 



T=0-001534/u/i/ (J0Q + 3) 



V 2VJOO + 676 / 



= 0*06120 second about. 



This result may be taken as approximately correct even for 

 roofed houses, because in a small house the roof gives stiffness 

 as well as adding inertia. 



When the height of the building is not more than twice or 

 three times the outside horizontal dimensions, the shearing 

 strains are important, and must be included in the calculations, 

 as we have done. But when the height becomes eight or more 

 times the horizontal dimensions, as in the case of a chimney, 

 then we may neglect shear stress and consider bending only. 



Tall Chimney. 

 Let 7* = 150 feet, a =10 feet, and c = 4 feet — that is, let us 

 consider a thick square chimney; then 



T=0-001534 + 150 



x/Kw+4 



= 2*301 seconds, about. 

 If the chimney has the same internal dimensions, but if the 

 brickwork is thinner than we have taken it, then a is less and 

 T becomes still greater. Even, therefore, without the heavy 

 top that some engineers have recommended to be added, this 

 structure vibrates too slowly to be sufficiently safe in a country 

 visited by frequent earthquakes, like Japan ; and we think it 

 quite likely that the first really severe shock which maybe expe- 

 rienced by the various chimneys recently erected in this country 

 will destroy them. It may here be noticed that even in 

 England it is not thought to be safe to connect a tall factory- 

 chimney with the main walls of a building ; so that, remem- 

 bering the very great difference in the time of vibration of the 

 chimney and the walls, such a connexion must be regarded as 

 exceedingly unsafe in Japan. 



Mountain. 

 Let us consider a conical mountain, then it is clear that in 

 its vibrations shearing-stresses need alone be considered. A 

 horizontal force P acting at the centre of gravity produces the 

 deflection 



'A P da 



f 



Til 



*2 N " 



