in an Earthquake Country. 213 



wish to drive in a nail without hurting the head with the 

 hammer, a block of wood is used as a cushion, the wood being 

 of service because, having an appreciable time of vibration, it 

 causes the duration of the impact to be lengthened, and so di- 

 minishes the force acting at any moment. Similarly, the baro- 

 meters in men-of-war are not now suspended direct to the 

 ship's sides, but to the end of a flexible lath, in order to prevent 

 the shock accompanying the firing of the guns breaking the 

 barometer-tube. In the same way, the lower parts of a struc- 

 ture having appreciable times of vibration cause the earth- 

 quake-shock to be altered in character, to be lengthened in 

 time, and therefore diminished in intensity before it reaches 

 the upper parts. Hence it is obvious that if small stones or 

 bricks set in bad common mortar are our building mate- 

 rials, it would be better to choose for the site a quaking bog 

 which was capable of supporting the weight of the building, 

 rather than to build the house direct from a rocky foundation ; 

 or if the ground is firm, there ought to be placed underneath 

 the house a foundation of yielding timber ; or some other 

 method should be sought for by means of which the time of 

 transmission of momentum through the joints may be in- 

 creased. 



Thus, there is a best time of vibration of the part of a struc- 

 ture below a joint, which depends on the strength of the joint; 

 and if the basement has a time of vibration different from this, 

 then we should advise that the building be kept low. For ex- 

 ample, it is desirable that houses with ordinary wall-thicknesses 

 built of bricks set in common mortar should not be more than 

 one or, at the very most, two stories high if there is a piled 

 or concrete foundation ; but if good cement be employed in- 

 stead of bad mortar in fastening the bricks together, then a 

 height of two or three stories may be employed probably with 

 comparative safety. 



Again, the horizontal vibration of the ground is given up 

 to a stone or brick building mainly by shearing-stress com- 

 municated from course to course, a kind of stress which mortar 

 is very unsuitable to transmit. Hence a stone or brick build- 

 ing subjected to horizontal shocks ought certainly to be built 

 with cement and not with ordinary mortar. In fact, in every 

 part it ought to be capable of resisting pulling as well as 

 crushing stresses. 



Every joint is a weak place ; and it is evident that if by in- 

 creasing the size of the building wc diminish the area of joints, 

 we shall be increasing the stability. Now in large masonry 

 structures larger stones are, as a rule, employed, and the joints 

 are made of less area. In this respect, then, may wc say that 



Phil. Mag. S. 5. Vol. 8. No. 48. Sept. 1879. Q 



