ON THE EARTHQUAKE AND TOLCANIC TKENOMENA OF JAPAN. 309 



ings, partly for this reason, and partly on account of their expense, are 

 not looked upon with great favour in Italy. The Ischian law specifies 

 Uiat if iron bands or chains are used they must act upon a large surface. 



7. Roofs. 



The advantage to bo gained by making the upper portions of any 

 structure light is very great. When a building with a heavy roof is 

 suddenly moved forwards, the roof by its inertia tends to remain at rest. 

 The result of this is the tendency to cause a fracture between the lower 

 part, which has been removed quickly and the upper part, which has 

 tended to remain at rest. In building regulations special reference is 

 made to roofs, which must always be light, the material recommended 

 being iron, zinc, or felt, ordinary tiles being only permissible for build- 

 ings which are only one story high and not habitations. Certaiu kinds 

 of tiles have sometimes been regarded as permissible, but these require 

 to be properly secui'ed, and it is specified that in such cases, above the 

 ceiling, there shall be a floor of planks. Tiles require to be especially well 

 fastened near the eaves. 



The difficulty with roofs made of sheet metal is first to secure them so 

 that they shall not be disturbed during severe gales, and, secoud, to pro- 

 tect the interior of the hoiises from heat. In Manila the fust end is 

 accomplished by a special system of bolting, whilst the latter is attained 

 by a series of false ceilings. 



The tie beams of trusses should extend at least two-thirds across the 

 thickness of the wall, if not over the whole thickness, and these rest upon 

 wall-plates. The form of truss recommended in Manila is the one with 

 a central post (king-post). For spaces greater than 7 metres, iron should 

 be used, and trusses must be so placed that they do not act upon weak 

 points in the walls. 



The Ischian law does not prohibit the use of flat roofs (ferrazzo'), but 

 it provides that the framing of the same shall be strong, and covered with 

 materials which are fairly light. 



The Commission who reported to the Government, however, con- 

 demned such roofs. 



8. Walls. 



Walls, like chimneys, require to be light and strong. If heavy, and 

 especially if loaded in their upper parts by copings and balustrades, they 

 may be fractured and shattered by their own inertia. The height to 

 which walls may bo taken with safety depends upon the material of 

 which they are constructed, the nature of the roof, &c. In Ischia it was 

 .'suggested to limit buildings to two stories, or a height of 7'55 metres. 

 The regulations, however, give 10 metres as a limiting height, and, if 

 they must be of simple masonry of tuff, to a height of 4 metres, with a 

 thickness of '70 metre. The committee suggested that external walls 

 should be at least 'SO metre in thickness, and that their uniformity 

 should in no way be broken by openings for chimneys, pipes, &c. 



The Ligurian regulations allowed three stories above the cellar, and a 

 height of IT) metres. The walls should be thick. If not built on the 

 barrack system they should be at least 60 centimetres and have a batter 

 of ^'y of their height. The Norcian regulations allowed two stories 



