300 Mr Bardie's Outline of' the 



greyish-white. It can with difficulty be obtained in slabs less 

 than four inches thick, while No. 1. may be had in plates less 

 than one-half an inch. 



The above rocks are all of them employed in architecture, and 

 are remarkably free working stones. Slabs of the slaty va- 

 riety, spanning from beam to beam in flat roofs, have been sub- 

 stituted by my friend Lieut. J. T. Boileau of the Bengal 

 Engineers, instead of the bridging joists and tiles in common 

 use ; and he has found that the roofs completed upon this prin^ 

 ciple have, in every instance, proved perfectly water-tight, an^ 

 that the terrace laid over the stones has invariably dried more 

 uniformly, and freer from cracks, than when bridging joists have 

 been used. By a series of experiments. Lieutenant Boileau has 

 demonstrated that no apprehension need be entertained in regard 

 to the transverse strength of the sandstones, objections on this 

 head having been urged against his plan, while the advantages 

 attending its adoption are invaluable, inasmuch as it secures 

 the buildings against fire, and, what is of still more importance;, 

 renders them safe from the attacks of white ants. The annexed 

 Table will shew the results of Lieutenant Boileau''s experiments, 

 which are interesting both in a mineralogical and economical 

 point of view. 



*:■ ij By a reference to the Table, it will been seen that the red- 

 sandstone does not lose much, if any, of its strength by being 

 saturated with water, which renders it particularly fit for the 

 'purposes of roofing ; and though, in its dry state, it is far infe- 

 rior to the salmon-coloured variety, (as 11 to 17 nearly), yet, 

 when wet, it is something superior to it, the latter variety losing 

 about one-half its strength by immersion in water. For the 

 cause we may look to the difference observable in the arrange- 

 ment of the component particles of either variety. 



1^5 



