28. Oil the Materials ufed 



the piece, opens the pores of the wood, and admits the par- 

 ticles of matter. By this accumulation of weight the wood 

 finks, and in 15 to 18 days a foft flime is formed over all its 

 furface; this gradually hardens into a craft, and diffufes it- 

 felf inwards : during the progrefs of formation the flitny 

 matter which is next to the wood receives the impreffion of 

 the fibres, and communicates their form to the mafs. Hence 

 Impreflions are conveyed to ftones, which are afterwards 

 formed when in a compact ftate. 



I am afraid it will be thought that I have exceeded the 

 limits I propofed to myfelf ; but the few particulars I have 

 related concerning the depofition of matter from water may 

 tend to explain fome part of the phenomena attending the 

 formation of fome iron-ftones. I (hall probably on a fu- 

 ture occafion give fome further obfervations concerning 

 the effects of this water, and the progrefs it makes in fubfti- 

 tuting one fpecies of matter in the room of another, and in, 

 the formation of ftrata. 



JFbrking of Iron-Jlone. 



When iron-done is found near the furface, it is only ne- 

 ceffary to unbare the foil and the fupcr-incumbent earths : 

 the ftone then prefents itfclf like a pavement more or lefs 

 inclined, and is eafily raifed by the application of wedges, 

 bars, &c. &c. But as all fecondary ftrata defcend, according 

 to the declivity impofed upon them by nature in their forma- 

 tion, it is obvious that, in proportion as the excavation ex- 

 pends at right angles to the line of level, the ftone be- 

 comes gradually buried under an accumulating depth of 

 garth, till fuch time as the expence of throwing this off 

 exceeds either the value of the ftone, or the expence at 

 which it could be procured by a different mode of operation. 

 ft therefore becomes ncceffary to make perforations by 

 means of horizontal galleries, extended under the foil, fa 

 as to fall in with the declivity of the ftrata : thefe catch the 

 inclining ftone for a ftretch of ico to 240 yards, according. 



to the exiftent circumftanccs of the mine, 



Thefc 



