of the Devon Iron-lVorls. 32^ 



bottom of the air-vault is only about four feet higher than 

 the level of the bottom of the furnaces. This vault has an 

 aperture at one end to receive the air from the blowing ma- 

 chine, and has two at the oppofite end, one of which re- 

 ceives the edu6lion-pipe, and the other is a door to give ad- 

 mittance occalionally into the vault. As the rock is ex- 

 tremely clofe and folid, the vault is dr\^, except that a little 

 water ouzes very gently from the fide next the bank in fmall 

 drops, and does not appear to exceed an Englifh pint in 

 24 hours. 



Thefe furnaces are provided with air, or blaft, as it is termed, 

 by the means of a fire-engine of the old, or Newcomen's 

 conftruction. The diameter of the fteam cylinder is 48|- 

 inches; and the fqnare area of its pifton being about 1866^ 

 fquare inches, the power of this fort of engine cannot be 

 rated at more than 71b. to the fquare inch, amounting in all 

 to about 130621b. This prnver was employed to work an 

 air-pump, or blowing cylinder, of 78 inches diameter, and 

 about 7 feet long. The number of fquare inches on the 

 pifton of the air-pump is 4778, and therefore this area^ 

 being multiplied by 2|, will produce 13139; being a re- 

 fiftance that nearly balances the above-rated power, and 

 fliows that the air, which was expelled from' the air-pump, 

 could not be condenfed more in the ordinary way of working 

 than with a compreflling power of about 3 Mb. on each fquare 

 inch. As the engine was not regulated, at lirft, to make a 

 longer ftroke than about 4 feet 8 inches, only one furnace 

 being ufed, the quantity of air expelled at each fl:roke of the 

 machine was about 155 cubic feet, which it difcharged 

 throuoh a valve into the air-vault about 16 times in a 

 minute. When two furnaces afterwards were blown, the 

 engine was regulated to work much quicker, and with a 

 longer ftroke. The air-vault is 7a feet long, 14 feet wide, 

 and 13 feet high ; and contains upwards of 13,000 cubic 

 feet, or above 8c times the contents of the air-pump. The 

 top, fides, and bottom of this vault, where the leaft fiflfure 

 could be difcovered in the beds of the rock, were carefully 

 caulked with oakum, and afterwards plaftered, and then co- 

 vered with pitch and paper. The intention of blowing into 



the 



