32 PHENOMENA obferved in tht AIR VAULT 



cavated, and formed by a mine drove in the folid rock, diftant 

 from the furnaces about 1 6 feet. The bottom of the air vault 

 is only about 4 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 machine, and has two at the oppofite end, 

 one of which receives the educflion pipe, and the other is a door 

 to give admittance occafionally into the vault. As the rock is 

 extremely clofe and folid, the vault is dry, except that a httle 

 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. 

 These furnaces are provided with air, or blaft, as it is term- 

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

 conftrudlion. The diameter of the fteam cylinder is 48 f inches ; 

 and the fquare area of its pifton being about 18667 fquare 

 inches, the power of this fort of engine cannot be rated at 

 more than 7 lb. to the fquare inch, amounting in all to about 

 13062 lb. This power was employed to work an air pump, 

 or blowing cylinder, of 78 inches diameter, and about 7 feec 

 long. The number of fquare inches on the pifton of the air 

 pump is 4778, and therefore this area, being multiplied by i\y 

 will produce 13 139. being a refiftance that nearly balances the 

 above-rated power, and fhows that the air, which was expelled 

 from the air pump, could not be condenfed more in the ordi- 

 nary way of working, than with a compreflTmg power of about 

 2| lb. on each fquare inch. As the engine was not regulated, 

 at firft, to make a longer ftroke than about 4 feet 8 inches, only- 

 one furnace being ufed,the quantity of air expelled at each ftroke 

 of the machine was about 155 cubic feet, which it difcharged 

 through a valve into the air vault, about 16 times in a minute. 

 When two furnaces afterwards were blown, the engine was re- 

 gulated to work much quicker, and with a longer ftroke. The air 

 vault is 72 feet long, 14 feet wide, and 13 feet high ; and con- 

 tains upwards of 13,000 cubic feet, or above 80. times the 



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