66 On the "various EffeBs produced hy 



raifed by the furnace, and diftribute it to the greateft advan- 

 tage. This is not the cafe with the floating piftons, where 

 a certain quantity of fpare wind is thrown out at every return 

 of the engine, left the great pifton and weight (hould be 

 blown out of the cylinder altogether; which, indeed, fome- 

 times happens. The only objecSlion which remains in force 

 againft the ufe of the water-vault, is the tendency which it 

 has to take up a confiderable portion of the water in folu- 

 tion, and introducing it into the furnace. A judicious ar- 

 rangement of the condu£ling-pipes would in fome meafure 

 obviate this, as well as the more dangerous tendency which 

 water has to rife in a pipe fpeedily emptied of its air by the 

 flopping of the engine : a ftream of water thus conveyed to 

 the furnace, would be produftive of the moft awful confe- 

 quences. 



The air afforded by the air-vanlt is much inferior to that 

 obtained in the former methods. This immenfe magazine 

 of comprefled air generates a confiderable portion of heat, 

 which greedily fcizes the damps, which are unavoidable in 

 underground excavations, and conveys them to the furnace. 

 The blaft is, however, fteady and uniform ; and when the 

 infide of the building is completely fecured againft the paf- 

 fage of air, it is produ£l:ive of confiderable effefts in the fur- 

 nace. In the fummer months, however, the air becomes fo 

 far debafed as to afl'eiSl the quality of the iron, and change 

 it from grey to white. Every change in the temperature of 

 the atmolphere during this period, is indicated by various 

 changes in the furnace. 



The largeft air-vault hitherto in ufe was excavated out of 

 folid rock at the Devon iron-works: the fiftlires of the rock 

 admitted confiderable quantities of water; and the fame de- 

 gree of damp would always prevent the poUibility of making 

 the fide-wails and roof air-tight by means of pitch and 

 paper, &c. 



Befides the various natures of blaft, as to the ftrength and 

 equality of the current afforded by different modes of con- 

 ftru£ting the blowing-machines, a variety in the quality of 

 the air obtained is alfo an invariable confequence : this is 



fufficientlv 



