J 14 On the Blajl of Iron Furnaces, 



a large fum of money, by flopping the works, and altering the blowing machinery. 

 Indeed, it has fincc been admitted, by all who have feen it at work, to be the mod fimple 

 and efFe£live method of equalizing the blaft of aoy yet put in praftice. 



This experiment led me, fome time afterwards, to apply a wind guage that I contrived, 

 to afcertain precifely the ftatc of the condenfation of the air thrown into the furnaces. I 

 found that a column of qulck-filver was raifed five inches, and fometimcs, though feldom, 

 fix inches, and, in the interval of the engine to receive air into the air pump, it fell only 

 half of an inch. At this time only one furnace was worked. But when two furnaces 

 were in blaft, the engine only raifed the mercurial guage about 4 inches, becaufe the Devon 

 company, for certain reafons, did not, while I continued a partner, think proper to 

 allow the blowing machinery to be completed, by the putting to work their feccnd boiler 

 of 20 feet diameter for the fire engine, according to my original defign, which, by adjufting 

 the machinery, would have enabled us to blow two furnaces, with two boilers, with as 

 much eSe£l:, in proportion, as one furnace with one boiler. This inftrument had the ad- 

 vantage of enabling the work people to difcover the real power of their blaft, and. know 

 the exa£l condition of the air valves, and the gearing of the blowing plfton ; for if thefe 

 were not tight, and in order, (although the engine might, to appearance, be doing well> 

 by making the fame number of difcharges of the air pump as ufual per minute), yet the 

 wind guage would not rife fo high, and would fhew that there was an imperfedtion fome- 

 where, by reafon of a quantity of air efcaping at the valves, or pifton, that could not fo 

 eafily otherwife be known. This contrivance vvas confidered as of much ufe, and was 

 afterwards always quoted in the company's journal books, to fliow the aftual ftate of the 

 blowing machine, in comparing the daily produce of the furnaces. 



I hope you will not think me tedious, when I explain to you another experiment, 

 which appears to me to be of confiderable importance to all manufadturers of caft iron. 



I had reafon to conjeflure, from my own obfervations on the effects of blowing 

 machinery on blaft furnaces, as well as from the knowledge I had acquired from my 

 father Dr. Roebuck, and from my communications with other experienced iron mafters> 

 that a great part of the power of fuch machinery was mifapplied in general pra£l:ice, by 

 throwing air into furnaces with much greater velocity than neceflary, and that, if this 

 velocity was, to a certain degree diminilhed, the fame power, by properly adjufting the 

 blowing machinery, of whatever nature, would be capable of throwing into the furnace^ 

 a proportionally greater quantity of air. For, " Since the Quantities of any fiuid, ijfuing 

 " through the fame aperture, are as the fquare roots of the prejfure " it follows, that it would, 

 require yowr times the preflure, or power, to expel double the quantity of air, through the 

 fame aperture, in the fame time : but if the area of the aperture was doubled, then the 

 quantity of air expelled by the fame power, and in the fame time, would be increafed in 

 the ratio of the fquare root of 2 to i, though its velocity would be diminifhed exadlly in 

 the fame proportion. Again : I confidered that the quantity and intenfity of heat, pro- 

 duced in blaft furnaces, and confequently its effe(fts in increafing the produce, might be 

 4 only 



