i i6 On tie Blaji of Iron Furnaces, 



confumption of materials, from which this large produce was obtained, was by no means 

 fo great as formerly. The furnace required very confiderably lefs fuel, lefs iron/lone, and 

 Icfs liinejione, than were employed to produce tjie fame quantity of iron by the former 

 method of blowing ; and according to the ftatements made out by the company's orders, 

 as great a change was efFe£ted in the oeconomical part of the bufinef;. 



From the fuccefs of this experiment, fa well authenticated, and continued for feveral 

 months, I am led to be of opinion, that all blaft furnaces, by a proper adjuftment of fuch 

 machinery as they are provided with, might, greatly and advantageoufly increafe their pro- 

 duce, by affuming this as a principle, viz. " That -with the giving power it is rather by a great 

 ^^ quantity of air thrown into the furnace, with a moderate veheity, than by a lefs quantity 

 ." thrown in with a greater velocity, that the great ejl benefit is derived, in the fmelting of iron- 

 ^ Jfo/ies, in order to produce pig-iron." However, it is by experiment alone, perhaps, that 

 we can be enabled to find out the exaft relations of power, velocity, and quantity of air. 

 requifite to produce a maximum of effedt *. 



But, an unfortunate difagreement among the partners of the Devon company, put it 

 out of my power to make further progrefs in this matter, by laying me under the neceffity, 

 two years ago, of withdrawing myfelf entirely from the concern. 



I have the honour to be, refpe£tfully, 

 SIR, 



Your mod obedient fervant", 



Edinburgh, June 20, I J gS. JOHN ROEBUCK. 



To Sir James Half, Bart. 



• If Q^be the quantity of a fluid, iffuing in a given time tlirough an aperture of the diameter D, V its 

 verocity, and P the power by which it is forced through the aperture : then the area of that aperture being 

 as D', the quantity of the fluid ifluing in the given time will be as VD', or VD' = Q. 



Again, this quantity multiplied into its velocity, will be as the momentum of the fluid expelled, or as 

 tbe power by which it is expelled, that is, V D' zz. P, or VD zi v'P- - 



Here, therefore, if D is given, V is as v'P, as Mr. Roebuck aflirras. Alfo, becaufe V ■=. ^, and 



v'P 



alio V — -~t Q^ DvP, fo that, while P remains the fame, Q_will in:reafe as D increafes, and V will 



diminifh in the fame ratio. 



The problem, therefore, of throwing the greateft quantity of air into the furnace, with a given power, 

 ftfiflly fpeaking, has no maximum, but the largeft aperture of which the engine can admit mutt be the beft. 

 It is probable, however, that theie is a certain vclociiy with which the air ought to enter into the furnace j. 

 this will produce a limitation of the problem, which, as Mr. Roebuck fuggefts, is not likely to be difcovered 

 but by experiment, J . P. 



In 



