Produflien of Steam under different Crcumjiancts. 87 



This letter arrived too late in the month for me to give an abridgejnent of fo much of Count 

 Rumford's excellent *Eflay VI. On the Management of Fire and the Economy of Fuel, as is appli- 

 cable to the enquiry it contains. If I can have the Count's permiflion to copy feme of the 

 engravings (of which I have no doubt), this fhall appear next month. In the mean time I 

 muft refer my correfpondent to the eflay of the fame author, on the propagation of heat in 

 Huids, of which accounts are given in this Journal, vol. I. 289. 341, 563. He will there 

 obferve that fluids communicate no perceptible quantity of their heat in ariy other way, than 

 by the actual contaft with folids, produced by the relative motions of their parts ; and from, 

 this leading fa£t it will follow, that the quantity of heat communicated to the boiler from a 

 given fire, will be governed by the arrangements for caufing the greateft number of particles 

 of heated air and flame to come into contaft with it, and for preventing the heat being con- 

 duced ofl^. It is very obvious, that the boiler he defcribes is not calculated to abforb all the 

 heat, but that from the fhortnefs and ftraitnefs of the channes, thofe heated fluids muft pafs 

 off at a very high temperature, to the confequent diminution of efFefl: and wafte of fuel. I 

 do not here minutely enquire what may be the beft figure for a fteam-boiler of iron, at the 

 ufual ftrength of lolb. to the fquare foot, to fuftain an internal preffure of between one-fixth 

 and one-fourth of an atmofphere, becaufe every one is aware of the advantages of a figure 

 approaching to the fpherical form, and becaufe other figures of much lefs ftrength will hold 

 very well under the circumftances above-mentioned. 



The principal queftion exclufive of the faving of heat, hereafter to be confidered, Is to dc- 

 termine how far the depth or fliallownefs of the water, in the boiler, may influence the produc- 

 tion of fteam. 



If the parts of a mafs of water could prefervfe the fame relative pofitions, during the appli- 

 cation of heat, and this heat were applied at the bottom under a depth of fix feet, it would 

 depend upon the conducing power of the fluid, whether the upper or lower parts fliould 

 give off" the greateft quantity of fteam in a given time. If the lower parts were to afFord the 

 greateft product, this would be done under a greater preffure and at a higher temperature ; 

 fo that it would become an obje£t of experimental refearch, whether fuch fteam, by giving out 

 heat to the fuperincumbent water, through which it muft rife, and rendering a portion of it 

 elaftic, might not be as effeftual in its ultimate operation, as if the heat had been employed 

 upon a fliallower mafs. But the ^£1 is, that the parts of fluids do not preferve their relative 

 fituation during the application of heat : the water in the boiler will expand and rife with 

 rapidity as it acquires heat, and, in all probability (for in this cafe alfo we are in want of fafts), 

 will give off a much larger portion of fteam while it circulates near the upper furface, 

 and is lofmg its elevated temperature, than while it glides along the bottom in the aft of re- 

 ceiving heat. From this view of the fubjeft, though I am difpofed on the whole to con- 

 clude that the fliallower mafs of water may, in many refpedls, deferve the preference, yet I 

 doubt whether the difference arifing from mere preffure be an obje£l of any confiderable im- 

 portance. As the medium temperature of the whole mafs of a fluid, heated from beneath, 



* PublJIhed before the Mmmencement of this JournaL 



b 



