Defer ipl'ton 9/ a pttU Tljion nvilh metallic Fiititigt. 3 55 



•THE contrivance to which my correfpondent refers, 13 part of a fteam engine, for which 

 H patent has been taken out. It is generally undcrftood that in pra£lice it is neceflary to' 

 apply the pacicing clofe round the pillon of a (team engine, in fuch a manner as to maice it 

 aft (Irongly againfl; the fides of the Cylinder, which mult occafion a confiderable degree of 

 friction : and when it has worlced loofc, it may be concluded that fome lofs of force muft 

 follow, from the efcape of elaftic fluid between the furfaces intended to be in contadh A 

 fimilar obfervation may be applied to the ufual leathering of pumps, in which the fri£lion 

 is rery great, and the refiflance to fevere prefijre very far from being eftedlual. Early in the 

 prcfent century, a contrivance was made by a Mr. Ilalkins to prevent fridion, by fubilitut- 

 ing quickfilver Inftead of leather ; which, for fcvcral ftatical reafons, and alfo, as I fufpedl, 

 from fome of a chemical nature, could not be brought into general ufe. The reader may 

 fee a very full defcription of this engine, in Defaguliers's Courfe of Experimental Philofo- 

 phy, II. 491. 



Mr. Cartwright's invention confids in ufing folid mafles of metal inftead of the packing 

 or leathering ; which, by means of fprings, adapt themfelves to the variations of diameter in 

 the cylinder, and, by their mode of application above each other, are expected to prevent 

 that efcape of fluid which would clfe take place through the intervals between the feveral 

 pieces. Imagine the pillon to confift of a circular plate of metal, nearly equal in diameter 

 to the cylinder in which it is to move. It will make no difference in the general confide- 

 ration of our fubjeft, whether this pifton have a valve in it or not. Upon the upper furface 

 of this pifton are laid three or more pieces of metal, which all together compofe a flat cir- 

 cular ring ; the ends of the feveral pieces nearly touching each other. A very moderate 

 portion of mechanical knowledge will fuggefl the manner in which thefe pieces might be 

 made to recede outwards, by means of fprings; fo as to occupy the circumference of a greater 

 circle externally, than that to which they would nearly correfpond when regularly prefled 

 inwards. If the pifton in this fituation be placed in its cylinder, it is evident, that the 

 pieces compofing the ring will be forced againft the concavity ; which, if of the proper cur- 

 vature, they will fit, and prevent any fluid from pafllng through, except at the interftices, 

 where the ring is rendered incomplete by being divided into parts. To remedy this, a 

 fecond ring is laid upon the firft, with its joints half-way between the joints of the former, 

 in the fame manner as we every day obferve in courfes of brick-work. This fecond ring, 

 being urged outwards like the firft, performs the fame function ; but any fluid that may 

 pafs downwards through the interftices of the upper ring, will be ftopped by the conti- 

 guous parts of the lower ; and, on the contrary, whatever fluid may pafs upwards through 

 the interftices of the lower ring will, for the fame reafon, be ftopped by the upper. 



From this defcription it is fufiiciently clear, that Mr. Cartwright is entitled to much 

 praife for the ingenuity and acutenefs difplayed in his contrivance. The practical value 

 of that contrivance,' refpefting which I am requefted to ftate the fads, will probably be af- 

 certained from the following confiderations : 



In the ufual ftuflSng or leathering, the elafticity of the organized matter made ufe of i« 

 fuppofed to a£t, in each individual part, fo far independently, that, if there be an irregularity 

 in the cylinder by variation of its curvature or magnitude, this irregularity will be followed 

 «p and fitted ty the elaftic material. But Mr. Cartwright's metallic packing pofleflfing a 

 determinate curvature, will fit only when the zone witii which it i» in contad poflefles the 

 Voi.,11.— Nov, 1798. 3B ^ fame 



