OX STEAM-ENGINES. 819 



forcing before it or carrying with it arvy of the metal or other M 1 "'^^ 1 ^ 

 fubftance employed, thai may have pafled by the pifton; taking m fleam-en- 

 care at the lame to provide another exit for the metal or other I»& 

 fubftance colleded at the bottom of the fleam veflel or working 

 cylinder to convey the fame into a refervoir kept at a proper 

 heat, whence it is to be conveyed to the upper fide of the 

 pifton by a fmall pump worked by the engine or by any other 

 contrivance. In order that the fluid metal or metals ufed with 

 the pifton may not be oxidated, I. always keep fome oil or 

 other fluid Jubilance on its furface, to prevent its coming in 

 contaft with the atmofphere ; and to prevent the neceffi.ty of 

 employing a large quantity of fluid metal, I generally make 

 my pifton of the depth of the column required, but of a dia- 

 meter a little lefs than the fteam veflel or working cylinder, 

 excepting where the packing or other fitting is neceflary to be 

 applied ; fo that, in fafil, the column of fluid metal forms only a 

 thin body round the pifton. In fome cafes I make a hollo vr 

 metallic pifton, and apply an altitude of fluid metal in the 

 infideof the working cylinder. 



" It may be neceflary, however, to ftate, that in apply- 

 ing my improved method of keeping the fteam velTels of fteam- 

 engines at any required temperature to the engine known by r 

 the name of Savary's, in any of its improved forms, in which 

 a feparate" condenfer has been introduced, I fometimes employ 

 oil (or any other fubftance lighter than water, and capable of 

 being kept fluid in the temperature employed, without being 

 converted into vapour,) in the upper part of the tube or pi^e 

 attached to the fteam veflel ; by which means fteam of any 

 temperature may be ufed without being expofed to the rifle of 

 partial condenfation by the admiffion of any colder body into 

 the fteam veflel ; for the oil, or ether fubftance employed for 

 this purpofe, foon acquires the requifite temperature j and to 

 prevent unnecefTary efcape of heat, I conftrucl: of, or line 

 with, an imperfect conductor of heat, that part of the tube or 

 pipe attached to the fteam veflel which may not be heated ex- 

 teriorly. And further, (as is already the practice in fome en- 

 gines, and therefore not excluflvely claimed by me,) I caufe 

 the water raifed by the engine to pafs off through another af- 

 cen ling tube than tire one attached to the fteam veflel, but 

 connected with it at fome part lower than the oil or other 

 U5£C employed in it is ever fuflfered to defcend to in the 



working 



