348 
Steam Engine, 
fixed in the boiler c having four openings, i i, which 
are represented in a plain view in Fig, 3 : ef\^ the me- 
tallic rod, bearing the weight K K, with which the safe- 
ty valve is loaded, and extending itself under that valve 
to f: g h is the vacuum valve consisting in a plane circu- 
lar plate, with a brass tube sliding across the rod, and pres- 
sed by a spiral spring to the safety valve a b (against 
which it has been well gi'ound in making it), closing in 
that situation the openings i i. 
Such being the consti'uction of the whole, it is evidcntj, 
that when the elasticity of the steam increases, the two 
valves, joined together, with the holes i i shut, make but 
one, opposing to the elasticity of the steam an united re- 
sistance, which is regulated by the weight k k, in the 
common way ; but, on the contrary, when by condensa- 
tion of the vapours a vacuum is produced, the external 
air in pressing through i i, upon the vacuum valve g h, 
forces it dowm, and opens to itself a passage into the boiler. 
The valve g h may easily be made conical, like the 
other, if that form should be prefereed ; but in different 
trials, I have found planes, if wdl turned and ground to- 
gether, join as perfectly as can be desired, being pres- 
sed by the united elasticity of the spring and the steam. 
Fig, 4. is the same contrivance adapted to a new kind 
of safety valve or piston^ which, though I originally in- 
tended it for the use of Papin’s digesters of a new con- 
struction,^ has been, in a larger size, applied by me to 
steam engines, and is described in the Philosophical Ma- 
gazine of December, 1803. f 
* Nicholson^s Journal, March, 1804. 
t The description of this contrivance being already published, 
it would be superfluous to repeat it. I only beg leave to add the 
following practical remark. A metallic piston, if \\eil turned and 
fitted into a cylinder of exactly the same kind of metal, will proba- 
bly have the same degree of expansion, especially if hollow, and 
eOiisequently will not inci'ease its friction in any increased degree 
