354 
Steam Engine » 
boiler presents to the fire, it is evident, that other circum- 
stances being the same, a boiler with tubes descending 
from its bottom, will be heated at least twice as soon as 
a cylindrical boiler of the same diameter, with a flat bot= 
tom. 
In order that a cylindrical boiler with flat bottom, sur- 
rounded by flame on all sides, might have the same ex- 
tent of surface exposed to the fire as a boiler with tubes, 
it would be necessary to give it a diameter greater than 
that of the boiler with tubes in the proportion of the 
square root of 1160.44, to the square root of 566.6'/, that 
is, of 17.171 to 12. 
Therefore, in order that a cylindrical boiler with a 
flat bottom, might have the same extent of surface ex- 
posed to the fire as our boiler with tubes, of twelve inches 
in diameter, it would be necessary to give it a diameter 
of 17.171 inches. 
But if the diameter of a boiler intended for producing 
steam be increased, it is necessary, at the same time, to 
increase its thickness, in order to increase its strength. 
The necessary increase of thickness, and the expence 
that it will occasion, can be easily calculated. 
The effort that an elastic fluid exerts against the sides 
of the containing vessel, is in proportion to the surface of 
a longitudinal and central section of the vessel, and con- 
sequently in proportion to the square of its diameter, the 
form remaining the same. Hence we may conclude, that 
a steam boiler of a cylindrical form with a flat bottom, 
which has the same extent of surface exposed to the fire 
as a boiler of twelve inches in diameter with tubes, should 
be at least twice as thick as this last, in order to have an 
equal degree of strength for resisting the expansive power 
of the steam. 
The boiler which I have the honour of presenting to 
the society, is particularly intended to serve as a steam 
