350 Steam Engine o 
pansive force of the steam, it is evident, that if the dia- 
meter be augmented (with a view to increase the surface 
of the bottom) a considerable expence is incurred on ac- 
count of the additional strength that must be given to the 
sides. 
Having been engaged in the year 171-6, in a set of expe- 
riments, in which I employed the steam of boiling water 
as a vehicle of heat ; I had a boiler made for this pur- 
pose, on a new construction, which answered well, and 
even beyond my expectations; and, as this boiler might 
be used with advantage in many cases, even where it is 
only required to heat liquids in an open boiler, this, and 
another motive, which it would be useless to mention in 
this place, have lately induced me to construct one here 
(at Paris) and to present it to the Insti ute. 
The object chiefly had in view in the construction of 
this boiler, was to give it such a form, that the surface ex- 
posed to the fire should be great in comparison with its 
diameter and capacity ; and this without having a great 
surface exposed to the cold air of the atmosphere. 
The body of the boiler is in the shape of a drum. It 
is a vertical cylinder of copper twelve inches in diameter, 
and twelve inches high, closed at top and at bottom by 
circular plates. 
In the centre of the upper plate thare is a cylindrical 
neck six inches in diameter, and three inches high, shut 
at top by a plate of copper three inches in diameter and 
three lines in thickness, fastened down by screws. 
This last plate is pierced by three holes, each about 
five lines in diameter. The first, which is in the centre 
of the plate, receives a vertical tube, which conveys wa- 
ter to the boiler from a reservoir which is placed above. 
This tube, which descends in the inside of the boiler, to 
vithin an inch above the circular plate which forms its 
bottom, lias a cock near its lower end. This cock is 
