368 JAMES WATT. 
possessing any figure, to give a clear idea of the mode of 
action in this little apparatus ; still I will attempt it. 
When gas escapes in a certain direction from the vase 
in which it is contained, this vase by means of reaction 
will be inclined to move in a diametrically opposite 
direction. The recoil of a gun loaded with gunpowder 
is a similar agency ; the gas engendered by the combus- 
tion of the saltpetre, the charcoal, and the sulphur, rushes 
into the air according to the direction of the gun; the 
reaction, produced in the rear, reaches the shoulder-of 
the person who fires it; it is on the shoulder then that 
the recoil must act. To change the direction of the. 
recoil, it would suffice to make the jet of gas issue in 
another direction. If the gun were closed at the end, 
and were pierced only by a lateral opening horizontally 
perpendicular to its direction, the gas from the powder 
would escape laterally and horizontally; therefore the 
recoil would act perpendicularly to the direction of the 
gun; and its force would be exerted on the arms and 
not on the shoulder. In the former instance, the recoil 
pushed the man who fired from the front to the rear, as 
if to upset him; in the second instance it would tend to 
turn him round on his own axis. Let the gun then be 
invariably fixed, and horizontally, to a vertical movable 
axis, and at the instant of being fired, it will alter its 
direction more or less, and it will also make the axis turn. 
Continuing the same arrangements, let us suppose the 
rotatory vertical axis to be hollow, but closed at its upper 
end; let it rise from the base as a sort of chimney from 
the caldron where gas is engendered; let there be 
besides a free lateral communication between the inte- 
rior of this axis and the interior of the gun’s barrel, so 
that, after having filled the interior of the axis, the steam 
