OF THE IRK, LOCOMOTIVE ENGINE. 505 
gravity ; and its centre of gravity will move in 
the same manner, as if the whole motive force of 
the steam was applied at that point. And, as the 
motive force of the steam always acts in a direc- 
tion perpendicular to that part of the surface of 
the boiler which is opposite to the aperture; and 
as the surface of the boiler, on which the steam 
thus acts, revolves about the centre of gravity of 
the Engine, it is evident, that the direction, in 
which the motive force of the steam acts upon 
the centre of gravity of the Engine, is continually 
changing. 
7. It is desirable to ascertain the force of the 
steam, which caused the explosion; but, the mo- 
tion of the Engine having been so much disturbed 
by breaking loose from the Tender, and by break- 
ing the six tie-beams, the calculations for this 
purpose must always be attended with some un- 
certainty. The only way, however, in which the 
force of the steam can be ascertained, with any 
degree of certainty, is to consider and estimate 
the several effects produced by the explosion, 
and then to calculate what must be the force of 
the steam necessary to cause the effects. 
When the plane of the top of the fire-box is 
