Moving Force. 163 
acknowledged, that the experiments which 
have been made to determine the effects of 
wind and water-mills do not agree with the 
computations of mathematicians; but this is 
no objection to the principles here maintained. 
Writers generally propose such examples with 
a view rather of illustrating the methods of 
calculation by algebra and fluxions, than of 
making any useful improvements in practice. 
They suppose the particles of the water to 
move in straight lines, and to strike the 
machine with a certain velocity ; and after 
that to have no more effect. As such suppo- 
sitions are evidently inconsistent with the 
known properties of a fluid, we are not at.a 
loss to account for a difference between expe- 
riment and theory ; and therefore it should 
seem unreasonable to assert, that certain 
authors of reputation have neglected the col- 
lateral circumstances of time, space, or velocity 
in the resolution of these problems, unless we 
are able to point out such omissions.”* But if 
the theory be applicable to speculative objects 
only, why are its conclusions laid down as rules 
to be adopted in practice? Mr. Smeaton 
objected to the practical application of the 
theory by the distinguished authors which he 
quoted, because they omitted to take into 
* Philos. Trans. 1778, p. 371. 
x 2 
