Moving Foree. 151 
upon which principle the most consistent 
explanation of the facts is to be obtamed m 
cases where the two measures disagree. 
It appears to me that Mr. Smeaton’s four 
maxims on undershot water-wheels may all be 
comprehended in one, expressed thus : That in 
cases where the maximum effect is produced, tt 
is nearly as the quantity of water multiplied 
by the effective head.* But the theory is 
founded on the supposition that in all cases the 
pressure at the circumference of the wheel is 
as (c+v)*, and if it were so, the maximum 
effect would, no doubt, be produced when 
v=tc. Bythe mere inspection, however, ‘of 
the results which I have stated above, it will be 
seen that the pressure at the circumference of 
the wheel is not as (c—v)* and therefore, the 
maximum effect cannot be produced when 
the wheel moves with one-third of the velocity 
of the water. 
‘I have to regret that I cannot ‘at present 
refer to M. Bossut’s experiments on water- 
wheels. It is observed, however, by M.du Buat, 
that according to these experiments, the maxi- 
mum ‘effect was produced when the velocity of 
the wheel was 4 that of the water, which corre- 
* Tt should be observed,’ that the maximum effect was 
not always produced at the same relative velocity. 
