MILL. 
the full effect desirable from the joint ope- 
rations of velocity and weight. In this 
kind of wheels it is proper that the float- 
boards should be confined both at their sides 
and at their extremities, so that the water 
may accompany all the way from the head 
down to the lowest part of the wheel, 
whence it should draw off with sufficient 
readiness to allow the i^ucceeding fall to 
supply its place, without being in the least 
retarded. It should be understood, that 
any quantity of water remaining in . the 
trough, at the bottom of a breast-wheel in 
particular, must tend more or less to oppose 
its motion, in the exact ratio with the dispo- 
sition of the fluid to become stagnate or sta- 
tionary. ■ 
The over-shot wheel is by far the most 
powerful ; both because it receives the wa- 
ter at the very commencement of descent, 
and that the buckets with which this kind 
of wheel is ordinarily furnished retain the 
power until they gradually discharge their 
contents, as these buckets successively be- 
come inferior parts of the circumference. 
It should be stated in this place, that much 
may be effected by allowing the water 
merely to flow upon the upper part of the 
wheel, into the superior buckets, whei eby 
an immense auxiliary power is erected as 
they successively become filled. Add to 
this the discovery made by Mr. Smeaton, 
that “ the more slowly any body descends 
by the force of gravity, while acting upon 
any piece of machinery, the more of that 
force will be spent upon it, and consequent- 
ly the effect will be the greater.” That 
effect is by no means increased in propor- 
tion to the velocity of the wheel’s motion ; 
on the contrary, Mr. Smeaton found, that 
when his wheel, which was two feet in dia- 
meter, revolved 20 times in a minute, its 
effect vi’as greatest : when it made only 18| 
turns the effect was irregular; and when so 
laden as not to make 18 turns, the wheel 
was overpowered by the load. He found 
that 30 turns in the minute occasioned a loss 
of about j^th, and that when turned 30 
times in a minute, the diminution of effect 
was nearly one-fourth of its powers. This 
proportion may be easily estimated on any 
wheel of greater extent, by computing the 
proportion of accumulated power lost by 
greater velocity than may be sufficient to 
load the wheel by means of the buckets 
being filled; observing that the progre.ss of 
a machine may be so much retarded as to 
cause the effect to be irrelevant of the pur- 
pose, although the madiine may be kept in 
VOL, IV. 
motion. Some machines do their work well, 
simply in consequence of a certain celerity, 
as is generally the case in a grinding appara- 
tus : thus also every person conversant in 
the practice of agriculture is sensible; that, 
when a plough is drawn at a certain pace, 
it will cut the soil regularly and freely, while, 
on the other hand, the same cattle proceed- 
ing at a very slow pace shall be more fati- 
gued with doing less work, and that work 
by no means so.nea'cly executed. All things 
considered, it will perhaps be found, that 
the great wheels of all machines ought to 
move at such a rate as to cause their cir- 
cumferences to pass over three feet in each 
second of time. We could instance several 
very large wheels, erected within the last 
five or six years, which scarcely make more 
than one revolution in the minute, but which 
operate so forcibly on the counter-wheels, 
as to give an astonishing degree of firmness 
as well as of regularity to their motions. 
The ma.ximum load for an overshot wheel 
is that which reduces the circumference of 
the wheel to its proper velocity, which is 
known by dividing the effect it ought to 
produce in a given time by the space in- 
tended to be described by the circumference 
of the wheel in the same time. The quo- 
tient will be the resistance overcome at the 
circumference of the wheel ; it is equal to 
the load required, including the friction and 
the. resistance of the machinery. So much, 
however, depends on the proper precautions 
for reducing the friction of the several 
moving parts, that too much stress cannot 
be laid on that highly important considera- 
tion. We therefore solicit those readers 
who may vid.sh to render themselves conver- 
sant in this branch of science, and especially 
if practical knowledge is in view, to refer 
to the ar ticle Friction, where they will find 
many very necessary points treated of with 
as much attention to their interests as our 
limits could allow. 
We may, in theor y, suppose a wheel to be 
capable of overcoming any resistance what- 
ever; yet we alw'ays find, in practice, that 
the wheel stops, or at least is incapable of 
progressive motion, when the ojrposition or 
load is equal to the sura of the water con- 
tained in all the buckets. In this we speak 
of overshot wheels, which designation in- 
cludes all that carry the water with them iu 
their descent, and do not depend so much 
on its velocity as its weight : hence many 
kinds of breast-wheels, which are construct- 
ed according to the above plan, ar e by many 
persons classed with overshots: the latter, 
li ’ 
