MECHANICS. 
706 
thrown quite out; and, being then ground, it falls through 
a fpout M, called the mill-eye, into the trough N. When 
the mill is fed too fall, the corn bears up the hone, and 
is ground too coarfe; and befides, it clogs the mill fo as 
to make it go too flow. When the mill is too flowly fed, 
it goes too fait, and the (tones, by their attrition, are apt 
to lirike fire againfi one another. Both which inconve- 
niencies are avoided by turning the pin L, backwards or 
forwards, which draws up or lets down the (hoe ; and lb 
regulates the feeding as the miller fees convenient. 
Sometimes, where there is a fufficient quantity of water, 
the cog-wheel in fig. 104. turns a large trundle, on whofe 
axis is fixed a horizontal wheel, with cogs all around its 
edge, turning two trundles at the fame time ; whofe axis 
orlpindles turn two millflones. When there is not work 
for them both, either may be made to lie quiet, by taking 
out one of the (laves of its trundle, and turning the va¬ 
cant place towards the horizontal cog-wheel. And there 
may be a wheel fixed on the upper end of the great up¬ 
right axle of this wheel for turning a couple of bolting- 
mills; and other work for drawing up the facks, fanning 
and cleaning the corn, (harpening of tools, &c. As the 
water adds upon an overfliot-mill both by impulfe and 
weight, fo does it likewife upon a bread-mill, or that 
where the water comes upon the bread or middle part of 
the wheel: and here, though the weight of the water is 
not fo great as in the oVerfhot-niill, being contained in the 
buckets of the lower quarter only; yet the impulfe of the 
water is much greater, the height of the water being in¬ 
creased nearly the femi-diameter of the great wheel, all other 
things being equal. If the height of the water remain 
the lame, the aperture of the pendock mud be enlarged 
to nearly twice the area, that the force may be the fame ; 
fo that, to produce the fame effect, twice as much water is 
neceffary for the bread-mill as for an overfliot one, every 
thing elfe being the fame. 
Mr. Fergufon obferves, that where there is but a final 1 
quantity of water, and a fall great enough for the wheel 
to lie under it, the bucket or overfliot wheel is always 
•ufed. But where there is a large body of water, with a 
little fall, the bread or floatboard wheel mud take place. 
As to the underfnot-mill, it is evident there can be only 
the impulfe from the water; and therefore, the height of 
the water remaining the fame, there mud be a larger aper¬ 
ture of the pendock for the difcharge of a greater quan¬ 
tity of water in the fame time, in order to produce the 
fame effect as the overihot or bread-mill; whence a 
greater expenfe of water will be made here than in any 
other mill, and can only be fupplied for a condancy by a 
river; and where this can be had, the underdiot is the 
eafied, cheapeft, and mod Ample, Itrudture of which a mill 
is capable. Dr. Defaguliers, having had occafion to exa¬ 
mine many underfnot and overfliot mills, generally found 
that a well-made overfliot-mill ground as much corn in 
the fame time as an underlhot-mill with ten times lei's 
water; fuppofing the fall of water at the overfliot to be 
twenty feet, and at the underfliot to be about fix or (even 
feet: and he generally obferved, that the wheel of the 
overfliot-mill was of fifteen or fixteen feet diameter, with 
a head of water of four or five feet, to drive the water 
into the buckets with fome momentum. 
Mr. Fergufon lias given the following directions howto 
condrufi water-mills, fo as to be in the greated degree of 
perfeftion.—When the fioatboards of the water-wheel 
move with a third part of the velocity of the water that 
afts upon them, the water lias the greated power to turn 
the mill: and, when the millltone makes about fixty revo¬ 
lutions in a minute, it is found to do its work the bed. 
For, when it makes but. about forty or fifty, it grinds 
too flowly ; and, when it makes more than feventy, it 
heats the meal too much, and cuts the bran Co fmall, that 
a great part of it mixes with the meal, and cannot be 
feparated from it by lifting or bolting. Confequently, 
the utmofl perfection of mill-work lies in making the train 
fo as that the millltone lhall make about fixty turns in a 
minute when the water-wheel moves with a third part of 
the velocity of the water. To have it fo, obferve the fol¬ 
lowing rules : 
1. Meafure the perpendicular height of the fall of wa¬ 
ter, in feet, above the middle of the aperture, where it is 
let out to aft by impulfe againd the fioatboards on the 
lowed fide of the underi'hot-wheel. 
2. Multiply this cooltant number, 64-2882, by the 
height of the fall in feet, and extraft the iquare root of 
the produft, which (hall be the velocity of the water at 
the bottom of the fall, or the number of feet the water 
moves per fecond. 
3. Divide the velocity of the water by 3 ; and the quo¬ 
tient Cliall be the velocity of the doats of the wheel in feet 
per fecond. 
4. Divide the circumference of the wheel, in feet, by 
the velocity of its floats ; and the quotient (hall be the 
number of leconds in one turn or revolution of the great 
water-wheel, on whofe axis the cog-wheel that turns the 
trundle is fixed. 
5. Divide 60 by the number of feconds in a turn of the 
water-wheel or cog-wheel; and the quotient (hall be the 
number of turns of either of thele wheels in a minute. 
6. By this number of turns divide 60 (the number of 
turns the millflone ought to have in a minute) ; and the 
quotient (hall be the number of turns the millltone ought 
to have for one turn of the water or cog-wheel. Then, 
7. As the required number of turns.of the millflone in 
a minute is to the number of turns of the cog-wheel in 
a minute, fo mud the number of cogs in the wheel be 
to the number of daves in the trundle on the axis of 
the millflone, in the neared whole number that can he 
found. 
Dr. Barker invented a curious kind of overfliot water¬ 
mill, which, without either wheel or trundle, performs the 
operation of grinding corn. This mill is reprefented in 
fig. 106. in which A is a pipe or channel that brings water 
from a refervoir to the upright tube B. The water runs 
down the tube, and thence into the horizontal trunk C C, 
which has equal arms; and, ladly, runs out through holes 
at d and e, opening on contrary fides near the ends of thole 
arms. Thele orifices d, e, have Aiders fitted to them, fo 
that their magnitude may be increafed or diminifned at 
pleafure. The upright fpindle D is fixed in the bottom of 
the trunk, and ferewed to it below by the nutg; and it 
is fixed to the upright tube by two crofs bars at f fo 
that, if the tube B and trunk C be turned round, the 
fpindle D will be turned alio. The top of the fniudle 
goes Iquare into the rynd of the upper mill-done H, as in 
common mills; and as the trunk, tube, and fpindle, turn 
round, the millltone is turned round thereby. The lower 
or quiefeent millflone is reprefented by I; and K is the 
floor on which it reds, in which is the hole L to, let the 
meal run through, and fall down into a trough M. The 
hoop or cafethat goes round the millflone reds on the 
floor K, and iupports the hopper, in the common way.. 
The lower end of the fpindle turns in a hole in. the bridge- 
tree F G, which fupports the millltone, tube, fpindle, and 
trunk. This tree is moveable on a pin at h ; and its other 
end is fupported by an iron-rod N fixed into ir, the top 
of the rod going through the fixed bracket O, and having.' 
a fc.rew-nut 0 upon it, above the bracket. By turning this 
nut forward or backward, the millflone israifed or lowered 
at pleafure. Whilfl the tube B is kept full of water from 
the pipe A, and the water continues to run out from the 
two ends of the trunk ; the upper millltone H, together 
with the trunk, tube, and fpindle, turn round. But, if the 
holes in the trunk were flopped, no motion would enfue, 
even though the tube and trunk were full of water. For, 
if there were no hole in the trunk, the prelfure of the wa¬ 
ter would be equal againd all parts of its fides within. 
But, when the water has free egrefs through the holes, its 
prelfure there is entirely removed ; and the prelfure againd 
the parts of the fides which are oppolite to the holes turns 
the machine. If any other kind of machinery is to be 
_ driven.; 
