making good work. When any accident of 
this kind happens, the perpendicular position 
of the spindle must be restored, by adjusting 
the bridge-tree with proper wedges put be- 
tween it and the brayer. 
It often happens that the rynd is a little 
wrenched in laying down the upper stone 
upon it, or is made to -sink a little lower on 
one side ot the spindle than on the other ; 
and this will cause one edge of the upper stone 
I to drag all round upon the other, while the 
opposite edge will not touch. But this is 
| easily set to rights, by raising the stone a 
j little with the lever, and putting bits of pa- 
i per, cards, or thin chips, between the rynd 
land the stone;. 
A less quantity of water will turn an over- 
j shot-mill (where the wheel has buckets in- 
stead of float-boards) than a breast-mill, where 
J the fall of water seldom exceeds half tire 
■ height of the wheel; so that where there is 
but a small quantity of water, and a fall great 
I enough for the wheel to lie under it) the 
bucket, or overshot, wheel, is always used : 
but where there is a large body of water with 
a little fall, the breast, or lloat-board, wheel 
must be used. Where the water runs only 
upon a small declivity, it can act hut slowly 
upon the under part of the wheel ; in which 
case the motion of the wheel will be slow : 
and therefore the floats ought to he very 
long, though not high, that a large body of 
water may act upon them ; so that what is 
wanting in velocity may be made up in 
power ; and then the cog-wheel may have a 
greater number of cogs, in proportion to the 
rounds in the trundle, in order to give the 
mili-stone a sufficient degree of velocity. 
It was the opinion of Smeaton, that the 
powers necessary to produce the same effect 
on an undershot-wheel, a breast-wheel, and 
an overshot-wheel, must be to each other as 
the numbers 2.4, 1.75, and 1. 
Practical rules for the construction of 
mills. — 1 . Measure the perpendicular height 
of the fall of water, in feet, above that part 
of the wheel on which the water begins to 
act, and call that the height of the fall. 
2. Multiply this constant number 64.2882 
by the height of the fall in feet, and the square 
root of the product will be the velocity of 
the water at the bottom of the fall, or the 
number of feet that the water there moves 
per second. 
3. Divide the velocity of the water by 
three, and the quotient will be the velocity of 
the floatrboards of the wheel, or the number 
of feet they must each go through in a se- 
cond, when the water acts upon them so as 
to have the greatest power to turn the mill. 
4. Divide the circumference of the wheel 
in feet by the velocity of its floats in feet per 
second, and the quotient will he the number 
of seconds in which the wheel turns round. 
5. By this last number of seconds'divide 
60, and the quotient will he the number of 
turns of the wheel in a minute. 
6. Divide 120 (the number of revolutions 
a mill stone four feet and a half diameter 
ought to have in a minute) by the number of 
turns of the wheel in a minute, and the quo- 
tient will be the number of turns the mill- 
stone ought to have for one turn of the 
wheel. 
7. Then, as the number of turns of the 
wheel in a minute, is to the number of turns 
ef the mill-stone in a minute, so must the 
! 
MltL; 
number of staves in the trundle, be to the 
number of cogs in the wheel, in the nearest 
whole numbers that can be found. 
10.5 
By these lules the following table is calcu- 
lated to a water-wheel 18 feet diameter, 
which may he a good size in general. 
THE MILL- WRIGHT’S TABLE. 
Height 
of the 
fall of 
water. 
Velocity of 
the fall of 
water per 
Second. 
Velocity of 
the wheel 
per se- 
cond. 
Revolutions 
of the wheel 
per minute. 
Revolutions 
of the mill- 
stone for 
one of the 
wheels. 
Cogs in the 
wheel, and 
staves in 
the trundle. 
Revolutions of 
the mill-stone 
per minute, by 
these staves 
and cogs. 
Feet. 
Feet. 
100 parts 
of a foot. 
Feet. 
100 parts 
of a foot. 
Revolu- 
tions. j 
100 parts 
of a rev. j 
Revolu- 
tions. 
100 parts 
of a rev. 
i 
Cogs. 
Staves. 
Revolu- 
tions. 
100 parts 
of a rev. 
1 
8 . 02 
2 . 67 
2 . 83 
42 . 40 
254 
6 
119 . 84 
2 
11 . 34 
3 . 78 
4 . 00 
30 . 00 
210 
7 
1 20 . 00 
3 
13 . 89 
4 . 63 
4 . 91 
24 . 44 
196 
8 
120 . 28 
4 
16 . 04 
5 . 35 
5 . 67 
21 . 16 
190 
9 
119 . 74 
5 
17 . 93 
5 . 98 
6 . 34 
18 . 92 
170 
9 
119 . 68 
6 
19 . 64 
6 . 55 
6 . 94 
17 . 28 
156 
9 
120 . 20 
7 
21 . 21 
7 . 07 
7 . 50 
16 . 00 
144 
9 
120 . od 
8 
22 . 68 
7 . 56 
8 . 02 
14 . 96 
134 
9 
119 . 34 
9 
24 . 05 
8 . 02 
8 . 51 
14 . 10 
140 
10 
119 . 14 
10 
25 . 35 
8 . 45 
8 . 97 
13 . 38 
134 
10 
120 . IS 
11 
26 . 59 
8 . 86 
9 . 40 
12 . 76 
128 
10 
120 . 32 
12 
27 . 77 
9 . 26 
9 . 82 
12 . 22 
122 
10 
119 . 8d 
13 
28 . 91 
9 . 64 
10 . 22 
11 . 74 
118 
10 
120 . 36 
14 
30 . 00 
10 . 00 
10 . 60 
1 1 . 32 
112 
10 
118 . 72 
15 
31 . 05 
10 . 35 
10 . 99 
10 . 98 
110 
10 
120 . 96 
16 
32 . 07 
10 . 09 
11 . 34 
10 . 58 
106 
10 
1 20 . 20 
17 
33 . 06 
11 . 02 
11 . 70 
10 . 26 
102 
10 
119 . 34 
18 
34 . 02 
11 . 34 
12 . 02 
9 . 98 
100 
10 
120 . 20 
19 
34 . 95 
11 . 65 
12 . 37 
9 . 70 
98 
10 
121 . 22 
20 
35 . 86 
11 . 95 
12 . 68 
9 . 46 
94 
10 
119 . 18 
1 
2 
S 
4 
5 
! 6 
7 
To construct a mill by this table, find the 
height of the fall of water in the first column, 
and against that height, in the sixth column, 
you have the number of cogs in the wheel, 
and staves in the trundle, for causing the 
mill-stone, four feet six inches diameter, to 
make about 120 revolutions in a minute, as 
near as possible, when the wheel goes with one- 
third part of the velocity of the water. And 
it appears by the 7th column, that the number 
of cogs in the wheel, and staves in the trun- 
dle, are so near the truth for the required 
purpose, that the least number of revolutions 
of the mill-stone in a minute is 118, and the 
greatest number never exceeds 121 ; which 
is according to the speed of some of the best 
mills. 
One of the most usual communications of 
motion in machinery, is by means of toothed 
wheels acting on each other. , It is of the 
greatest consequence to have the teeth so 
formed, that the pressure by which one of 
them urges the other round its axis is con- 
stantly the same. This is by no means the 
case when the common construction of a 
spur-wheel, acting in the cylindrical staves of 
a lantern, or trundle, is used. "The ends of 
teeth should never be formed of parts of cir- 
cles, but of a particular curve, called the epi- 
cycloid, which is formed by moving the cir- 
cle, called the generating circle, round the 
circumference of another circle, while it 
turns also round its own centre ; then any 
point will describe an epicycloid. 
Emerson observes, that the teeth of wheels 
ought not to act upon each other before they 
arrive at the line which joins their centres ; 
and though the inner or under sides of the 
teeth may he of any form, yet it is better to 
make both sides alike, which will serve to 
Bb 3 
make the wheel turn backwards. The more 
teeth that work together the better ; at least 
one tooth should always begin before the 
other has done working. The teeth ought to 
be so disposed as not to trouble or hinder one 
another before they begin to work. 
If the cogs of a wheel and rounds of a trun- 
dle could be put in as exactly as the teeth are 
cut in the wheels and pinions of a clock, then 
the trundle might divide the wheel exactly, 
that is to say, the trundle might make a given 
number of revolutions for one of the wheel, 
without a fraction. But as any exact num- 
ber is not necessary in mill-work, and the 
cogs and rounds cannot be set in so truly as 
to make all the intervals between them equal, 
a skilful mill-wright will always give the wheel 
what ke calls a hunting-cog ; that is, one 
more than what will answer to an exact divi- 
sion of the wheel by the trundle. And then 
as every cog comes to the trundle, it will take 
the next staff, or round, behind the one winch 
it took in the former revolution ; and by that 
means will wear all the parts of the cogs and 
rounds which work upon one another equally, 
and to equal distances from one another, in a 
little time. See Flour mill. 
Mills, Bark, like most other mills, are 
worked sometimes by means of horses, at 
others by water, and at others by wind. One 
of the best mills we have seen described for 
these purposes is that invented by Mr. JBag- 
nall, of Worsley, in Lancashire : this ma- 
chine will serve not only to chop hark, to 
grind, to riddle, and pound it, but to beam, 
or work green hides and skins out of the mas- 
tering, or drench, and make them ready for 
the ouse, or bark-liquor; to beam sheepskins 
and other skins for the skinner's use; and to 
scour and take off the bloom from tanned 
