J A C 
T the ninth letter of the alphabet, used as 
-*• ) a numeral, signifies no more than one, 
and stands for so many units as it is repeated 
times: thus, 1, one; II, two; III, three, 
&c. and when put before a higher nume- 
ral, it subtracts itself, as IV, four; IX, nine, 
See. but when set after it, so many are added 
to the higher numeral, as there are rs added; 
thus VI is 5 + 1, or six ; VII, 5 + 2, or 
seven ; VIII, 5 + 3, or eight. The antient 
Romans likewise used If) for 500, CI3 for 
1000, If) f) for 5000, CCI33 for 10,000, 
1333 for 50,000, and CCCIQ33 for 100, 
000. Farther than this, as Pliny observes, 
they did not go in their notation ; but when 
necessary, repeated the last number, as 
CCCI333, CCCI333 for 200,000 ; CCC- 
J333>CCCl33f), 0CCI333 for 300,000 ; 
and so on. 
JACK, in mechanics, an instrument in 
common use for raising heavy timber, or 
very great weights of any kind, being a pow- 
erful combination of teeth and pinions, and 
the whole inclosed in a strong wooden stock 
or frame BC, and moved by a winch or han- 
d ' e IIP; the outside appearing as in Plate 
Miscel. fig. 131. In fig. 132, the wheel or 
rack work is shewn, being the view of the 
inside when the stock is removed. Though 
it is not drawn in the just proportions and 
dimensions, for the rack AB must be sup- 
posed at least four times as long in proportion 
to the wheel Q, as the figure represents it; 
and the teeth, which will be then four times 
more in number, to have about three in the 
inch. Now if the handle HP is seven inches 
long, the circumference of this radius will be 
44 inches, which is the distance or space the 
power moves through in one revolution of 
the handle ; but as the pinion of the handle 
has but four leaves, and the wheel Q suppose 
20 teeth, or five times the number, there- 
fore to make one revolution of the wheel Q, 
it requires five turns of the handle, in which 
case it passes through 5 times 44 or 220 
inches; but the wheel having a pinion R of 
three leaves, these will raise the rack three 
teeth, or one inch, in the same space. 
Ilence, then, the handle or power moving 
220 times as fast as the weight, will raise or 
balance a weight of 220 times its own energy. 
Yon. If. 
J A C 
And if tins is the hand of a man who can sus- 
tain 50 pounds weight, he will, by the help 
of this jack, be able to raise or sustain a 
weight or force of 11000 pounds, or about 
rive tons weight. 
This machine is sometimes open behind 
from the bottom almost up to the wheel Q, to 
let the lower claw, which in that case is turned 
up as at B, draw up any weight. When the 
weight is drawn 01 pushed sufficiently high, 
it is kept from going back by hanging the 
end of the hook S, fixed to a staple, over 
the curved part of the handle at h. 
The Society of Arts rewarded Mr. Mo- 
cock of Southwark, with a premium of 20 
guineas, for his contrivance to prevent a jack 
from taking a retrograde course whenever 
the weight by any accidental circumstance 
overbalances the power. The improved 
jack only differs from those in common use 
in this respect, that it has a pall or clock, and 
ratchet, applied in such manner as to stop 
the motion of the machine as soon as it be- 
gins to run back again. As the difference 
in the mechanism is very trifling, the im- 
provement may be easily applied to any 
common jacks already made. 
Jack is also the name of a well-known 
engine,in the kitchen, used for turning a spit. 
Here the weight is the power applied, acting 
by a set of pulleys ; the friction of the parts, 
and the weight with which the spit is charged, 
are the forces to be overcome ; and a steady 
uniform motion is maintained bv means of a 
fly. 
The common worm-jack is represented at 
Plate Miscel. fig. 130. ABC is the barrel 
round which the cord QR is wound; KL 
the main wheel, commonly containing 60 
teeth ; N the worm-wheel of about thirty 
teeth, cut obliquely ; LM the pinion, of 
about 15 ; O the worm or endless screw, con- 
sisting of two spiral threads, making an angle 
of sixty or seventy degrees with its axis; X 
the stud, and Z the loop of the worm-spindle ; 
P a heavy wheel or fly, connected w ith the . 
spindle ot the endless screw to make the mo- 
tion uniform ; DG the struck wheel fixed 
to the axis FD ; S,S,S, are holes in the frame, 
by which it may be nailed to a board, and 
thence to any wall, the end D being pe,r- 
J A C 
mitted to pass through it ; HI the handle 
going upon the axis ET, to wind lip the 
weight when it has run down. R is a box 
of fixed pulleys, and V a corresponding one 
of moveable pulleys carrying the weight,- 
The axis ET is fixed in the barrel AC, which 
axis being hollow, both it and the barrel 
turn round upon the axis FD, which is fixed 
to the wheel KL, when it turns in the order 
BTA ; but cannot turn the contrary way, by 
reason of a catch nailed to the end AB, which 
lays hold of the cross-bars in the wheel LK. . 
The weight by means of the cord QR, ia 
consequence of its descent, carries about the 
barrel AB, which by the action of the catch- 
carries the wheel KL, and this moves the 
pinion LM and wheel N, the latter moving 
the worm O and the fly P. Also the wheel; 
LM carries the axis FD with the wheel DG, 
which carries the cord or chain that goes 
abotit the wheel or pulley at the head of the 
spit. But when the handle H gives motion, 
to the axis in a contrary order to that given 
by the weight, the catch is depressed ; so that 
although the barrel BC moves and winds the, 
cord upon it, the wheel DG continues at rest. 
The time which the jack will Continue in 
motion depends upon the number of pulleys, 
at R and V : and as these increase or de- 
crease, so must the weight which communi*, 
cates the motion, in order to perform the 
same work in the same time. 
Jack, smoke, is an engine used for the 
same purpose as the common jack ; and is so 
called from its being moved by means of 
the smoke, or rarefied air, ascending the, 
chimney, and striking against the sails of 
the horizontal wheel AB (Plate Miscel. fig, 
129), which being inclined to the horizon, is 
moved about the axis of the wheel, together 
with the pinion C, which carries the wheels, 
D and E; and E carries the chain F, which 
turns the spit. The wheel AB should be 
placed in the narrow part of the chimney • 
where the motion of the smoke is swiftest* f 
and where also the greatest part of it must 
strike upon the sails. The force-of this ma- , 
chine depends upon the draught of the chim- 
ney, and the strength of the fire. 
Smoke-jacks are sometimes moved by 
mqans of spiral flyers wiling about a vertical • 
