MECHANICS. 
24.) the edge of the paper A C will form a 
spiral line round the cylinder, which will 
give the thread of the screw. It being 
evident that the winch must turn the cy- 
linder once round, before tlie weight of re- 
sistance can be moved from one spiral wind- 
ing to another, as from d to c ; therefore, as 
much as the circumference of a circle 
described by the liahdle of the winch is 
greater than the interval or distance be- 
tween the spirals,'so pinch is the force of the 
screw. Thus, supposing the distance of the 
spirals to be half an inch, and the length of 
the winch twelve inches, the circle describ- 
ed by the handle of the winch where the 
power acts, will be 76 inches nearly, or 
about 152 half inches ; and consequently 
152 times as great as the distance between 
the spirals ; and therefore a power at the 
handle, whose intensity is equal to no more 
than a single pound, will balance 152 pounds 
acting against the screw ; and as much ad- 
ditional force as is sufficient to overcome 
the friction, will raise tlie 152 pounds ; and 
the velocity of the power will be to the ve- 
locity of tlie weight, as 152 to 1. Hence 
it appears, that the longer the winch is, and 
the nearer the spirals are to one another, so 
much the greater is the force of the screw. 
A maciiine for .shewing the force or 
power of the screw may be contrived in the 
following manner : let the wheel C have a 
screw, (fig. 25.) on its axis, working in the 
teeth of the wheel D, which suppose to be 
48 in number. It is plain, that for every 
time the wheel C and screw are turned 
round by the winch A, the wheel D will be 
moved one tooth by the screws ; and there- 
fore, in 48 revolutions of tlie winch, the 
wheel D will be turned once round. Then, 
if the circumference of a circle, described 
by the handle of the winch A, be etjual to 
the cireiimferencc of a groove round the 
wheel D,the velocity of the handle will be 
48 times as great as the velocity of any 
given point in the groove. Consequently, 
if a line G goes round the groove, and has a 
weidit of 48 pounds hung to it, a power' 
equal to 1 pound at the handle will balance 
and support the weight. To prove this by 
experiment, let the circumferences of the 
grooves of the wheels C and D be equal to 
one another; and then if a weight H, of one 
pound, be suspended by a line going round 
the groove of the wheel C, it will balance 
a weight of 48 pounds hanging by the line 
G ; and a small addition to the w'eight H 
will cause it to descend, and so raise up the 
otlier weigliC 
If a line G, instead of going round il*" 
groove of the wheel D, goes round its axle 
I, the power of the machine will be as much 
increased as the circumference of the groove 
exceeds the circumference of the axle : 
which supposing it to be six timc.s, then one 
pound at H will balance six times 48, or 
288 pounds, hung to the line on the axle : 
and hence the power or advantage of this 
maciiine will be as 288 to 1. That is to say, 
a man, who by his natural strength could 
lift an hundred weight, will be able to raise 
288cvTts. by this engine. If a system of 
pullics were applied to the cord H, the 
power would be increased to an amazing 
degree. ATlien a screw acts in a wheel in 
this mannfer, it is called an endless screw. 
When it is not employed in tuining a 
wheel, it consists of two parts : the first is 
called the male, or outside screw, being cut 
in such a manner as to have a prominent 
part going round the cylinder in a spiral 
manner ; which prominent part is called 
the thread of the screw ; the other part, 
which is called the female, or inside screw, 
is a solid body, containing, a hollow cy- 
linder, whose concave surface is cut in the 
same manner as the convex surface of the 
male screw, so that tlie prominent parts of 
the one may fit the concave parts of the 
other. A very considerable degree of 
friction always acts against the power in a 
screv/ ; but this is fully compensated by 
other advantages ; for on this account the 
screw continues to sustain a weight, even 
after the power is removed, or ceases to 
act, and presses upon the body against 
which it is driven. Hence the screw will 
sustain very great weights, insomuch, that 
several screws, properly applied,, would 
support a large building, whilst the founda- 
tion was mending, or renewed. 
The screw is of extensive use in the 
printing-press, and in the press for coining 
money, and in a great variety of other pur- 
poses. It has lately been employed in the 
flour mills in .America, for pushing the flour 
which comes from the mill-stones to the end 
of a long trough, from which it is conveyed 
to otlier parts of the machinery, in order to 
undergo the remaining processes.- In this 
case, the spiral threads are very large in 
proportion to the cylinder on which they 
are fixed. As tlie lever used with the 
screw moves through a large .space when 
compared with the velocity of its other e.x- 
tremity, or of any body which it puts in 
motion ; the screw is of vei’y great use in 
subdividing any spate into a great number 
