Encyclopaedia Britannica, or, a Dictionary of arts, sciences, and miscellaneous literature : enlarged and improved. Illustrated with nearly six hundred engravings > Volume 18, RHI-SCR

AMUSEMENTS OF SCIENCE*1 565
jchanic a]f0 be obferved, that in all experiments upon the tno-
'onT" ^°n ^oc^‘es’ 011 which there is much friftion, as where
,^LmJ a fledge is employed, the refults are never fo uniform as
52 under other circumftances.
in pulley, p. Upon the pulley we {hall fay little, as it is in
every body’s hands, and experiments may be tried upon
it without any particular apparatus. It fhould, however,
be diftindlly inculcated, that the power is not increafed
by a fixed pulley. For this purpofe, a wheel without
a rim, or, to fpeak with more propriety, a number of
fpokes fixed in a nave fhould be employed (fig. 61.).
Pieces like the heads of crutches fhould be fixed at the
ends of thefe fpokes, to receive a piece of girthweb,
which is ufed inflead of a cord, becaufe a cord would
be unfleady $ and a ftrap of iron with a hook to it
fhould play upon the centre, by w'hich it may fome-
times be fufpended, and from which at other times a
weight may be hung.
Let this fkekton of a pulley be hung by the iron
ftrap from the tranfom of the frame *, fallen a piece of
web to one of the radii, and another to the end of the
oppofite radius. If two perfons of equal weight pull
thefe pieces of girthweb, they will balance each other*,
or two equal weights hung to thefe webs, will be in
equilibrio. If a piece of girthweb be put round the
aftermoft radius, two equal weights hung at the ends
of it will remain immoveable j but if either of them be
pulled, or if a fmall additional weight be added to
either, it will defcend, and the web will apply itfelf
fucceflively to the afcending radii, and will detach it¬
felf from thafe which are defcending. If this move¬
ment be carefully confidered, it will be perceived that
the web, in unfolding itfelfj adls in the fame manner up¬
on the radii, as two ropes would, if they were hung to
the extremities of the op polite radii in fucceflion. The
two radii which are oppofite, may be confidered as a
lever of the fir ft kind, when the centre is in the middle
of the lever ; as each end moves through an equal fpace,
there is no mechanical advantage. But if this fkeleton-
pulley be employed as a common b/ock or its mo¬
tions and properties will be entirely different.
* '61, 10. Nail a piece of girthweb to a. port, at the di*
fiance of three or four feet from the ground ; fallen the
other end of it to one of the radii (fee fig. 61.). Fallen
another piece of web to the oppofite radius, and let a
perfon hold the fkeleton-pulley fufpended from the web;
hook weights to the ftrap that hangs from the centre.
The end of the radius to which the fixed girthweb is
faftened will remain immoveable ; but if the perfon pulls
the web which be bolds in his hand upwards, he will be
able to lift nearly double the weight which he can raife
from the ground by a fimple rope without the machine,
and he will perceive that bis hand moves through twice
as great a fpace as the weight defcends : he has therefore
the mechanical advantage, which he would have by a
lever of the fecond kind. Let a piece of web be put
round the under radii, let one end of it be nailed to the
pod, and the other be held by the perfon, and it will
reprefent the application of a rope to a moveable pulley;
if its motion be carefully confidered, it will appear that
the radii, as they fucceffively apply themfelves to the
web, reprefent a feries of levers of the fecond kind.
Upon the wooden road lay down a piece of girth¬
web ; nail one end of it to the road ; place the pulley
upon the web at the other end of the board, and bring¬
ing the web over the radii, let the perfon taking hold Mechanic
of it, draw the loaded fledge fallened to the hook at K‘^ciea-
the centre of the pulley ; he will draw nearly twice as .
much in this manner as he could without the pulley.
Here the web lying in the road fhews more dillin£l-
ly, that it is quiefeent where the lowefl radius touches
it ; and if the radii, as they tread upon it, are obferved,
their points will appear at reft, while the centre of the
pulley will proceed as fall as the fledge, and the top of
each radius fueceflively will move twice as far as the
centre of the pulley and the edge.
If a perfon holding a flick in his hand, obferves the
relative motions of the top and the middle, and the bot¬
tom of the flick, whilft he inclines it, he will fee that
the bottom of the flick has only half the motion of the
top. This property of the pulley has been confidered
more at large, becaufe it elucidates the motion of a
wheel rolling upon the ground ; and it explains a com¬
mon paradox, which appears at firft inexplicable, the
bottom of a rolling wheel never moves upon the road.
This is afferted only of a wheel moving over hard
ground, which, in fad, may be confidered rather as lay¬
ing down its circumference upon the road, than as mo¬
ving, upon it.
II. The inclined Plane and the Wedge.
The irtclined plane is to be next confidered. When S3
a heavy body is to be raifed, it is often convenient to
lay a floping artificial road of planks, upon which it
may be pulhed or drawn. This mechanical power,
however, is but of little fervi.ee without the aftiftance
of wheels or rollers ; we fhall therefore fpcak of it as it
is applied in another manner, under the name of the
wedge, which is in fad a moving inclined plane ; but
if it be required to explain the properties of the inclin¬
ed plane by the panorganon, the wooden road may be
raifed and fet to any inclination required, and the fledge
may be drawn upon it as in the former experiments.
Let one end of a lever, N (fig. 59.), with a wheel at p;g> ^
one end of it, be hinged to the poll of the frame, by
means of a gudgeon driven or ferewed into the poll.
To prevent this lever from deviating Tideways, let a flm
of wood be conneded with it by a rail, which fhall be
part in the lever, but which may move freely in a hole
in the rail. Tire other end of this flip mull be faftened
to a flake driven into the ground at three or four feet
from the lever, at one fide of it, and towards the end
in which the wheel is fixed (fig. 62.), in the fame man¬
ner as the treadle of a common lathe is managed, and
as the treadle of a loom is feme times guided.
12. Under the wheel of this lever place an inclined
plane (fig. ?9.) on the wooden road, with rollers under
it, to prevent fridion ; fallen a rope to the feremoft end
of the wedge, and pafs it through the pulleys (P 4 and
P 3), as in the fifth experiment; let a perfon draw the
fledge by this rope over his fhoulder, and he will find,
that as it advances it will raife the weight upwards ; the
wedge is five feet long, and elevated one foot. Now, if
the perpendicular afeent of the weight, and the fpace
through which he advances, be compared, he will find
that the fpace through which he has paffed will be five
times as great as that through which the weight has
afeended ; and that this wedge has enabled him to raife
five times as much as he could raife without it, if his
ftrength were applied as in experiment 1. without any*
mechanical