MECHANICS. 
70® 
("pokes, to receive a piece of girth-web, which is ufed in- 
ftead of a cord, becaufe a cord would be unlteady ; and a 
(trap of iron with a hook to it fhould play upon the cen¬ 
tre, by which it may fometimes be fufpended, and from 
which at other times a weight may be hung. Let this 
lkeleton of a pulley be hung by the iron (trap from the 
tranfom of the frame; fatten 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 girth- 
web, they will balance each other; or two equal weights 
hung.to thefe webs will be in equiiibrio. If a piece of 
girth-web be put round the aftermoft radius, two equal 
weights hung at the ends of it will remain immoveable; 
blit, 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 itfelf from thofe which are defcending. If this 
movement be carefully confidered, it will be perceived 
that the web, in unfolding itfelf, adts in the fame manner 
upon the radii as two ropes would if they were hung to 
the extremities of the oppofite radii in fuccelTion. The 
two radii which are oppofite, may be confidered as a lever 
of the firft kind, when the centre is in the middle of the 
lever ; as each end moves through an equal (pace, there is 
no mechanical advantage. But, if this (keleton-pulley be 
employed as a common block or tackle, its motions and 
properties will be entirely different. 
10. Nail a piece of girth-web to a poft, at the diflance of 
three or four feet from the ground ; fallen the other end 
of it to one of the radii; (fee fig. 78.) Fatten 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 girth-web is fattened will remain 
immoveable; but, if the perfon pulls the web which he 
holds 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 his hand moves through twice as great a fpace as the 
weight defcends : he has therefore the mechanical advan¬ 
tage 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 poll, 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 fucceflively apply 
themfelves to the web, reprefent a feries of levers of the 
fecond kind. 
Upon the wooden road, fig. 73, lay down a piece of 
girth-web; nail one end of it to the road ; place the pul¬ 
ley upon the web at the other end of the board, and, bring¬ 
ing the web over the radii, let the perfon taking hold of 
it, draw the loaded fledge fattened to the hook at the cen¬ 
tre 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 (hows more diltindtly that it is 
quiefcent where the lowed radius touches it; and, if the 
radii, as they tread upon it, areobferved, their points will 
appear at reft, while the centre of the pulley will proceed 
as faft as the fledge, and the top of each radius fucceflively 
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 re¬ 
lative motions of the top, the middle, and the bottom, 
of the flick, whilft he inclines it, he will fee that the bot¬ 
tom 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 common paradox, which ap¬ 
pears at firft inexplicable, that the bottom of a rolling wheel 
never moves upon the road. This is afterted only of a wheel 
moving over hard ground, which, in faff, may be confi¬ 
dered rather as laying down its circumference upon the 
road than as moving upon it. 
11. The inclined plane is to be next confidered. When 
a heavy body is to be raifed, it is often convenient to lay 
a (loping artificial road of planks, upon which it may be 
puflied or drawn. This mechanical power, however, is 
but of little fervice without the aflittance of wheels or rol¬ 
lers ; we (hail therefore (peak of it as it is applied in an¬ 
other manner, under the name of the wedge , which is in 
faff a moving inclined plane ; but, if it be required to 
explain the properties of the inclined plane, by the panor¬ 
ganon, the wooden road may be raifed and fet to any in¬ 
clination required, and the fledge may be drawn upon it 
as in the former experiments. 
Let one end of a lever, N, fig. 79. with a wheel at one 
end of it, be hinged to the poft of the frame, by means of 
a gudgeon driven or fcrewed into the poft. To prevent 
this lever from deviating fideways, let a flip of wood be 
conneffed with it by a rail, which (hall be part in the 
lever, but which may move freely in a hole in the rail. 
The other end of this flip mult be fattened 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, in the fame manner as the treadle of a common lathe 
is managed, and as the treadle of a loom is fometimes 
guided. Under the wheel of this lever place an inclined 
plane on the wooden road, with rollers under it, to pre¬ 
vent fridtion ; fatten a rope to the foremolt end of the 
wedge, and pafs it through the pulleys P4 and P3, as in 
the fifth experiment; let a perfon draw the fledge by this 
rope over his ftioulder, 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 
af'cent of the weight, and the fpace through which he ad¬ 
vances, be compared, he will find that the fpace through 
which he haspafled will be five times as great as that through 
which the weight hasafcended ; and that this wedge has ena¬ 
bled him to raife five times as much as he could raife with¬ 
out it, if his ftrength were applied, as in Experiment 1, 
without any mechanical ad vantage. By making this wedge 
in two parts hinged together, with a graduated piece to 
keep them afunder, the fame may be adjufted to any 
given obliquity; and it will always be found, that the 
mechanical advantage of the wedge may be afcertained by 
comparing its perpendicular elevation with its bafe. If 
the bafe of the wedge be 2, 3, 4, 5, or any other number 
of times greater than its height, it will enable the perfon 
to raife refpedtively, 2, 3, 4, or 5, times more weight than 
he could do in Experiment 1. by which his power is efti- 
mated. 
12. The fcreui is an inclined plane wound round a cy¬ 
linder; the height of all its revolutions round the cylin¬ 
der taken together, compared with the fpace through 
which the power that turns it paflfes, is the meafure of its 
mechanical advantage. Let the lever ufed in the lalt ex¬ 
periment be turned in fuch a manner as to reach from its 
gudgeon to the lhaft of the panorganon, guided by an at¬ 
tendant lever as before (fig. So). Let the wheel reft upon 
the lowed helix, or thread, of the fcrew ; as the arms of 
the (haft are turned round, the wheel will afcend, and 
carry up the weight which is fattened to the lever. As 
the fituation of the fcrew prevents the weight from being 
fufpended exaftly from the centre of the fcrew, proper 
allowance mutt be made for this in eftimating the force 
of the fcrew, or determining the mechanical advantage 
gained by the lever. This can be done by meafuring the 
perpendicular afcent of the weight, which in all cales is 
ufeful, and more expeditious than meafuring the parts of 
a machine, and eftimating its force by calculation ; be- 
caufe the different diametersof ropes, and other fmall cir- 
cumftances, are frequently miltaken in ettimates—both 
methods fliould be employed, and their refults compared. 
The fpace pafled through by the moving power, and by 
that which it moves, are infallible data tor eftimating the 
powers of engines. 
The Panorgan may be employed in afcertaining the re- 
fiftance of air and water ; the ftrength of different perfons j 
and in a great variety of amufing and ufeful experiments. 
In 
