MECH 
ftrength and weights of each individual, but alfo accord¬ 
ing to the different manner in which that ftrength or 
weight is applied, we mutt in the firtt place eftablifh one 
determinate inode of applying human force to the ma¬ 
chine, as well as a method of determining the relative force 
of each individual, whole ftrength is employed in letting 
it in motion. 
Hang a common long fcale-beam (without fcales or 
chains) from the top, c r tranfom, of the frame, fo that one 
end of it may come within an inch of one fide or poft of 
the machine. Tie a rope to the hook of the fcale-beam, 
where the chains of the fcale are ufually hung, and pal's 
it through the pulley P3, which is about four feet from 
the ground ; let the perfon pull this rope from 1 towards 2, 
turning his back to the machine, and pulling the rope 
over his iho.ulder, as (liown at lig. 74-. As the pulley may 
be either too high or two low to permit the rope to be ho¬ 
rizontal, the perfon who pulls it ftiould be placed ten or 
fifteen feet from the machine, which will lelfen the angu¬ 
lar dire&ion of the cord, and thus diminifh the inaccu¬ 
racy of the experiment. Hang weights to the other end 
of the fcale-beam, till the perfon who pulls can but juft 
walk forward, pulling fairly without knocking his feet 
againlt any thing. This weight will eltimate the force 
with which the perfon can draw horizontally by a rope 
over his (boulder. 
Let a youth who tries this, walk on the board with dry 
fhoes ; let him afterwards chalk his fhoes, and then try it 
with his (hoes foaped. He will find that he can pull with 
different degrees of force in thefe different circumftances. 
When he makes the following experiments, however, let 
the fhoes be always dry, that the fame degree of force may 
always be exerted. 
2. The lever L (fig. 75.) is paffed through a focket, 
((hown feparately ate,) in which it can be fluffed from one 
of its ends towards the other, fo that it may be fattened at 
any place by the ferew of the focket. This focket has two 
gudgeons, upon which both the focket and the lever 
which it contains can turn. The focket and its gudgeons 
can be lifted out of the hole in which it plays between 
the rails R, R, fig. 75. and may be put into other holes 
at R, R, fig. 77. 
Hook the cord that comes over the perfon’s fhoulder to 
the end I of the lever L, fig. 75. Loop another rope to 
the other end of this lever, and let the perfon pull as before. 
Perhaps it ftiould be pointed out, that the perfon rnuft 
walk in a direction contrary to that in which he walked 
before, viz. from 1 towards 3, (fig. 73.) The height to 
which the weight afeends, and the diftance to which the 
perfon advances, ftiould be carefully marked and mea- 
fured ; and it will be found, that he can raife the weight 
to the fame height, advancing through the fame fpace as 
in the former experiment. In this cafe, as both ends of 
the lever moved through equal fpaces, the lever only 
changed the direction of the motion, and added no me¬ 
chanical power to the direfl ftrength of the perfon. 
3. Shift the lever to its extremity in the focket 5 the mid¬ 
dle of the lever will now be oppofite to the pulley, fig. 76. 
hook to it the rope that goes through the pulley P 3, and 
fatten to the other end of the lever the rope by which the 
perfon is to puil. This will be a lever of the fecond kind, 
in ufing which, the refjlanee is placed between the centre of 
motion or fulcrum and the moving power. He will now raife 
double the weight that he did in Experiment 2. and he 
will advance through double the fpace. 
4. Shift the lever, and the focket which forms the axis, 
(without (hitting the lever from the place in which it was 
in the focket in the laft experiment,) to the holes that are 
prepared for it at R, R, fig. 77. The free end of the 
lever E will now be oppofite to the rope, and to the pul-- 
ley, (over which'the rope comes from the fcale-beam.) 
Hook this rope to if, and nook the rope by which the per- 
lon pulls to the midclle of the lever. The effect will now 
be different from what it was in the laft two experiments ; 
the perfon will advance only half as far, and will raife only 
A N I C S. 699 
half as much weight as before. This is called a lever of 
the third kind. 
The experiments upon levers may be varied at pleafure, 
increafingor diminiftiing the mechanical advantage, fo as 
to balance the power and the reliftance, to accuftotn the 
learners to calculate the relation between the power and 
the effect in different circumftances j always pointing out 
that whatever excefs there is in the power, or in the re¬ 
finance, is always compenfated by the difference of fpace 
through which the inferior paffes. 
The experiments which we have mentioned are fufiici- 
ently fatisfaflory to a pupil, as to the immediate relation 
between the power and the refiftance ; but the different 
fpaces through which the power and the refiftance move 
when one exceeds the other, cannot be obvious, ur.lefs 
they pafs through much larger fpaces than levers will 
permit. 
5. Place the (ledge, fig, 73. on the fartheft end of the 
wooden road ; fallen a rope to the fledge, and conduct it 
through the lowed pulley P 4, and alfo through the pul¬ 
ley P 3, (b that the perfon may be enabled to draw it by 
the rope patted over his (boulder. This (ledge rnuft now 
be loaded, till the perfon can but juft advance with fliort 
fteps lteadily upon the wooden road ; this rnuft be done 
with care, as there will be but juft room for him befide 
the rope. He will meet the (ledge exactly on the middle 
of the road, from which he mult ttep alide to pafs the 
(ledge. Let the time of this experiment be noted. It is 
obvious that the perfon and the (ledge move with equal 
velocity; there is therefore no mechanical advantage ob¬ 
tained by the pulleys. The weight that he can draw will 
be about half a hundred, if the weight be about nine 
(tone ; but the exaft force with which the perfon draws is 
to be known by experiment firft. 
6. To the largeft drum, D, fatten a cord, and pafs it 
through the pulley P downwards, and then through the 
pulley P 4, to the fledge placed at the end of the wooden 
road which is fartheft from the machine. Let the perfon, 
by a rope fattened to the extremity of one of the arms of 
the capftan, and patted over his (boulder, draw the capftan 
round ; he will wind the rope round the drum, and draw 
the (ledge upon the road. To make the fledge advance 
twenty-four feet upon its road, the perfon mutt have 
walked circularly 144 feet, which is fix times as far, and 
he will be able to draw about three hundred weight, which 
is fix times as much as in the laft experiment. 
It may now be pointed out, that the difference of fpace, 
palled through by the power in this experiment, is exactly 
equal to the difference of weight which the perfon could 
draw without the capftan. 
7. Let the rope be now attached to the fmallerdrum G ; 
the perfon will draw nearly twice as much weight upon 
the (ledge as before, and will go through double the fpace. 
3 . Where there is a number of perlons, let five or fix of 
them, vvhofe power of drawing (eltimated as in Experi¬ 
ment 1.) amounts to fix times as much as the force of the 
perfon at the capftan, pull at the end of the rope which 
was fattened to the (ledges; they will balance the force of 
the perfon at the capftan ; either they or he, by a fudden 
pull, may advance; but, if they pull fairly, there will be 
no advantage on either fide. In this experiment the rope 
(hould pafs through the pulley P 3, and ftiould be coiled 
round the larger drum. And it mult alfo be obferved, 
that, in all experiments upon the motion of bodies on 
which there is much friflion, as where a fledge is em¬ 
ployed, the refults are never fo uniform as under other 
circumftances. 
9. Upon tire pulley we (hall (ay little, as it is in every 
body’s hands, and experiments may lie tried upon it with¬ 
out any particular apparatus. It Ihould, however, be dif- 
tinflly 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, may be employed, as in fig. 78. Pieces like 
the heads of crutches (hould be fixed at the ends of thefe 
fpokes., 
