M E C IT A N I C S. 
6? 5 
of either wheel at pleafure, by Hiding the clutch-box to¬ 
wards that wheel. 
We have feen a very ingenious application of the live 
and dead pulley to this purpofe, for a crane in a cotton- 
mill, to take up and down the goods, work people, &c. 
It was invented by Mr. Henry Strutt, and has been ap¬ 
plied in his cotton-mills at Belper, Dsrbyfhire. In this 
machine it was neceflary to have a motion which could 
be turned either way at pleafure, to draw up or let down 
the bafket ; but the double wheel-work above defcribed 
was evidently improper, from the bidden jerk it would 
have given at the inbuilt of changing- the motion. It was 
effected in this manner i an axis which- gave motion to 
the crane-barrel has two pair of live and dead pulleys 
ppon it, and alfo a brake-wheel to Hop the motion, which 
is fituated between the two pair; an endlefs ftrap is con¬ 
duced to each pair, being turned by a long drum placed 
parallel to the axis of the pulleys, and kept in confia.nt 
motion by the mill. One of thele endlefs ftraps is croffed 
between the drum and its pulleys, but the other is not; 
therefore one pair of the live or dead pulleys is always 
revolving in one direction, and the others are turning in 
an oppofite way. Both ft raps are conducted through 
guides fixed to a fliding-rail, by which the ftraps can be 
fhifted, both at once, ftdeways. When this rail is in a po¬ 
rtion that the ftraps are both upon their dead pulleys, 
the axis and brake-wheel are at reft, and in this pofition 
the rail has a tendency to remain, unlefs forced by hand. 
On moving the rail one way from the quiefcent point, one 
of the ftraps is thrown on ks live pulley, and the fpindle 
turns with it, winding up the bafket. By moving the 
rail in the other direction beyond its quiefcent point, this 
itrap is fhifted on to its dead pulley, and becomes inactive ; 
but the other (trap operates on its live pulley, to turn 
the fpindle in the oppofite direction, and lets down the 
bafket. 
Logwood rafping-engines, fcrew-preftes, and fome other 
machines, require a motion to work them forwards to a 
certain extent, and then the direction is to be reverfed to 
draw them back, which requires but very little power to 
effeCt it. In this cafe the motion may be effected by a 
pair of cog-wheels turning each other, and thus commu¬ 
nicating the motion for one dire Cion in which it is to 
perform the work. A couple of puileys are fixed on the 
refped'tive*axes near the cog-wheels, and an endlefs ftrap 
connects them ; but the ftrap is fo long, that, when the 
cog-wheels are in gear, the ftrap hangs flack, and does 
not operate ; but to reverfe the movement, the fockets for 
one of the gudgeons of the driving fpindle or axis is made 
to fhift, that the diftance between the centre of the two 
wheels may be increafed, fo as to difengage the teeth of 
the wheels, and the ftrap becomes tight, and turns the 
wheels back ; but, on bringing the wheels together again, 
the ftrap becomes flack, and the wheels refume their ori¬ 
ginal courfe. 
Of Friction, and of the Rigidity of Ropes. 
I. The friction generated in the communicating parts 
of machinery, oppofes fuch a reftltance to the impelling 
power, and is fo injurious to the machine itfelf, that an 
acquaintance with the nature and effeCts of this retarding 
force, and with the method of diminiftiing its effeCts on 
machinery, is of infinite importance to the practical me¬ 
chanic. 
The fubjeCt of friction has, been examined at great 
length by Anion tons, Bulfinger, Parent, Euler, and Boflut; 
and has lately occupied the attention of our ingenious 
countryman Mr. Vince of Cambridge. 
He found that the friction of hard bodies in motion is 
an uniformly retarding force, and that the quantity of 
friction ccnfidered as equivalent to a weight drawing the 
.body backwards is equal to — 4 *~ - a where M is the 
tyOL, XIV. No. 1004.. 
moving force exprefled by its weight, W the weight of 
the body upon the horizontal plane, S the fpace through 
which the moving force or weight defceijded in the time t, 
and g-io-oSj feet, the force of gravity. Mr. Vince alfo 
found that the quantity of friction incrcafes in a Ic-fs ratio 
than the quantity 0! matter or weight of the body ; and 
that the friCtion of a body does nor continue the fame 
when it has different furfaces applied to the plane oil 
which it moves, but that the lmalleft furfaces will have the 
leaft friction. 
Notwithftanding the attempts of preceding pbilofophers 
to unfold the nature of friction, it was referved for the 
celebrated Coulomb to furjnount the difficulties "hich 
are irifeparable ffosi fuch an inveiiigation, and to give an 
accurate and fatisfactory view of this difficult branch of 
mechanical philofophy. See p. 624. As it would be fo¬ 
reign to the nature of our work to follow this ingenious 
philofopher through his numerous and varied experiments, 
we fhal! only .prelent the reader with the interefting refults 
to which they led. 
The friction of homogeneous bodies, or bodies of the 
fame kind, moving upon one another, is generally fup- 
pofed to be greater than that of heterogeneous bodies ; 
but Coulomb has fliown that there are exceptions to this 
rule. He found, for example, that the friCtion of oak. 
upon oak was equal to —-— of the force of predion 5 the 
2-34 
friftion of pine again ft pine-and that of oak againft 
178 
1 _ j 
Pine — * The friction of oak againft copper was -, 
and that of oak againft iron nearly the fame. 
It was generally fuppofed that, in the cafe of wood, the 
friction is greateft when the bodies are dragged contrary 
to the courfe of their fibres ; but Coulomb has ihown that 
the friction is in this cafe fotnetimes the fmalleft. When 
the bodies moved in the direction of their fibres, the fric¬ 
tion was -of the force with which they were prefled 
-• 34 - 
together; but, when the motion was contrary to the courfs 
of the fibres, the friction was only —— • 
3-76 
Prop. LXXVIII. The longer the rubbing furfaces remain in 
contact, the greater is their frittion. —When wood was moved 
upon wood according to the direction of the fibres, the 
friCtion was increafed by keeping the furfaces in contaCfc 
Tor a few feconds ; and, when the time was prolonged to 
a minute, the friCtion feemed to have reached its fartheft 
limit. But, when the motion was contrary to the courfe 
of the fibres, a greater time was neceflary before the fric¬ 
tion arrived at its maximum. When wood was moved 
upon metal, the friCtion did not attain its maximum till 
the furfaces had continued in contaCt for five or fix days ; 
and it is very remarkable, that, when wooden furfaces 
were anointed with tallow, the time requifite for produc¬ 
ing the greateft quantity of friCtion is increafed. When 
the furfaces of metallic bodies were moved upon one 
another, the time of producing a maximum of friCtion 
was not changed by the interpolition of olive-oil; it was 
increafed, however, by employing fwine’s greafe as an 
unguent, and was prolonged to five or fix days by be- 
finearing the furfaces with tallow. 
Prop. LXXIX. FriBion is in general proportional to the 
force with which the rubbing furfaces are prefect together ; and 
is, for the mojl part, equal to between \ and 5 of that force. 
In order to prove the fir It part of this proportion, Cou¬ 
lomb employed a large piece of wood, whole furface con¬ 
tained three fquare feet, and loaded it fucceflively with 
74 pounds, 874 pounds, and 2474 pounds. In thefs 
cafes the friCtion was fucceflively —~, —— 9 -i— of the 
*■46 z‘ib zzi 
8 I fores 
