MECHANIC S. 
brafs which contains the pivot, fhould be made in two 
halves, and put together with fcrews, that the halves may¬ 
be (crewed clofer as the focket enlarges by wearing : but 
this is (till an imperfect method, becaufe the pivot can 
never fit accurately after having been worn ; a conical 
form is therefore ufed for the pivots of axles requiring 
great accuracy, as thefe may be always made to fill their 
lockets, by prefiing the cone farther into its focket. The 
cone is ufed in many turning-lathes, udiilft others are 
made very nearly cylindrical, with a (boulder; and, as the 
collar is of hard (feel, they do not wear in any fenfible de¬ 
gree. Their advantage over the cone is, that they have 
no drift endways upon the oppofite centre, as the cone 
has ; though this is fo (light in an acute cone, as to be of 
no importance in fmall machinery. In heavy works, fuch 
as the gudgeons of water-wheels, a conical figure would 
be highly improper, and has no advantage to recommend 
it ; as fuch gudgeons feldom have any brafs fcrewed down 
over them, their own weight being fufficient to keep them 
clown, and they always fit true as they wear away. The 
molt accurate and fimple of all pivots is that which is 
fimilar to a piece of work while turning in a lathe; the 
axis having a fmall hole made in each end of it, and the 
fiipports formed by (harp conical points, received into the 
holes; and one of them mu ft be adjuftable by a fcrew, to 
make it always fit the length of the fpindle. It is ufual 
to make the conical points on the ends of two fcrews, 
either of which may-then be adjufted. The fame thing 
may be accompliftied by making conical points at the 
ends of the fpindle, and forming the holes .or its reception 
in ends of the two fixed fcrews, which can at all times be 
fcrewed up as the parts wear. It is the mo(t perfttft of 
all methods; but is not adapted to bear any great (train, 
becaufe the fcrews will get loofe; and all the objections 
Co the conical fpindle apply to it. 
The pivot at the lower end of a vertical (haft, which 
bas a great weight to fultain, as in a heavy horfe-wheel, is 
stiade of an hemifpherical figure, and received into a pro¬ 
per cavity. A cylindrical pivot, having a flat end, is 
frequently ufed for large and heavy upright axes ; but 
it is difficult to keep oil fupplied to them, as the great 
weight prefl'es it out from between the adding furfaces, 
and the gudgeon burns. To avoid this, fome mechanics 
uiake a cleft acrofs the lower face of the gudgeon, exaftly 
in the manner of a (crew-head. This getting full of oil, 
the oil is conlfantly fupplied to the adding furfaces. We 
have feen an horizontal windmill, having a vertical axis 
ibo feet high, with fails and wheels of immenfe weight, 
all bearing upon one pivot. This was with the greatelfc 
difficulty kept in order ; and it was neceffary to keep a 
fmall ltream of cold water always running into a pan 
which furrounded the gudgeon, to keep it cold. This 
method of watering, inltead of oiling, a gudgeon, is alio 
ufed in paper-mills; but it cannot be recommended as a 
good method. 
A great number of machines depend upon reciprocating 
motions, fuch as pump-mills, faw-mills, &c. Where the fir it 
mover has a circular motion, as a water-wheel, the reci¬ 
procating movement will be mo(t conveniently produced 
by means of a crank ; becaufe it commences the change 
of motion by degrees, and does not fuddenly urge the 
parts into motion in a contrary direction ; nor fuddenly 
check the movement again ; but effects both changes 
without violence. It is proper, in fuch cafes, to regulate 
the motion of the fir It mover by a fly-wheel, otherwife 
the refiltance of the work, at the lnlfant of the change of 
motion, is fo fmall, that the machine would accelerate in 
that period, and then be checked again. The fame may 
be accompliftied by having feveral of the reciprocating 
movements, and thele act alternately, that, when one re¬ 
quires the molt power, the others take the lealt, fo as to 
equalize the refiltance to the firtt mover, and make the 
motion uniform. All reciprocating machines labour un¬ 
der great ,difad vantages, troin the circurnltance that a 
great mafs of matter mult be put ia motion, and this 
6 n 
motion deftroyed again. Thus, in a Angle pump forcing 
water through a great height of pipes, the column of wa¬ 
ter is, at every ftroke the pump makes, put in rapid mo¬ 
tion, which is wholly loft during the return of the pump- 
bucket for another ftroke, when freffi impetus mult be 
given to the water: now, by applying a double acting 
pump, or two or three pumps aiding at intervals, and the 
water regulated by an air-veffel, the motion will be very 
ealy, becaufe the column of water will be in conftant mo¬ 
tion through the pipes, and the momentum, once given to 
it, will continue as long as the machine is at work, in- 
Itead of requiring a repetition of it at every ftroke. 
In every machine, the aftion of the moving power is 
transferred to the working-point through the parts of the 
machinery, which are material, inert, or heavy ; or, ta 
defcribe it more accurately, before the neceffary force 
can be excited at the working-point of the machine, the 
various connedfting forces mult be exerted in the different 
parts of the machine: and, in order that the working- 
point may follow out the impreffion already made, all the 
connecting parts or limbs of the machine mult be moved 
in different directions, and with different velocities. Force 
is neceffary for thus changing the ftate of all this matter, 
and frequently a very conliderable force. Time mult alfo 
elapfe before all this can be accomplilhed. This often 
confumes, and really vvaltes, a great part of the impelling 
power. Thus, in a crane worked by men walking in a 
wheel, it acquires motion by flow degrees; becaufe, in 
order to give fufficient room for the action of the number 
of men or cattle that are neceffary, a very capacious wheel 
mult be employed, containing a great quantity of inert 
matter. All of this mult be put in motion by a very mo¬ 
derate preponderance of the men : it accelerates (lowly, 
and the load is raifed. When it has attained the required 
height, all this matter, now in conliderable motion, muft 
be flopped. This cannot be done in an inftant, with a 
jolt, which would be very inconvenient, and even hurt¬ 
ful : it is therefore brought to reft gradually. This aifo 
confumes time. Nay, the wheel mull get a motion ia 
the contrary dire< 5 tion, that the load may-be lowered into 
the cart or lighter; and this can only be accompliftied by- 
degrees. Then the tackle muft be lowered down again, 
for another load, which alfo muft be done gradually. All 
this waltes a great deal both of time and force, and ren¬ 
ders a walking-wheel a very improper form for the fir(fc 
mover of a crane, or any machine whofe ufe requires fucli 
frequent changes of motion. The fame thing obtains, 
although in a lower degree, in the (team.engine, where 
the great beam and pump-rods, fometimes weighing many 
tons, muft be made to acquire a very brilk motion in op¬ 
pofite directions, twice in every working ltroke. It ope¬ 
rates, in a greater or a lets degree, in all engines which 
have a reciprocating motion in any of thtir parts. Pump- 
mills are of neceftity fubjedft to this inconvenience. In 
the famous engine at Marly, about hs of the whole 
moving power of forne of the water-wiieels is employed 
in giving a reciprocating motion to a let of rods and 
chains, which extend from the wheels to a cittern about 
three-fourths of a mile diftant, where they work a let of 
pumps: thus the engine is, by fuch injudicious conllruc- 
tion,a monument ot ufelefs magnificence, and the Struggle 
of ignorance with the unchangeable laws ot nature. See 
Mar Li, p. 385. In machines all the parts of which con¬ 
tinue the direction of their motion unchanged, the inertia 
of a great mafs of matter does no harm ; but, on the con¬ 
trary, contributes to preferve the ileadinels of the mo¬ 
tion, in fpite of fmall inequalities of power or refiltance, 
or unavoidable irregularities of force in the interior part. 
But, in all reciprocations, it is highly prejudicial to the 
performance ; and, therefore, conlti uCtions which admit 
fuch reciprocation without necefiity, are avoided by all 
the intelligent engineers. 
In many compound machines it is of confequence to be 
able to detach part of the movements while the others con¬ 
tinue in motion. Thus, in cotton-fpiiining machines, it 
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