M E C H A N I C S. 
There is alfo great room for ingenuity and good choice 
In the management of the moving power, when it is fuch 
as cannot immediately produce the kind of motion re¬ 
quired for effecting the purpofe. We mentioned the con¬ 
vention of the continued rotation of an axis into the re¬ 
ciprocating motion of a pifton, and the improvement 
which was thought to have been made on the common 
and obvious contrivance of a crank, by fubftituting a 
double rack on the pifton-rod, and the inconvenience 
arifing from the jolts occafioned by this change. We 
have feen a great forge, where the engineer, in order to 
avoid the fame inconvenience arifing from the abrupt mo^ 
tion given to the great fledge-hammer of feven hundred¬ 
weight, refilling with a five-fold momentum, formed the 
wipers into fpirals, which communicated motion to the 
hammer almott without any jolt whatever ; but the refult 
was, that the hammer role no higher than it had been 
raifed in contaft: with the wiper, and then fell on the iron 
bloom with very little effect. The caufe of its inefficiency 
was not guefled at; but it was removed, and wipers of 
the common form were put in place of the fpirals. In 
this'operation, the rapid motion of the hammer is abfo- 
lutely neceffary. It is not enough to lift it up ; it mult 
be tojfed up, fo as to fly higher than the wiper lifts it, and 
toftrike with great force the ftrong oaken fpring which is 
placed in its way. It compreffes this fpring, and is re¬ 
flected by it with a confiderable velocity, fo as to hit the 
iron as if it had fallen from a great height. Had it been 
allowed to fly to that height, it would have fallen upon 
the iron with fomewhat more force (becaufe no oaken 
fpring is perfectly elaltic) ; but'this would have required 
more than twice the time. 
In employing a power which of necelfity reciprocates , 
to drive machinery which requires a continuous motion 
(as in applying the fleam-engine to a cotton or a grift 
mill), there alio occur great difficulties. The neceffity of 
reciprocation in the firlt mover waftes much power ; be¬ 
caufe the inftrument which communicates fuch an enor¬ 
mous force mull be extremely firong, and be well fup- 
ported. The impelling power is wafted in imparting, and 
afterwards deftroying, a vaft quantity of motion in the 
working beam. The fkilful engineer will attend to this, 
and do his utmoft to procure the neceffary ftrength of 
this firft mover, without making it a vaft load of inert 
matter. He wilTalfo remark, that all the ftrains on it, and 
on its fupports, are changing their directions in every ftroke. 
This requires particular attention to the mannerof fupport- 
ing it. If we obferve the fteam-engines which have been 
long ereCted, we fee that they have uniformly fhaken the 
building to pieces. This has been owing to the ignorance 
or inattention of the engineer in this particular. They 
are much more judicioufly ereCted now, experience hav¬ 
ing taught the moft ignorant that no building can with. 
Itand their defultory and oppofite jolts, and that the great 
movements muft be fupported by a frame-work indepen¬ 
dent of the building of mafonry which contains it. The 
gudgeons of a water-wheel fhould never reft on the wall of 
the building. It fhakes it; and, if fet up foon after the 
building has been erefted, it prevents the mortar from 
taking firm bond ; perhaps by Ihnttering the calcareous 
cryftals as they form. When the engineer is obliged to 
reft the gudgeons in this way, they Ihould be fupported 
by a block of oak laid a little hollow. This foftens all 
tremors, like lprings of a wheel-carriage. This practice 
would be very ferviceable in many other parts of the con- 
ltruCIion. 
It will frequently conduce to the good performance of 
an engine, to make the aftion of tlie refilling work un¬ 
equable, accommodated to the inequalities of the impel¬ 
ling power. This will produce a more uniform motion 
in machines in which the momentum of inertia is inconfi- 
dcrable. There are fome beautiful fpecimens of this kind 
of adjuftment in the mechanilm of animal bodies. 
Another circumftance is worth notice, and fhould al¬ 
ways be attended to where it will not interfere with more 
Vol. XIV, Wo. 1003. 
m 
important confiderations; this is, the direElion in which 
any force is given to, and taken from, any piece of wheel- 
work ; fuppofe, for inftance, a water-wheel turning its 
axis, upon which is fixed a cog-wheel to give motion to a 
fecond wheel, for the purpofe of driving any machinery; 
now, if this fecond cog-wheel is applied on that fide of the 
firlt cog-vyheel which is afeending, it will be oppofite to 
that fide of the wheel which is loaded with water, and is 
confequently defeending. In this Itate the gudgeons of 
the water-wheel will have to bear (in fome cafes) double 
the ftrain of the power of the machine ; becaufe the po wer, 
which is the weight of the water, is applied on one fide 
the centre of the wheel, and is taken off ,by turning the 
fecond cog-wheel on the other fide : the centre, or ful¬ 
crum, therefore, bears the whole power, and alfo the re¬ 
action to that power, in addition to the weight of its own 
parts; in the fame manner as the fulcrum of a fteelyard 
or balance-beam bears the whole of the weight fufpended 
from either end, and its own weight alfo. On the other 
hand, fuppofe the fecond wheel applied on the defeending 
fide of the water-wheel, this being on the fame fide of the 
centre, the prefliire thereon will be far iefs than the power 
of the machine. In fome cafes (but not in a water-wheel), 
by the proper arrangement of the wheel-work, the power 
may be made to operate to lift the centres, and thus in part . 
relieve them from the weight of the wheel, fo as actually 
to diminilh the preffureof friftion of the pivots, when, by 
a contrary application, it would have increafed it in the 
fame degree. Similar advantages will attend the precau¬ 
tion of adapting the pofitions of different wheels upon their 
fliafts to the different weights or ftrains they have to bear, 
fo that the gudgeons at the two ends of any lhaft may have 
an equal drift or preffure upon them. This will caufe 
them to wear equally, and to have lefs friftion, becaufe 
they may be made fmaller than where no fuch care is taken, 
ftill having fufficient ftrength. It is accomplilhed by con- 
fidering the drift or prelfure upon the centre of every 
wheel upon any axis, and placing the two gudgeons or 
pivots of the axis at a diftance from each of the wheels, 
proportionate to the drift upon its centre. Thus, fup¬ 
pofe a lhaft has a cog-wheel fixed upon it, and a ftnall 
wheel or pinion alfo fixed upon it at fome diftance from 
the wheel, the power is given to the axis by wheel-work 
operating upon the teeth of the pinion, and the re-aftion 
to this power is given by fome machinery which the teeth 
of the large wheel aftuates. In this cafe the drift on the 
centre of the pinion will be very confiderable, becaufe the 
power is applied near the centre of the axis ; but the wheel 
tranfmitting the power at a greater radius will, perhaps, 
have much lefs drift on its centre (the proportion depend¬ 
ing in fome degree upon the direction in which the power 
and re-aftion are applied, as Hated in our lalt obfervation): 
if this is the cafe, the gudgeon at that end of the lhalt, 
where the pinion is placed, Ihould be lengthened out, fo 
as to give the bearing point at a greater diftance from it 
than the wheel, which Ihould have its gudgeon placed 
much nearer to it, becaufe lefs ftrain is to be borne. By 
this means the drift upon the two ends of the lhaft will 
be equally divided between them ; and, though this pro¬ 
portion of the centre cannot be always accomplilhed with¬ 
out inconvenience, the engineer Ihould always have it in 
view 9 and then, where-it is not practicable, lie Ihould at¬ 
tain the fame end by apportioning the ftrength or diame¬ 
ter of the gudgeons to the relative drains they have to bear. 
An endlefs belt or ltrap is a very general method of 
tranfmitting rotatory motion : it is ulually employed in 
cafes where a very quick motion is to be created, and the 
re-aftion to be overcome is nearly equable. In fuch cafes 
it has the advantage of wheel-work from its fimplicity 
and the eafe of its motion. Some curious properties be¬ 
long to the endlefsJlrap ; viz. that the pulley or rigger it 
works upon muft be largelt in the middle, that is, the 
diameter muft be greater in the middle of the pulley than 
at the edges, becaufe the ltrap always rides on to the largelt 
diameter oi the pulley, and, if this is not in the centre, 
8 H it 
