MECHANICS. 773 
fion of boiling water does it appear that the denfiry of 
fleam exceeds i-io,oooth of the denfity of water. Dcfa- 
guliers fays that it is above 14,000 times rarer than wa¬ 
ter. But no diltinct experiment ihows fo great an ex- 
panfion of water when converted into (team at 212 0 . Mr. 
Watt never found it under the preflure of the air more 
than 1800 times rarer than water. 
The moving force during the afcent of the pifton muft 
be confidered as refulting chiefly, if not folely, from the 
preponderating weight of the pit pilton rods. The of¬ 
fice of this is to return the fteam-pifton to the top of the 
cylinder, where it may again be prefled down by the air, 
and make another working-ftroke by raifing the pump'- 
rods. But the counter-weight at E has another fervice 
to perform in this ufe of the engine ; namely, to return 
the pump-piftons into their places at the bottom of their 
refpeft'ive working-barrels, in order that they alfo may 
make a working-itroke. This requires force independent 
of the fritffion and inertia of the moving parts; for each 
pifton muft be pufhed down through the water in the bar¬ 
rel, which muft rife through the pifton with a velocity 
whofe proportion to the velocity of the pifton is the fame 
with that of the bulk of the pifton to the bulk of the 
perforation through which the water rifes through the 
pifton. 
From this general confideration of the afcent of the pif¬ 
ton, we may fee that the motion differs greatly from the 
defcent. It can hardly be fuppofed to accelerate, even if 
the fteam in the cylinder were in a moment annihilated. 
For the refiftance to the defcent of the pifton is the fame 
with the weight of the column of water, which would 
caule it to flow through the box of the pump-pifton with 
the velocity with which it really rifes through it, and 
muft therefore increafe as the fquare of that velocity in- 
creafes; that is, as the fquare of the velocity of the pif¬ 
ton increafes. Independent of fridion, therefore, the ve¬ 
locity of defcent through the water muft foon become a 
maximum, and the motion become uniform. In fuch a 
pump as is generally ufed, this will happen in lei's than 
the 10th part of a fecond. The fridion of the pump will 
diminifh this velocity a little, and retard the time of its 
attaining uniformity. But, on the other hand, the fup- 
ply of fteam which is neceffary for this motion being fuf- 
ceptible of no acceleration from its previous motion, and 
depending entirely on the brilknef's of the ebullition, an 
t almoft inilantaneous flop is put to acceleration. Accord¬ 
ingly, any perfon who obferves with attention the work¬ 
ing "of a fteam-engine, will fee that the rife of the pifton 
and defcent of the pump-rods, is extremely uniform, 
whereas the working ftroke is very fenftbly accelerated. 
But we may obferve, that the counter-weight is different 
during the two motions of the pump-rods. While the 
machine is making a working-ftroke, it is lifting not only 
the column of water in the pump, but the abfolute weight 
of the piftons and pifton-rods alio; but, while the pump- 
rods are defcending, there is a diminution of the counter¬ 
weight by the whole weight loft by the immerfion of the 
rod in water. The wooden rods which are generally ufed, 
foaked in water, and joined by iron ftraps, are heavier, 
and but a little heavier, than water; and they are gene¬ 
rally about one-third of the bulk of the water in the 
pumps. Thefe two motions complete the period of the 
operation; and the whole may be repeated by (hutting 
the fteam-cock and opening the injedion-cock whenever 
the pifton has attained the proper height. We have been 
very minute in our attention to the different circum- 
flances, that the reader may have a diftind notion of the 
ftate of the moving forces in every period of the opera¬ 
tion. It is by no means fuflicient that we know in ge¬ 
neral that the injection of cold water makes a void which 
allows the air to prefs down the pifton, and that the re- 
admiflion of the fteam allows the pifton to rife again. 
This lumping and flovenly way of viewing it has long 
prevented even the philofopher from feeing the defeds of 
she eonftrudion, and the methods of removing them. 
We now fee the great difference between Savery’s and 
Newcomen’s engine in refped of principle. Savery’s was 
really an engine which raifed water by the force of fteam ; 
but Newcomen’s raifes water entirely by the preflure of 
the atmolphere, and fleam is employed merely as the moll 
expeditious method of producing a void, into which the 
atmofpherical preflure may impel the JirJi mover of his ma¬ 
chine. The elafticity of the (team is not the fir (I mover. 
We fee alfo the great fuperiority of this new machine. 
We have no need of fteam of great and dangerous elalti- 
city ; and we operate by means of very moderate heats, 
and confequently with much ('mailer quantities of fuel; 
and there are no bounds to the power of this machine. 
How deep foever a mine may be, a cylinder may be em¬ 
ployed of fuch dimenfions that the preflure of the air on 
its pifton may exceed in any degree the weight of the co¬ 
lumn of water to be railed. And laftly, this form of the 
machine renders it applicable to almoft every mechanical 
purpofe; becaufe a (kilful mechanic can readily find a 
method of converting the reciprocating motion of the 
working-beam into a motion of any kind which may fuit 
his purpofe. Savery’s engine could hardly admit of fuch 
an immediate application, and feems almoft reftrided to- 
raifing water. 
Inventions improve by degrees. This engine was firft 
offered to the public in 1705. But many difficulties oc¬ 
curred in the execution, which were removed one by one ; 
and it was not till 1712 that the engine feemed to give 
confidence in its efficacy. The molt exad and unremit¬ 
ting attention of the manager was required to the precife 
moment of opening and (hutting the cocks; and negled 
might frequently be ruinous, by beating out the bottom 
of the cylinder, or allowing the pifton to be wholly drawn 
out of it. Stops were contrived to prevent both of thefe 
accidents; then firings were ufed to conned the handles 
of the cocks with the beam, fo that they ffiould be turned 
whenever it was in certain poiitions. Thefe were gradu¬ 
ally changed and improved into detents and catches of 
different ffiapes ; at laft, in 1717, Mr. Beighton, a very in¬ 
genious and well-informed artilt, Amplified the whole of 
thefe fubordinate movements, and brought the machine 
into the form in which it has continued, without any 
material change, to the prefent day. In the machine, 
thus improved, an upright bar of timber, t, is feen hanging 
by a chain : this is (ufpended from a concentric arch r s , 
of the working-beam. The bar is called the plug-beam. ; 
and it muft rile and fall with the pifton, but with a (lower 
motion. The ufe of this plug-beam is to give motion to 
the different pieces which turn the cocks. We have faid 
that the injedion-water is furniflied from a ciftern placed 
above the cylinder, at W. When the ciftern cannot be 
l'upplied by pipes from fotne more elevated fource, its 
water is raifed by the machine itfelf. A fmall lifting- 
pump ik, called the jack-head, or j a quelle, is worked by a 
rod xi, fufpended from a concentric arch ax, near the 
outer end of the working-beam. This forces a fmall por¬ 
tion of the pit-water along the rifing-pipe i L M into the 
injedion-ciftern. 
The engine continued in this form for many years. 
The only remarkable change introduced, was the manner 
of placing the boiler. It is no longer placed below the 
cylinder, but at one fide ; and the (tream is introduced 
by a pipe from the top of the boiler into a flat box im¬ 
mediately below the cylinder. The ufe of this box id 
merely to lodge the regulator, and give room for its mo¬ 
tions. This was a very conliderable improvement. It 
greatly reduced the height of the building, which was 
formerly a tower. The wall which fupportrd the beam 
could hardly be built with fuflicient ftrength for with- 
(ianding the violent (hocks which were repeated without 
ceafing ; and the buildings feldom lalted longer than a 
few years. But the boiler is now fet up in an adjoining 
(lied, and the gudgeons of the main beam reft on the top 
of upright polts, which are framed into the joilts which 
fupport the cylinder. Thus the whole moving parts of 
s tits 
