9 
MECHANICS. 
774 
of common air before the fteam-cock was fliut, and if 
none has entered fince, the pifton will defcend to the very 
bottom of the cylinder. And this may be frequently ob- 
ferved in a good fteam-engine, where every part is air¬ 
tight. It fometimes happens, by the pit-pump drawing 
air, or fome part of the communication between the two 
(trains giving way, that the pifton comes down with fuch 
violence as to knock out the bottom of the cylinder. 
The only obfervation which remains to be made on the 
motion of the pifton in defcending is, that it does not 
begin at the inlfant the injection is made. The pifton 
was kept at the top by the preponderancy of the outer 
end of the working-beam, and it muft remain there till 
the difference between the elafticity of the fteain below it 
and the prefl'ure of the atmofphere exceeds this preponde¬ 
rancy. There muft therefore be a fmall fpace of time be¬ 
tween the beginning of the condenfation and the begin¬ 
ning of the motion. This is very fmall, not exceeding the 
third or the fourth part of a fecond ; but it may be very 
diftinftly obferved by an attentive fpeffator. He will fee, 
that the inlfant the injeftion-cock is opened, the cylinder 
will fenfibly rife upwards a little by the prelfure of the 
air on its bottom. Its whole weight is not nearly equal 
to this prelfure; and, inftead of its being necelfary to 
fupport it by a ftrong floor, we muft keep it down by ftrong 
joilts loaded with heavy walls. It is ufual to frame thefe 
joilts into the polls which carry the axis of the working- 
beam, and are therefore loaded with the whole ftrain of 
the machine. This riling of the cylinder (hows the in- 
ftantaneous commencement of the condenfation; and it 
is not till after this has been diftinffly obferved that the 
pifton is feen to ftart, and begin to defcend. 
When the manager fees the pifton as low as he thinks 
proper, he Units the inje£tion-cock, and opens the fteam- 
cock. The fteam has been accumulating above the water 
in the boiler during the whole time of the pifton’s defeent, 
arid is now rulhing violently through the puppet-clack. 
The moment, therefore, that the iteam-cock is opened, 
it rufltes violently into the cylinder, having an elafticity 
greater than that of the air. It therefore immediately 
blows open the fnifting-valve, and allows (at lealt) the 
water which had come in by the former injection, and 
what arofe from the condenfed fteam, to defcend by its 
own weight through the eduftion-pipe degk, to open the 
valve h , and to run out into the hot well. And we muft 
ealiiy fee that this water is boiling hot; for, while lying 
in the bottom of the cylinder, it will condenfe fteam till 
it acquires this temperature, and therefore cannot run 
down till it condenfes no more. There is (till a wafte of 
fteam at its firl't admiflion, in order to heat the infide of 
the cylinder and the injedted water to the boiling tempe¬ 
rature: but the fpace being fmall; ar.d, the whole being 
already very warm, this is very foon done; and, when 
things are properly conftrudted, little more fteam is wanted 
than what will warm the cylinder; for the edudtion-pipe 
receives the injedlion-water even during the defeent of the 
pifton, and it is therefore removed pretty much out of the 
way of the fteam. 
This flrft puff of the entering fteam is of great fervice; 
it drives out of the cylinder the vapour which it finds 
there. This is feldom pure watery vapour; all water 
contains a quantity of air in a flare of chemical union. 
The union is but feeble, and a boiling heat is fufficient 
for difengaging the greateft part of it by increafing its 
elafticity. It may alio be difengaged by Amply removing 
the external prefl'ure of the atmofphere. This is clearly 
feen when we expofe a glafs of water in an exhaufted re¬ 
ceiver. Therefore the fmall fpace below the pifton con¬ 
tains watery vapour mixed with all the air which had been 
dilengaged from the water in the boiler by ebullition, and 
all that was feparated from the injedftion-water by the di¬ 
minution of external prefl'ure. All this is blown out of 
the cylinder by the firft puff of fteam. We may oblerve 
in this place, that waters differ exceedingly in the quan¬ 
tity of air which they hold in a ftate of folution. All 
fpring-water contains much of it; and water newly 
brought up from deep mines contains a great deal more, 
becaufe thefolution was aided in thefe fituations by great 
preffures. Such waters fparkle when poured into a glafs. 
It is therefore of great confequence to the good per¬ 
formance of a fteam-engine to ufe water containing little 
air, both in the boiler and in the injeftion-ciltern. The 
water of running brooks is preferable to all others ; and, 
the freer it is from any faline impregnation, the lefs air it 
generally contains. Such engines as are fo unfortunately 
fituated that they are obliged to employ the very water 
which they have brought up from great depths, are found 
greatly inferior in their performance to others. The air 
colledted below the pifton greatly diminifhes the accele¬ 
rating force ; and the expulfion of fuch a quantity requires 
a long-continued blaft of the belt fteam at the beginning 
of every ftroke. It is advifeable to keep fuch water in a 
large (hallow pond for a long while before ufing it. 
Let us now confider the ftate of the pifton. It is evi¬ 
dent that it will ftart, or begin to rife, the moment the 
fteam-cock is opened ; for at that inftant the excefs of at- 
mofpherical prefl'ure, by which it was kept down in op- 
pofition to the preponderancy of the outer end of the 
beam, is diminiflied. The pifton is therefore dragged up¬ 
wards, and it will rife even although the fteam which is 
admitted be not fo elaftic as common air. Suppofe the 
mercury in the barometer to ftand at 30 inches, and that 
the preponderancy at the outer end of the beam is -£th of 
the prefl'ure of the air on the pifton, the pifton will not 
rife if the elafticity of the fteam is not equal to 30— 
that is, to 26‘7 inches nearly ; but, if it is juft this quan¬ 
tity, the pifton will rife as faft as this fteam can be (Ap¬ 
plied through the fteam-pipe; and the velocity of its 
alcent depends entirely on the velocity of this fupply. 
This obfervation is of great importance; and it does not 
feem to have occurred to the mathematicians who have 
paid molt attention to the mechanifm of the motion cf 
this engine. In the mean time, we may clearly fee that 
the entry of the fteam depends chiefly on the counter¬ 
weight at E ; for, fuppofe there was none, fteam no ftronger 
than air would not enter the cylinder at all; and, if the 
fteam be ftronger, it will enter only by the excefs of its 
ftrength. Writers on the fteam-engine (and even fome 
of great reputation) familiarly fpeak of the fteam giving 
the pifton a pufi: but this is icarcely poflible : during the 
rife of the pifton, the fnifting-valve is never obferved to 
blow ; and we have not heard any well-attefted accounts 
of the pifton-chains ever being flackened by the upward 
prefl'ure of the fteam, even at the very beginning of the 
ilroke. During the riling of the pifton the fteam is (ac¬ 
cording to the common conception and manner of fpeak- 
ing) fucked in, in the fame way that air is fucked into a 
common fyringe or pump when we draw up the pilton ; 
for in the fteam-engine the pifton is really drawn up by 
the counter-weight. But it is Itill more lucked in, and 
requires a more copious fupply, for another reafon. As 
the pifton defeended only in confequence of the inlide of 
the cylinder’s being fufficiently cooled to condenfe the 
fteam, this cooled lurface muft again be prefented to the 
fteam during the rife of the pilton, and muft condenfe 
fteam a fecond time. The pifton cannot rife another inch 
till the part of the cylinder which the pilton has already 
quitted has been warmed up to the boiling point, and 
fteam muft be expended in this warming. The inner l'ur- 
face of the cylinder is not only of the heat of boiling wa¬ 
ter while the pifton rifes, but is alfo perfectly dry ; for the 
film of water left on it by the afeending pifton muft be 
completely evaporated, otherwife it will be condenfing 
fteam. That the quantity thus wafted is confiderable, 
appears by the experiments of Mr. Beighton. He found 
that five pints of water were boiled off in a minute, and 
produced 16 ftrokes of an engine whole cylinder con¬ 
tained 113 gallons of 282 inches each ; and he thence con¬ 
cluded that fteam was 2886 times rarer than water. But in 
no experiment made with fcrupulous care on the expan- 
iiosi 
