214 
MAGAZINE OE SCIENCE AND ART. 
two to four feet of ashes, and by these means a 
high degree of temperature is preserved, which is 
essential to the proper working of these engines ; 
whereas, in the Boulton and Watt engine the cylin¬ 
der, steam-pipe, and boiler are in general left 
without any kind of covering, and in consequence 
a great loss of heat is incurred. 
Fifth . The steam-expansion valve, the equili¬ 
brium-valve, and the exhaustion-valve in the 
Cornish engine, are what are usually called 
u double-heat ” valves, and are made of large di¬ 
mensions for the’ free passage of the stream ; 
whereas, the common conical or mitre-valve, 
usually employed in the Boulton and Watt engine, 
effect only a partial opening for the steam, and 
require great force to work them. 
Sixth. In the Cornish system the equilibrium, 
valve is placed near the top of the cylinder, and 
usually by the side of the steam-exhaustion valve, 
communicates with the bottom by means of a pipe; 
whereas, in the Boulton and Watt engine, the 
equilibrium-valve is usually placed near the bot¬ 
tom of the cylinder, and communicates with the 
top by means of a pipe. There is a slight disad¬ 
vantage in this latter arrangement, but it is hardly 
worthy of notice. 
Seventh. In the Cornish engine, the air pump 
and condenser with their appendages are placed on 
the opposite arm of the balance beam to that at 
which end the cylinder is placed; whereas in a 
Boulton and Watt engine, they are placed on the 
same side, with the cylinder. There is an advan¬ 
tage in the Cornish plan, inasmuch as the effective 
stroke of the air pump piston, and the consequent 
exhaustion of the condenser, take place when the 
engine is going “ in doors ” ; consequently at the 
most favourable moment for withdrawing the con¬ 
densed steam and air contained in the condenser; 
but this effect does not take place in the Boulton 
and Watt engine, where the steam piston and air 
pump piston descend at the same moment. 
Eight. The speed of the Cornish engine seldom 
exceeds 7 or 8 strokes per minute, and is frequently I 
much less, the number of strokes being regulated 
by an apparatus called the “ cataract.” The Boul¬ 
ton and Watt engine is generally calculated to 
move at the rate of 220 to 240 feet in a minute, for 
- no other reason, apparently, than because that is the 
usual speed of a horse when walking! 
Such then are the chief points of difference 
which exist between the common form of the Boul¬ 
ton and Watt engine, and the modified and im¬ 
proved form of the Cornish high pressure expan¬ 
sive and condensing steam engine. There are some 
minor points of difference in the details of the 
working parts, blit these are not of sufficient im¬ 
portance to require alluding to in this place. 
But an important feature in the Cornish engine 
remains to be noticed. This is the boiler, the form 
and dimensions of which differ materially from the 
waggon-headed boiler of a Boulton and Watt en¬ 
gine. The Cornish boiler consists of a cylinder, 
or rather of two cylinders of great length and small 
diameter, and possesses eminently the property of 
strength resulting from its form and mode of con¬ 
struction. The furnace or fire grate is contained 
in the internal or fire tube, the bottom of which is 
placed within a few inches of the bottom of the 
boiler, and the flame and heated air make a long 
circuit in the flues, previous to passing off into the 
chimney stack. The furnace itself is of small 
dimensions, and the combustion of the fuel is 
carried on at a slow rate $ the door of the furnace r 
which is also of small dimensions is only opened 
when the fireman requires to place fresh fuel on the 
fire, so that all the air required for combustion en- 
I tors by the ash-pit. and passes through the fuel. 
The mode of firing differs also from that usually 
I adopted: as instead of throwing on a mass of coal 
1 on the top of the fire, the fuel is placed in front of 
it, and is thus ignited gradually: one advantage of 
this method of firing being that the dense black 
smoke which is usually seen issuing from a steam 
engine chimney shaft, and which represents so much 
loss of coal, is in the Cornish system entirely obvi¬ 
ated, and the whole of the products of combustion 
are made available. 
In consequence also of the internal of fire-tube 
being always surrounded with water, it is not liable 
to be burnt by the action of the fire, more particu- 
| larly as any deposit from the water falls to the bot- 
| tom of the boiler, which is not exposed to the direct 
I action of the flame. A blow-off cock situated at. 
I the bottom of the boiler gets rid of any sediment 
I that may he contained in the water. 
It is one-of the faults of cylindrical boilers that 
have large fire tubes, that the space left for steam is 
exceedingly confined. This delect, however, is ob¬ 
viated by the use of a steam-tube, or receptacle for 
the steam, being a tube placed above the boilers in 
a transverse direction. The use of this tube is ob¬ 
vious; it forms a chamber for the steam to pass into 
from the boilers, before it passes into the steam 
pipe: the steam is thus rendered more pure and dry, 
being deprived of the superabundance of aqueous 
matter with which it is necessarily saturated while 
in the boiler. 
As contrasted with the Cornish boiler, the wag- 
gon-headed boiler of Boulton and Watt’s engines, 
is inferior in most respects. Its form renders it 
notoriously weak, and unable to sustain any great 
pressure, while in consequence of the furnace be¬ 
ing placed under this boiler, its bottom is very 
liable to he burned from the deposit forming a hard 
crust on the part exposed to the greatest heat. This 
is particularly the case if the water should hold in 
solution sulphate of lime or other calcareous matter. 
It possesses, however, one advantage over the Corn¬ 
ish boiler, namely, that in the waggon-headed boiler 
there is generally a large amount of steam-room ; 
it also admits of being more readily cleansed than 
the Cornish boiler. 
Having thus, instituted a liasty comparison 
between the two descriptions of engines and 
boilers, the writer will proceed to examine the 
parts of the Cornish engine more in detail, and 
for this purpose will give a description of a large 
steam-pumping engine of this kind, erected at 
Yauxhall for the Yauxhall Water Works Com¬ 
pany, under his superintendence, in 1842. This 
engine was designed for the purpose of lifting a 
large body of water to a small height, and was 
situated on the bank of the river Thames at 
Yauxhall. 
The steam-cylinder had an internal diameter 
of 64 inches, and the piston a length of stroke 
of 11 feet 0 inches. The cylinder was sur¬ 
rounded with a jacket or casing of iron, leaving 
an intermediate space of one inch Tound the cy¬ 
linder. this space being kept constantly supplied 
with steam from the boiler by means of a sepa¬ 
rate pipe.^ In addition to this steam-jacket the 
