192 TlMEHRI. 
or any other causes. This is often aggravated and concealed by the 
brick work of boilers improperly seated, such as inaccessible 
flues, broad seatings &c. , this latter is perhaps responsible for more ex- 
pensive repairs, annoying stoppages, and disastrous explosions, than 
any other defe6t, although it is not one of the most difficulty to discover 
when boilers are properly seated ; the effect of this bad style of seating is 
invariably aggravated by being faced with lime, instead of mud, or fire- 
clay. I met with a most interesting example of this kind of corrosion on a 
multitubular boiler at Columbia, which, if properly seated would doubt, 
less have been in good condition whereas from the forementioned defects' 
the plates along the side flue seating cover, and round the front wall were 
reduced in thickness from about \ an inch to an £ and in many cases 
less the -^ of an inch thick by corrosion which was never noticed, in- 
deed it scarcely could be noticed when the flues were so small that they 
could not be entered, and could only be cleaned by using a long rake ; 
otherwise the boiler was in such good condition, and of such splendid 
material, that the §300 spent in rectifying the mischief done has proved 
to be money well invested. 
External corrosion is often rapid but rarely so rapid as to become 
dangerous in twelve months. 
It is more or less severe according to the conditions it is working under ; 
for instance, if there be a leakage in a seam where the seatings are 
broad, and where the plates are continually moist, oxidation goes on 
very rapidly, particularly if the fuel gives off sulphurous fumes, or other 
acid gases which may become condensed, and so increase the corrosive 
action going on. 
Dampness in flues may be due to many other causes than leakage not 
attended to ; natural dampness in foundations ; the flues being lower than 
the surrounding yard floor; flues not being drained, and other causes ; 
not the least of which is the condensation of the steam given off by the 
screen megass : indeed this latter cause has been found to give rise to 
corrosion on what were looked upon as perfectly dry seatings with no 
trace of leakage. Fortunately this corrosion is most conspicuous in low 
pressure boilers, whose temperature is little above that of 220 degrees 
F., whereas the freedom of high pressure boilers from this oxidation 
may be attributed to the fact of their higher temperature ; this also is 
one of the many reasons why a boiler should not be emptied whilst 
under pressure, but allowed to cool down, to a few pounds steam pressure, 
thus materially helping to evaporate the damp collected, and absorbed 
