190 TlMEHRI. 
and thus have a clearer understanding of the condition to be fulfilled by 
the seatings. From a consideration of the statistics of boiler explosions 
that have taken place of recent years it will be seen that the four 
principal causes from which boilers give way, and the proportion of 
these causes of failures, are 1st. 49 per cent, to defective condition ; 
2nd. 18 per cent, to shortness of water, about 10 per cent, of these have 
defective fusible plugs ; 3rd. 25 per cent, to malconstruction ; and 4th, 8 
per cent, from excessive pressure. Defective condition embraces 
corrosion in various forms ; due to chemical action when ordinary rust 
is formed ; to galvanic action when the stay heads are eaten out, or 
when pit holes are formed in the tubes, or plates ; it is mechanical 
action when grooves are formed by the alternate bulging, and straining, 
assisted by any acidity that may be present in the water, this may be 
introduced with the condensed water from the engine cylinders, or the 
overflow from the vacuum pumps mixing with the trench water with 
what the boilers are fed, especially when this water is nearly stagnant 
bush water. Sweets finding their way into feed water, although this 
has an anti-scaling effect in cases using feed water impregnated with 
lime, it is not to be tolerated in Demerara where our water is free from 
lime. Stagnant bush water particularly when mixed with sea water is 
disastrously active on either iron or steel. Fortunately some of their 
chemical actions can be counteracted by the use of neutralising agents 
as it is never of such a form, and appearance, to escape the notice of 
the experienced eye, although the plates are sometimes so generally and 
evenly wasted as to escape the notice of an unskilled, or indifferent 
attendant. Boilers standing and not working should be cleaned out, and 
filled with pure water containing a good amount of soda dissolved in it. 
Some few months since I had occasion to condemn a boiler rendered 
dangerous through this even corrosion, indeed so even that when the 
boiler was rolled out of its seating to an ordinary eye it seemed perfect 
but a few blows with the hand-hammer revealed the fact that corrosion 
over the bottom had reduced the plates from half an inch to an eighth, 
and in some places only a sixteenth ; it was the seam that particularly 
told an unmistakable tale ; this peculiar corrosion is generally found to 
be most severe on heat giving surfaces, such as bottoms of externally 
fired boilers and furnaces of internally fired boiler. It is often 
caused by a peculiar class of mine water, and by most of 
our peaty waters here, both of which sometimes contain free 
oxygen, which attacks the plates readily. This particular feed 
