Report of Society's Meetings. 191 
water is generally so pure that little or no sediment is seen. As a 
preventative against the ravages of this enemy a few days feed from 
water charged with calcium sulphate will give a coating of scale which 
will for a time proteft the plates, this coating been renewed as required. 
It will therefore be seen that the non mineral feed water in this colony 
is not always an unmixed blessing. Whenever this peculiar corrosion 
is seen the only way to gauge depth, and extent, is by the use of either 
the hammer, or drill, as other modes of gauging and sounding are 
often misleading. 
Corrosion is also seen in a commoner, but perhaps not so formidable a 
type being generally found in fittings, and scattered patches over 
different parts of the boiler in which it is much more conspicuous, and 
has a more dangerous appearance, consequently it has a greater 
tendency to intimidate owners into having it attended to, the somewhat 
erratic forms taken by this particular corrosion are very difficult to 
account for. 
It is occasionally found deepest in the coldest plate, which leads us 
to think it is the cold plates, such as shell bottom plates immediately 
over seating blocks, and other cold places where the corrosion is 
effefted, it also invariably attacks the back end plates of multitubular 
boilers just above water line and at the tube ends particularly at the 
front, but sometimes at the back of the boiler, but seldom both ends in 
the same boiler. 
In other cases it will be found to most seriously affe6t the heat giving 
surfaces ; the most conspicuous case of this I ever met with was on 
L'Union estate, where the intensity of the fire, direction of the draught, 
and the position of the fire bridge, could readily be determined from the 
water side of the flue; the corrosion in this case also attacked the tops of 
the galloway tubes. I may mention that the flues of this boiler had no 
provision for expansion, consequently the mechanical aftion formed 
grooves, and cracks, some 3 feet in length, and for the whole depth of the 
angle iron connecting the two tubes to the boiler end. It is fortunate, 
however, that wherever these a6tive corrosive agents are at work their 
mischievous results are easily detected, for where corrosion is very 
aftive there will be no hard tenacious scale to conceal it, from the simple 
fa£t that so long as the iron is being oxidised, or dissolved as it were, the 
sediment will shell off with the oxide. 
Leaving internal corrosion, we meet with a worse enemy in the form 
of an external corrosion set up by dampness in the flues due to leakage 
