150 Transactions. 
of engine. By mechanical means its action can be greatly retarded and kept 
within safe limits, but I do not think there is at present any known remedy 
for its perfect prevention. 
In using surface condensers the same water is being continually converted 
into steam and reconverted into water. Has this anything to do with lifting 
the water above its true level? Is there a large, or any, portion of the air 
extracted with this continual distillation ? 
Secondly.—The effect of using surface condensed water and high-pressure 
steam in the boiler is to destroy the plates of the boiler, either by galvanic 
action or from some electrical influence. I am inclined to believe more in the 
former because we have the brass tubes of the condenser and the copper pipes 
forming the negative pole, and the boiler and hot well forming the positive 
pole, the sea water circulating in the condenser and used to supplement the 
feed, forming a saline solution as a medium. On leaving out the zinc plates 
for a few trips, streaks of black oxide of iron were discovered about the 
superheater, and other parts in the boiler, especially where the greatest heat 
was. The superheater was cleaned and painted, and zine plates replaced in 
the water space of the boiler. Since then no injurious effects have taken 
place. 
Another strange phenomenon is the deposition of a calcareous substance 
thrown against the top of the shell of the boiler as if one were to take 
a handful of mortar and throw it against the wall, but these deposits only 
require removing about once in two months. Since leaving off the use of — 
tallow for lubricating the cylinders these deposits on the upper part of the 
shell are scarcely noticeable. 
I must not forget to mention another important matter in reference to the 
preservation of the boiler, especially where exposed to the action of the steam, 
viz.,—the application of Portland cement, put on in the same way as white- 
wash ; it is the best preservative that I am acquainted with, and I am 
indebted to Mr. James Stewart, C.E., for this hint. 
In conclusion I may state that some persons imagine that compound 
engines are complicated. This is not so; neither is there any difficulty in 
starting or stopping them. When we find boats of 3,000 tons steaming 10 
knots on a consumption of 18 tons per 24 hours it speaks well for this class 
of engine. The “ Adriatic,” the largest steamer afloat next to the “ Great 
Eastern,” has compound engines, and has just made the quickest trip across 
the Atlantic on record. There is nothing whatever to prevent any of the 
steamers running on the New Zealand coast from being compounded, with 
results equal to those here stated, and it is also satisfactory to know that 
there are special facilities for converting them in Auckland. 
