268 REPORT—1840. 
is yet in our power greatly to retard the corrosion of the harder 
iron at the expense of the softer, so that the engineer is thus 
given a principle of guidance in the combination of different 
*‘makes’’ or sorts of cast iron in the same structure, when it 
may be desirable partially to preserve some parts at the expense 
of others of less structural importance*. Instances of such 
cases, and of the applicability of the principle here given, will 
at once occur to practical men. 
269. The engineer of observant habit will soon have per- 
ceived, that in exposed works in iron, equality of section or 
scantling, in all parts sustaining equal strain, is far from in- 
suring equal passive power of permanent resistance, unless, in 
addition to a general allowance for loss of substance by corro- 
sion, this latter element be so provided for, that it shall be 
equally balanced over the whole structure; or, if not, shall be 
compelled to confine itself to portions of the general structure, 
which may lose substance without injuring its stability. 
The principles we have already established sufficiently guide 
us in the modes of effecting this; regard must not only be had 
to the contact of dissimilar metalst, or of the same in dis- 
similar fluids}, but to the scantling of the casting and of its 
parts§, and to the contact of cast iron with wrought iron or 
steel, or of one sort of cast iron with another||. Thus, in a 
suspension bridge, if the links of the chains be hammered, and 
the pins rolled, the latter, where equally exposed, will be eaten 
away long before the former. In marine steam-boilers, the 
rivets are hardened by hammering until cold; the plates, there- 
fore, are corroded through round the rivets before these suffer 
sensibly, and in the air-pumps and condensors of engines work- 
ing with sea water, or in pit vork, and pumps lifting mineral - 
ized or “bad”? water from mines, the cast iron perishes first 
round the holes through which wrought-iron bolts, &e. are in- 
serted. And abundant other instances might be given, showing 
that the effects here spoken of are in practical operation to an 
extent that should press the means of counteracting them on 
the attention of the engineer. 
270. I have not yet been enabled to extend this part of the 
inquiry to fresh water, but have reason to suppose it would not 
be in such case attended with equally striking results from 
facts before stated with respect to zinc and iron in contact in 
fresh water ; the same forces, however, still will operate with 
like results, only differing in degree. It seems not improbable, 
* Report, § 134. T § 244—261. Pg War 
§ Sect. 179—183. i| § 263—268. 
