228 REPORT—1840. 
is precisely the condition of any casting reaching through a 
considerable depth of water at the mouth of a tidal river. The 
water is salter below than above, the part of the casting im- 
mersed therein (the lower end of a cast-iron pile for instance), 
will therefore be in an opposite electric condition to that of the 
portion above, and the amount of corrosion of the positive ele- 
ment due to the kind of iron, and the state of the water, will be 
further increased or ‘‘ exalted”’ by the negative condition of the 
opposite end, which will be itself in the same proportion pre- 
served. This principle extends to very many practical cases, 
as to iron plates, &c., partly immersed in a solvent fluid, and 
partly exposed to moist air, &c. ; and it suggests the importance 
of giving increased scantling to all castings intended to be so 
situated, to allow for this increased local destruction of material. 
158. In section (138) of my previous report, I stated the fol- 
lowing as important desiderata for experimental answers as 
touching the subject, viz., 
I. The rate of progression of corrosion in sea and fresh 
water, with reference to increasing depth. 
II. The comparative amount of corrosion in the same water 
at various temperatures, within the limits of climate 
and season. 
III. The determination of the relation between the saltness 
of water and its corroding effects on iron. 
I am now enabled to state the law governing each of these 
conditions, as deduced experimentally. 
159. And first, with regard to depth: if water held in solu- 
tion or combination the same volume of air or oxygen (not 
constitutional) at every depth, then the amount of corrosive action 
of the menstrua on iron or any other metal standing in its rela- 
tion to air and water, at any given depth, will be to the amount 
of corrosion of the same iron at any other depth, inversely as 
the depth. This supposes the water stagnant, and that fresh 
supplies of combined air are derived from the surface, and not 
from a lateral current. It also only applies to moderate 
depths ; and it is possible that at very great depths this law 
might not hold true. 
160. We do not know with any certainty whether deep 
waters, or the ocean, contain the same proportions of combined 
air at all depths; but within twenty-four feet I have not been 
able to find any difference in the volume of combined air from 
that at the surface in either sea or river water. 
161. This determination, of course, will not apply where a 
constant supply of water, holding new quantities of combined 
air, is brought in laterally, as in a tide-way or river. In this case, 
