ON THE ACTION OF AIR AND WATER UPON IRON. 3 
The barometric pressures— 
fn Dublin ou a 30849 Annual 
In London. . . . ~~. . - 29°880 f mean pressures* 
All other circumstances being the same, the rate of corrosion of iron exposed 
to the ordinary atmospheric influences may be expected to vary in increase 
or decrease thus :— 
Ist. Directly as the volume of rain and dew falling on it in a given time, 
these fluids being supposed to contain similar amounts of combined air 
and free oxygen. 
Ind. Directly as the elevation of temperature with equal moisture. 
3rd. Directly as the barometer pressure. 
The two last do not vary enough in our climate to produce very marked 
results, and probably the volume of rain and dew in a given time will be a 
tolerably exact measure of corrosion in any part of Great Britain. 
The rate of corrosion will be rather greater in a crowded city-(ceteris pa- 
ribus), and greater over the sea than in the open country, the latter, owing to 
the presence of saline particles frequently in the air. 
The series of tables is therefore now complete, and I would venture to hope, 
present to the engineer sufficient data to enable him to predict the term of 
durability and allow for the loss by corrosion of iron in all conditions when 
entering into his structures. 
Their completion has involved no slight labour, having required more than 
five thousand accurate weighings to be made, without reference to other ex- 
periments. 
287. In all the tables which follow and relate to the second period of im- 
mersion of cast iron and the standard wrought iron bar, viz. Tables I., III., V., 
and VII., the dimensions and weight of each specimen are given previous to im- 
mersion, its weight after the second exposure to corrosion, and the loss of metal 
for a unit of surface ; and by comparing this in every case with the results given 
in column 10 of tables of second Report, having regard to the difference in 
absolute time of exposure in the first and second periods, the results given in 
columns 8 and 9 of this Report have been found, viz. the amounts of incre- 
ment or decrement of corrosion of the same surface of the same iron when 
exposed at the first and second periods. Of course this information does not 
apply to the tables of wrought iron immersed for the first time only, The 
other information conveyed will be sufficiently obvious from the headings of 
the columns. The amount of corrosion of all the wrought iron and steel 
have been referred to the standard bar a 58, so that the whole suite of tables 
are comparable. The characters of corrosion of these have also been given 
as minutely as brevity permitted, and in a few set words throughout. 

Discussing the results given in these tables, we are enabled to draw the 
following conclusions as regards, first, 
Cast Iron. 
288. The rate of corrosion is a decreasing one, at least where the coat of 
plumbago and rust first formed has been removed prior to second immersion, 
’ which was unavoidable in these experiments; but, as I shall hereafter show, 
where this coating remains untouched, the rate of corrosion remains much 
more nearly uniform, and is nearly proportionate to the time of reaction in 
given conditions. In some cases, however, even with this coating removed, 
an increment in the rate of corrosion has taken place; and it is observable 
* It is uncertain that the pressures given are strictly the ‘Annual means,” the Dublin 
Registers are incompletely reduced. 
BQ 
