238 REPORT—1840. 
not very far astray where no special perturbations or causes of 
corrosion supervene. 
191. This gives a period of duration of about two hundred 
years to the cast iron wharf wall, lately constructed at Black- 
wall, London, before the castings shall have become so atte- 
nuated and fragile as to be useless. 
192. Having been recently in correspondence with Col. Pas- 
ley, Royal Engineers, and Lieut. Symonds, of the same corps, 
regarding the state of the iron taken up from the wreck of the 
Royal George, with specimens of which I have been favoured by 
the former gentlemen, an opportunity has occurred, through in- 
formation for which I am indebted to Lieut. Symonds, of controll- 
ing or testing the accuracy of these results in a very decisive 
way. A number of guns have been taken up from the wreck of 
the Edgar, which were upwards of one hundred and twenty-nine 
years under water. The depth to which they were corroded 
from their original dimensions, which were known, was found 
to be seven-eighths of an inch on the average. Now let us ap- 
ply the results of our experiments to see what depth of cor- 
rosion they predict under the circumstances. Turning to Table 
No. VI., we find the average loss for hard gray iron with the 
skin removed (as in a bored gun) per square inch of surface in 
387 days = 13°55 grains in foul sea water = 1951°2 grains per 
square foot, which, multiplied by 123°4, the times 387 days is 
contained in 130 years, and divided by 7000 = the grains ina 
pound avoirdupois, give a loss by corrosion of 344 |b. per square 
foot; but the actual loss has been 32°81 Ibs. per square foot, 
that being the weight of a square foot of cast iron seven eighths 
of an inch thick. This strikingly close result is corroborated 
by others on iron from the Royal George, immersed for fifty- 
eight years, corroded from half an inch to three fourths of an 
inch in depth on the average, and together prove that the re- 
sults of these experiments may be relied on practically. 
193. I have now to make a few remarks upon the supple- 
mentary tables which accompany the five first tables of experi- 
ments, viz. those containing the results of corrosion of cast iron 
covered with various paints and varnishes. These were mostly 
such as are generally in use for such purposes, and were laid on 
with great care, so as to cover the whole surface completely, and 
leave as few microscopic pores as possible in the covering. The 
general results are these : of the ten sorts of paints or varnishes 
tried, there is not one that will completely prevent corrosion,- 
nor one that will remain perfectly adherent or undecom- 
posed for a single year under water. In foul water, fresh or 
