270 REPORT—1840. 
I have heard) his specific gravities were taken by weighing 
equal bulks ; cubes, in fact, cut from the mass of cast iron by 
the chisel and file, a method in itself not susceptible of much 
accuracy, but rendered much more liable to error from the 
liability to variable condensation of volume of the iron in the 
processes of chipping and filing; a rough crystalline broken 
surface effectually prevents an exact specific gravity being taken 
of cast iron by the usual method of weighing such a specimen 
suspended in water; and no cutting out of the specimen for 
weighing by any method is allowable, except by the lathe or 
planing-machine, which operate so quietly, that no condensa- 
tion of volume is likely to take place. 
273. Dr. Thompson’s results give the specific gravity of hot- 
blast iron greater than that of cold-blast. Mr. Fairbairn’s, on 
the contrary, give the specific gravity of cold-blast iron as the 
greater, and to the latter conclusion my own results tend. I have 
entire confidence in the correctness of the specific gravities I 
have given, from the method and precautions taken, and the ac- 
curacy of the instrument used in the weighings—a balance of 
Troughton’s construction, readily sensible, when loaded, to the 
third decimal place. 
274. A correct knowledge of the specific gravities of cast iron 
is important in several respects to the engineer, but most of all 
so from the fact, that Messrs. Fairbairn’s and Hodgkinson’s ex- 
periments on the strength of hot- and cold-blast iron seem to in- 
dicate that the ultimate strength of cast iron is in the ratio of 
some function of the specific gravity, a view more recently also 
confirmed by Mr. Richard Evans’s experiments on the strength 
of anthracite pig-iron. 
Now the conditions rendering the specific gravity of the same 
cast iron variable, are 
I. The bulk of the casting. 
II. The depth or head of metal under which it has been cast. 
III. The temperature at which the iron has been “ poured,” or 
run into the mould. 
IV. The rate at which the casting has cooled. 
The determination of the law governing the change in each 
of these cases is a work of some labour and difficulty, which has 
been partly attempted. 
275. In Table XII. the results are given of the experi- 
ments I have made on Scotch, Welsh, and Staffordshire cast 
irons, showing the increase of density produced in large cast- 
ings at every two feet in depth, down to fourteen feet in depth 
of casting. These experiments were made on pieces cut at every 
two feet from a shaft or cylinder of four inches in diameter, cast 
