510 
LORD OXMANTOWN ON THE REFLECTING TELESCOPE. 
they were taken off and spoiled, and in the meantime another process suggested itself, 
which seemed to be decidedly preferable. 
It was evident that the flaws of so frequent occurrence in the plates formerly cast, 
and also their extreme brittleness, arose from the contraction of the metal in some 
places more than others, just at the time of transition from the fluid to the solid state. 
The edge of the plates always became solid first, and the central portions thus pre- 
vented from contracting were strained when no longer ductile. Were it possible, 
therefore, to satisfy the following conditions, viz. that heat should be abstracted 
rapidly and equally from the lower surface of a fluid disc of speculum metal, so that 
it should solidify from the bottom upwards in strata, or rather infinitely thin laminae, 
the surface being the last to solidify, we should have a perfect casting; for the par- 
ticles in that case being deposited, not uniformly, indeed, owing to the unknown ac- 
tion of the forces of crystallization, but in such a way as to fill up the interstices, 
there would be no flaws ; and the temperature being uniform in a horizontal direc- 
tion, and in the vertical varying in regular gradation from the lower surface to the" 
upper, there would be no strain. 
This, I believe, is the true principle upon which the most perfect castings can be 
obtained : its truth has been fully proved by practice ; and, although in the arts for- 
tunately it is not necessary often to attend to such minutiae, as the materials em- 
ployed are in some measure ductile, and therefore adapt themselves to the unequal 
strain to which they may be subjected, still it seems probable that the not unfrequent 
failure of large castings under a pressure much less than they were calculated to 
bear is due to this cause. The management of speculum metal may be regarded as 
an extreme case, where all the defects of manipulation are strikingly developed. 
There are evidently two ways in which it might be possible to attain the required 
adjustment of temperature; the one by cooling the lower surface of a mould contain- 
ing the liquid speculum metal, while the heat of the upper remained undiminished ; 
the other by constructing the mould itself, so that the lower surface should absorb 
the heat rapidly and the upper retain it. Both were tried ; the first by making the 
mould itself of cast iron, in which the metal was fused, and then exposing its lower 
surface to the action of a jet of cold water; the result justified the theory, but the 
mould very frequently cracked, and where this occurred before the speculum metal 
had become perfectly solid, the casting was spoiled ; discs were, however, obtained of 
different sizes, the largest eighteen inches diameter, which was merely by chance, as 
a mould of that size almost invariably cracks before the completion of the process. 
The experiment, therefore, is not worth repeating, particularly as the other plan is 
simple and succeeds perfectly. It is obviously to make the lower surface of the 
mould of iron, and the upper sides of sand ; at first a simple disc of iron was tried, 
but although the castings were sound, there was this defect; that bubbles of air were 
often entangled between the iron and speculum metal, producing cavities which it 
was tedious to grind out : the iron disc was therefore replaced by one made of pieces 
