LORD OXMANTOWN ON THE REFLECTING TELESCOPE. 
509 
intermediate brass chiseled out, as represented in the figure. They were then filled 
with melted brass, which was easily effected by imbedding the whole in sand ; clear- 
ing the sand completely out of the spaces, and pouring the brass into each through a 
hole in a flat surface of sand packed in a small flask ; when the brass cooled, the con- 
traction was sufficient to draw the joint firmly together. This process may, possibly 
in engineering, prove useful as a means of connecting large masses of metal : I am 
not aware of its having before been put in practice. Conceiving that any further 
yielding in the joints was now impossible, the speculum was again placed on the 
lathe and turned to a radius of fifty-four feet, and new plates of speculum metal pre- 
pared for it. 
Notwithstanding the small dimensions of these plates, none exceeding nine inches 
square, more difficulty was originally experienced in casting them than could have 
been anticipated. A great many unsuccessful attempts were made to cast them in 
sand, according to the directions of Mr. Edwards and others ; they were seldom free 
from flaws, and, although cooled very slowly in an oven, they were extremely brittle, 
sometimes flying in pieces the moment they were touched, and generally breaking in 
the attempt to heat them again for soldering. The cause of this brittleness was 
evident, as the broken parts when replaced no longer fitted exactly, the metal there- 
fore had been in a state of tension, owing no doubt to the edges of the plates beco- 
ming solid sooner than the centre. 
The next plan tried was this : a number of equidistant thin plates of iron were im- 
mersed in a square crucible of cast iron filled with fluid speculum metal, so as to di- 
vide the whole mass into plates of equal thickness: this failed altogether, the plates 
of speculum metal were full of flaws; their contraction had been prevented by the 
unavoidable irregularities of the plates of iron, and crucible. Another plan was then 
tried, more successful than any of the preceding. A circular sawing machine was con- 
structed, the blades were of soft iron, and while revolving were always partially im- 
mersed in emery and water; with this a block of speculum metal was without much 
difficulty cut into plates, which were perfectly free from flaws, and not liable to crack. 
Still their texture was not uniform ; for about three quarters of an inch from their 
edges the arrangement of the particles was different from what it was in the re- 
mainder; so much so, that the edge of the plates for that distance evidently resisted 
the action of emery more than the remainder, and therefore probably a speculum 
made of such plates would not eventually be as true as if they were free from that 
defect. Upon this point, however, I cannot speak quite decisively ; from what I have 
observed I have strong reasons to think it is so ; but were it of any importance to 
determine it, farther experiments would be necessary. I have a speculum two feet 
aperture formed of such plates ; it was however polished long ago, and I have not a 
polishing tool of the construction I now make use of, of the proper size ; so it has 
not had a fair trial. The same plates were also originally used for the three-feet spe- 
culum I am describing, but when it became necessary further to secu'-p the joints. 
