508 
LORD OXMANTOWN ON THE REFLECTING TELESCOPE. 
on of the plates of speculum metal ; fig - . 5. the speculum complete, faced with sixteen 
plates of speculum metal. The whole depth of the brass work was five inches and a 
half, and weight about four hundred and fifty pounds ; the sides I B (fig. 2.) were made 
square and true, and then tinned, and the whole bolted together with iron bolts, as re- 
presented in the figure ; the temperature was then raised till the tin was in fusion, 
and the bolts tightened to the utmost, so as to make close joints ; in this state the 
speculum was turned, plated with speculum metal, and polished; but it was found 
that the joints had not been sufficiently secured, owing to which the bracing had not 
produced its proper effect, and flexure was quite perceptible by its effects in the dif- 
ferent positions of the telescope ; on a close examination, there was reason to suspect 
that the solder was not everywhere perfect, though it had been so originally ; that in 
the rough operation of bolting to the lathe and turning, it had been in some places 
detached, and had not subsequently united, though it had necessarily been fused 
when the plates of speculum metal were soldered on: it was evident, therefore, that 
tin alone was not to be depended upon in putting together a speculum of this size. 
The iron bolts, of course, contracting less than the brass, had added nothing to the 
security of the joints ; brass bolts had been tried formerly in constructing a speculum 
two feet diameter, but they were not strong enough to bring the joints close, and 
were replaced by iron ones. 
The plates of speculum metal were, therefore, taken off, and the joints secured in 
the following manner. The whole mass of brass work was imbedded in casting sand 
everywhere in contact with it; over it there were about three inches of sand. The 
sand was then removed from the centre, so as to expose a circular surface of about 
one inch and a half in diameter, and about forty-five pounds of very hot melted brass, 
in the same proportions as the brass speculum, were poured upon it in a continued 
stream from a perpendicular height of about ten inches : the sand having been so 
arranged, that after the melted brass had reached the depth of two inches, the re- 
mainder continued to flow off; the melted brass as it began to cool was therefore 
continually replaced with hot brass ; and from the height at which it was poured, it 
was necessarily in immediate contact with the surface of the cold metal, and soon 
completely fused it, and perfectly united it in that place. This was repeated in thirty- 
four different places, marked by dots in the figures: it was also tried in the places 
marked c, c, (fig. 3.) but failed. When the redundant brass had been removed with a saw, 
and the surface made smooth, a slight hair crack was perceptible at the boundary of 
the fused metal, proceeding no doubt from expansion and contraction after the brass 
had ceased to be fluid, but before it had become ductile. This process is made use 
of by the brass-founder in stopping holes in defective castings; he calls it burning. 
The failure of this process at the places marked c, c, is a fact which it would be im- 
portant to keep in view in disposing of the parts of a large speculum. As it was de- 
sirable that some further attempt should be made to secure the joint at c, c, holes of 
about one inch and a half in diameter were bored completely through at c, and the 
