AOT) A SELECTION EEOM THE OBSEEVATIONS MADE WITH THEM. 685 
free from the defects of the one already finished. We had satisfied ourselves that the 
fissures were owing to our having employed the bed of hoop-iron in the state in which it 
was when taken from the turning-lathe. The surface, though nicely turned, was not as 
smooth as the surface of a solid disc would have been : a slight yielding at the edges of 
the hoop-iron, and a slight spreading under the pressure of the tool, had produced little 
irregularities; and although the surface had been carefully “black-washed,” the specu- 
lum-metal had encountered too much friction in the act of contracting after it had 
become nearly sohd, and thus had been filled with superficial rents. On the first occa- 
sion there had been no fissures, but the bed of hoop-iron had been ground ; the remedy 
was therefore obvious. 
The third speculum was successfully cast ; but on opening a small aperture, and look- 
ing into the oven before it was quite cold, it was observed that the speculum was cracked 
through the middle. The temperature of the speculum was found not to be quite uni- 
form ; and that chcumstance, taken in connexion with the direction of the crack, seemed 
to point out the cause : the ends of the oven, from want of room, had been made thinner 
than the sides. The first speculum had probably been strained by the same cause, and 
rendered more fragile. 
The oven being ready, an attempt was made to cast a fourth speculum, which failed. 
We had each time, before the bed of hoop-iron became cold, saturated it with tallow to 
prevent the formation of rust between the hoops, which would have rendered the sur- 
face impervious to air; but just before it was again employed it was made red-hot, and 
the tallow burned out. On this occasion, by an oversight, the bed of hoop-iron had 
not been sufficiently heated, and there remained some of the tallow unconsumed, which, 
being vaporized in large quantities, produced an ebullition which made the casting as 
porous as pumice-stone. This speculum, of course, was not annealed, and the following 
day it was in small fragments. 
The fifth speculum, being in every respect a perfect casting, without the slightest 
blemish and of a proper cm’vature, was ground and polished in about a month. It is 
desirable that the bed of hoop-iron when the metal is poured should be warm, so as to 
prevent the possible deposition of moisture; but if much hotter than this, it at once 
dries up the sand, and it is difficult to make the mould secure. In the whole of the 
operation I have described, one of the difficulties is to time each stage. If the mould 
was prepared too long, the bed of hoop-iron might become cold and damp ; on the other 
hand, if the mould was not ready when required, it might be hazardous to keep the 
crucibles so long at the pouring-heat. It may, perhaps, be as well to add that the 
crucibles, when in the pouring-gimbals, require to be thoroughly skimmed, as particles of 
coal falling upon the hoop-iron would be immediately entangled in metal not rising to 
the top : the skimming should be done promptly, lest the metal fall below the proper tem- 
perature. Any considerable delay in drawing the speculum, when solid, into the anneal- 
ing-oven would be fatal; therefore there should be ample capstan power to overcome 
the difficulty which usually is experienced in detaching the speculum from the mould. 
