LORD OXMANTOWN ON THE REFLECTING TELESCOPE. 
511 
of hoop-iron placed side by side, with their edges up, tightly packed in an iron 
frame: the edges were brought to a smooth surface of the proper curve either by the 
file or lathe, whichever was the most convenient. A metallic surface was thus con- 
structed everywhere porous ; as however close the hoop-iron had been packed, the 
interstices suffered air to pass freely through. So successful was this expedient, that 
of sixteen plates cast for the three-feet speculum, not one was defective : the following 
particulars require to be attended to. The disc of hoop-iron should be as thick as 
the speculum to be cast upon it, so as to cool it with sufficient rapidity ; it requires 
to be warm, so that there may be no moisture deposited upon it from the sand ; it 
may be heated to 212° without materially lessening its cooling power. The metal 
should enter the mould by the side, as is usual in iron founding, but much quicker, 
almost instantaneously ; one second is sufficient for filling the mould of a nine-inch 
plate or speculum. As to the temperature of the metal, this can best be ascertained 
by stirring it with a wooden pole occasionally after it has become perfectly fluid ; 
when the carbon of the pole reduces the oxide on the surface of the metal, rendering 
it brilliant like quicksilver, the heat is sufficient. When the metal has become solid 
in the ingate or hole through which it enters the mould, the plate is to be removed 
quickly to an oven heated a little below redness, to remain till cold, which, where the 
plates are nine inches diameter, should be three or four days at least. 
The metal which had filled the ingates having been separated from the plates by a 
file, they were fitted to the brass speculum by grinding each separately by hand upon 
it till brought to the same curve. The surface of the brass speculum was then scraped 
and tinned, and when cold all resin was removed by washing it first with spirits of 
turpentine, and then soap and water. The lower surfaces of the plates which fitted 
the brass speculum, were carefully scraped all over with broad flat chisels of thin 
steel, perfectly hard ; it is necessary that no spot should be passed over unscraped, 
and that the plate should be kept perfectly dry till soldered to the speculum. The 
plates were then arranged in their places on the brass speculum, which had been 
previously placed in an oven so constructed that the temperature could be gradually 
and equally raised. The bottom of the oven consisted of a cast-iron plate four feet 
diameter, set in brick work, as if it was intended for a sand bath ; the brass speculum 
rested upon this, with loose bricks between to protect it from radiation, so that it 
should acquire heat solely from the heated air ; the top of the oven had a cover in 
four pieces, that one might be opened at a time. In about eight hours the tin on the 
speculum was fused, and then melted resin was poured in between the plates ; tin in 
fusion was also applied in the same manner, and the plates moved a little backwards 
and forwards. As soon as from the aspect of the edges of the plates it was certain 
that the tin was acting on the speculum metal, the fire was almost all withdrawn, and 
the temperature was not suffered to rise higher. The joints of the plates were now 
made straight, and kept open about one-twentieth of an inch by chips of wood ; the 
whole was then suffered to cool gradually, and in five days it was ready to be ground. 
