116 



the same curvature. This process required several weeks, 

 and it was then ground smooth by a frame filled with con- 

 cave blocks of sandstone. The bed of hoops so prepared 

 being set exactly level, and heated sufficiently to blue its sur- 

 face (for the purpose of burning out the tallow with which its 

 interstices are filled, when not in use, to protect it from oxida- 

 tion), the wooden pattern of the speculum was placed on it, 

 and founders' sand rammed round it to its top. The mould 

 thus formed was about a foot deep, the thickness of the spe- 

 culum, 5^ inches in this instance, being determined by the 

 quantity of metal melted. By this arrangement, as Dr. Ro- 

 binson formerly explained to the Academy, the fluid metal 

 which comes in contact with the hoops is chilled at once into 

 a dense sheet about half an inch thick; the air which might 

 be entangled with it in pouring, escaping through their inter- 

 stices. The circumference sets much more slowly in conse- 

 quence of the inferior conducting power of the sand ; and the 

 upper surface, which is only in contact with air, remains so 

 long fluid, that the greatest part of the shrinkage occurs 

 there ; its tendency to crack the cast is prevented, and the 

 coarse structure which it produces is confined to a place 

 where it is unimportant. 



On this occasion, besides the engrossing importance of the 

 operation, its singular and sublime beauty can never be for- 

 gotten by those who were so fortunate as to be present. 

 Above, the sky, crowded with stars and illuminated by a most 

 brilliant moon, seemed to look down auspiciously on their 

 work. Below, the furnaces poured out huge columns of 

 nearly monochromatic yellow flame, and the ignited crucibles, 

 during their passage through the air were fountains of red 

 light, producing on the towers of the castle and the foliage of 

 the trees, such accidents of colour and shade as might almost 

 transport fancy to the planets of a contrasted double star. 

 Nor was the perfect order and arrangement of every thing 

 less striking : each possible contingency had been foreseen, 



