692 EAEL or EOSSE ON THE CONSTEHCTION OE SPECULA OF 6-FEET APEETUEE, 
curved rule about 6 inches long. A pair of these little rules were made from calculated 
ordinates, and ground together. In adjusting the gauges to the cm’ve so traced, nothing 
was employed but the file and scraper. The gauges were then slightly ground together 
ndth the finest emery and in very small quantity, and care was taken to distribute it 
evenly with a camel’s-hair pencil. The grinding-tool was from time to time adjusted 
roughly to the curvature on a turning-lathe, which was accomplished with great facility, 
as the slide-rest was governed by a guide of the same curvature as the gauge, and there 
were adjusting-screws in the face-plate, by which the tool was made to run perfectly true 
each time it was replaced. When the curvature of the speculum was nearly exact, the 
remaining little changes in the radius of the tool were made with the file. We had no 
means of optically measuring the focus of the 6 -feet mirrors while on the engine ; there- 
fore further precautions were taken. A brass wedge was made, about 3 inches long, at 
one end 5 -^th less than the vers sine of the circle of curvature, and at the other end 
■j-^-oth greater. This wedge was cut into three parts, equal, greater, and less than the 
vers sine. A straight edge was then laid upon the speculum ; and it was considered per- 
fect when one of these just touched the straight edge, another passed under it without 
touching, and the third did not pass at all. So accurately was the adjustment as to 
focus made in this way, that neither of the specula differed from the calculated focus 
more than inch. 
The machinery is precisely on the same principle as that which we employ in working 
3-feet specula, already described in the ‘ Transactions.’ Instead of belts there are chains, 
workmg in V grooves. The driving-wheels are of wood, in several layers, the grain 
being disposed radially. The chains are made tight by straining-pulleys, and act per- 
fectly. This species of machinery is neither compact nor elegant : when originally 
designed, its principal recommendation was facility of construction and facility of altera- 
tion, both important qualities where it was doubtful whether machinery unguided by 
hand, acting independently of the sense of touch, would answer at all. Some are sur- 
prised that machinery so rude should be employed, and successfully, in a mechanical 
operation where the utmost precision is required, a precision almost fabulous, and they 
compare it with the beautiful machinery in the mills where textile fabrics are made : 
but in figuring specula everything depends upon the principle ; and so long as certain 
motions are communicated to the tool and speculum, machinery can do no more. The 
tool is raised and lowered by a screw passing through a carriage which moves upon a 
railway over-head upon the principle of the travelling crane, and the same mechanical 
arrangement removes the speculum with its lever supports from the grinding-machine 
to the truck upon which it is conveyed to the telescope. The screw is obviously the 
best mechanical power to employ, as its action begins and ends slowly, and there is 
therefore less danger of breaking the speculum. As a further precaution in raising 
or lowering the grinding-tool, thin wooden wedges between the tool and speculum 
are gradually introduced and withdravm. In the final adjustment of the speculum 
to focus, the operation is much facilitated by a judicious management of the second 
