304 
ME. W. LASSELL ON POLISHING THE 
I was obliged then to use some other arrangement or modification. I was not able to 
alter my machine and carry out my experiments fully before my return home, and it is 
only of late that I have had leisure again to return to the subject. But I have succeeded 
so perfectly and completely, even beyond my hopes, and by processes so simple, so 
certain, and so pleasurable, that I am desirous to place on record and before the world 
the means by which this has been accomplished. 
In the machine I am about to describe, those familiar with the subject will probably 
recognize little that is new , for I have not hesitated to adopt parts of other machines 
that have been contrived, and rearrange or simplify them as I thought best for the 
required result. In describing, however, this new machine, I am desirous not to say 
any thing in disparagement of that which I invented many years ago (above referred to) ; 
for with that machine, especially since I applied to it the elegant improvement of a 
train of wheels for producing uniform axial motion of the polisher (a condition I had 
indeed attempted to secure by less efficient means) invented by Mr. De La Hue, I have 
produced many surfaces, on various specula up to 12 inches diameter, which I have never 
been able to surpass, and which are indeed so perfect that I cannot discover in them 
any error whatever. Still I have found it difficult, though not impossible, to use 
Mr. De La Bue’s train for specula as large as 24 inches diameter, the strain on the 
wheels (being levers of the third kind) endangering the teeth. It was in applying this 
arrangement to polishing the four-foot that, although I had purposely had the wheel 
and pinion on which is the greatest strain made of cast steel, the machine broke down, 
and I was obliged to give up its use. 
Description of the more recently constructed Machine. 
Throughout the several figures the same letters generally indicate the same parts of 
the machine. Those figures which represent that part of the machinery supporting the 
speculum are on a scale of 1^ inch to the foot, or one eighth the full size. 
Plate 50. fig. 1 represents a firm support of wood or masonry for the cast-iron frame B 
of fig. 2, to which all this part of the machinery is fixed ; and it will be seen that there 
is provision made for attaching it to a wall ; but that method is not so convenient as 
placing it on a firm and independent base. The form of the plate and bracket B will 
be understood from the several views of it in figures 1 to 4. On its upper surface 
are two Y-shaped planed grooves shown at B l . On this travels a cast-iron plate, C, with 
V-shaped ribs fitting the grooves of the frame B, and, depending from its centre, is cast 
a hollow tube accurately bored inside (C 2 , figs. 1 & 2). There is also a boss cast on its 
under surface (C 3 , fig. 2) to afford a firmer support to the axis of the pinion (or wheel) 
of 26 teeth working through it. In the upper and lower plates of frame B are two 
wide recesses or grooves shown in plan at B 2 , fig. 4. These allow the downward pro- 
jecting tube of plate C to pass backward and forward within certain limits, as the 
plate C travels along the V-shaped grooves. Fig. 5 shows a turned shaft fitting the 
bored tube of plate C with two toothed wheels keyed upon it, the upper wheel having 
