IOWA ACADEMY OF SCIENCES. 
193 
glass to one side. When as round as it is possible to cut it 
with a glass cutter fasten it to a small face plate of a lathe 
by means of little soft wax balls. A wooden face plate 
held in a chuck will serve the purpose. For the cutting 
instrument use a sheet-iron strip some two inches wide, 
and of a length a little more than the circumference of the 
disk when finished. Bend it into a circular form, and after 
having put emery paste on the inner side, hold it, by means 
of the hands, firmly around the circumference of the rapidly 
revolving disk. The emery paste is made by mixing emery 
powder with water. A course emery should be used at 
first, and the final cutting should be done with fine emery. 
The complete apparatus is shown in plate xv. The ad- 
vantages of the cone suspension over the old methods are: 
first— it gives a rigid support, and if accurately constructed 
it will always hang parallel to the surface of the water, 
and the disk will come away from the water ail at once 
instead of separating the water a little at a time. Second — 
the cone gives stability to the apparatus, which adds much 
to the accuracy as well as to ease of manipulation. 
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