FREED FROM THE ACTION OF GRAVITY. 23 
iron, a metal whose prolonged contact with oil does not stain 
it as copper does. The diameter of the disc is about 35 milli- 
metres, and the axis is formed of an iron wire about 17 millimetre 
thick. This axis is fixed by its lower end into a hole pierced in 
the middle of the plate of glass which forms the bottom of the 
yessel ; this hole is closed below by.a small plate of iron cemented 
to the glass. The upper end of the axis is screwed to a larger 
wire, which forms the prolongation of it, and which, held with a 
moderate degree of friction [a frottement doux| ,in a piece of which 
I shall speak hereafter, receives at its other extremity the handle 
by means of which the disc is turned. When the whole system is 
in place the disc ought to be half way up the vessel. The square 
plate of glass which closes the vessel above is pieced with two 
openings, each furnished with an iron neck, which is closed with 
a stopper of the same metal. One of these openings is in the 
middle of the plate, and its diameter is 55 millimetres; it is 
through the stopper which closes it that the rod passes, a 
frottement doux, which receives on the one side the axis of the 
disc and on the other the handle. (See fig. 2.) The other open- 
vessel as a lid upon a box. The upper edges of the vessel have 
been ground with emery all together after their being placed 
in the frame, so that the upper plate of glass fits exactly upon 
them; and by rubbing these edges and the metallic stoppers 
with a little oil, the vessel, when the plate and stoppers have 
