Beck^ on Tivo Neiv Forms of Reversible Compressors. 5 
to a brass plate by the heads of two screws ; spaces were then 
to be made for the heads of these screws in each opposing 
plate, when the two pieces of thin glass could be brought close 
together. This simple plan has great advantages : — the 
pieces of glass are easily cut, as no great nicety of size is re- 
quired, and they can be changed or replaced without difficulty 
and with little loss of time. 
The following are the mechanical means by which I have 
endeavoured to fulfil the requirements already stated : — 
Eig. I shows two brass plates furnished with thin glass and 
Pig. I. 
connected by four small rods ; this is in fact the commonest 
parallel rule motion, and very nearly a vertical one, for when 
the plates are near together the position of the connecting 
rods is such as to cause no perceptible end movement. The 
plates, when about Vuth of an inch from each other, are 
kept apart by two springs, which supply the separating power_, 
the compressing force being given by a small screw, with a 
conical end attached to one plate, working into a corresponding 
recess on the other plate ; the first hold the point of the screw 
gets is at the bottom, so that the plates are drawn together 
as the screw advances. When the inner surfaces of the thin 
glasses want cleaning, or when the object or the glasses have 
to be changed, the screw must be brought quite back, and 
the plates will then separate to the extent of a full inch. 
In the other compressor (Fig. II) the thin glasses are se- 
cured to two circular plates of brass ; the one fits the ring, and 
the other the counter ring of a brass cell ; w^hen these are 
screwed together, the plates approach each other, and when 
unscrewed they are separated by springs. Two upright pins 
in a flat brass plate pass through both the circular brass plates 
to prevent their turning round whilst the cell is screwed or 
unscrewed, and a small milled head screwing against a push- 
piece holds the cell itself firmly in a sunken ring that it drops 
into. 
Both these compressors supply the wants I commenced 
