rubber blanket. It has withstood the exposure perfectly well for a year,
without injury. The gauge, made from flat rubber, is altogether so cheap
and so convenient that I am now experimenting with one of this
description having a black cloth covering upon the outside. The balloon
is of spherical shape, the black cloth covering is of cylindrical shape,
and I hope that this device will serve every necessary purpose. A
sectional view of the air-cushion is offered as a part of this
communication.

_The Frame, which Contains the Plate Glass_, is made of thick board or
plank, with the broad side of the board at right angles to the surface
of the glass. A rabbet is made for the reception of the glass, and four
strips of strap iron, overlapping both the glass, and the wood, and
screwed to the wood, keep the glass in position. Strips of rubber are
interposed between the glass and the wood and between the glass and the
iron. The frame is hinged to the back-board by separable hinges, so that
the glass can be unhinged from the pad without removing the screws.
Hooks, such as are used for foundry flasks, connect the frame with the
pad upon the opposite side. A frame made in this manner is very stiff
and springs but little, and its depth serves an excellent purpose. The
air-cushion and the frame are so mounted that they can be easily turned
to make the surface of the glass square with the direction of the sun's
rays. It is necessary to have a tell tale connected with the apparatus,
which will show when the surface of the glass has been thus adjusted.
The shadow of the deep frame is an inexpensive tell-tale, and enables
the operator to know when the adjustment is right. I have now described,
in detail, the construction of the air-cushion with its back-board, as
well as that of the frame which holds the plate glass, and I think it
will be evident that the first cost of the materials of which they are
made is comparatively little, and that the workmanship required to