That it had been so cut was not to be imagined, and on
further examination I found that the fracture had occured at a line
which separated a surface in the strong sunlight from a surface in the
shade, at a fold in the rubber. I saw that all of the rubber which had
been continuously exposed to the intense sunlight had changed color and
had become whiter than before, and that that portion of the balloon had
lost its strength. I then returned to the use of the mercury gauge, and
took the precaution to cover my pad with cotton flannel, as a protection
from the light and from other sources of destruction. This pad is upon
the roof of the Institute; and is exposed to all weathers. As a
protection from the rain and the snow, the whole is covered again with a
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.
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