pressure is sufficient. Upon the top of the pad is laid a piece of
common cotton flannel with the nap outward, and with its edges tacked
along the under edge of the back-board. The cotton flannel is not drawn
tight across the top of the pad. The reason for employing a cotton
flannel covering is this: When the sheet rubber has been exposed for a
few days to the strong sunlight, it loses its strength and becomes
worthless. The cotton flannel is a protection against the destruction of
the rubber by the sunlight. I first observed this destruction while
experimenting with a cheap and convenient form of gauge. I used, as an
inexpensive gauge, an ordinary toy balloon, and I could tell, with
sufficient accuracy, how much pressure I had applied, by the swelling of
the balloon. This balloon ruptured from some unknown cause, and I made a
substitute for it out of a round sheet of thin flat rubber, gathered all
around the circumference. I made holes about one-quarter of an inch
apart, and passing a string in and out drew it tight upon the outside of
a piece of three eighths of an inch pipe, I then wound a string tightly
over the rubber, on the pipe, and found the whole to be air-tight. This
served me for some time, but one day, on applying the pressure, I found
a hole in the balloon which looked as if it had been cut with a very
sharp knife. 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