A pressure of one inch of mercury is sufficient for any
work that I have yet undertaken. With particularly good paper, a lower
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.
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