| In the Palm Glass demonstration, the palm of the hand
is laid across the top opening of the small glass jar placed on the base
plate of the vacuum pump. In this particular apparatus atmosphric pressure
acting on the area of about 5 square inches produced a net force of about
75 pounds, and the hand can be removed only with some difficulty. Used
in this way, the apparatus is sometimes called a Cupping Glass, after the
eighteenth century practice of drawing blood to the surface of the body
by means of a partial vacuum.
A varient is the Bladder Glass demonstration in which the hand is replaced by a section of animal bladder tied over the mouth of the glass jar.