machine.

[Illustration]

The type of machine which we now illustrate has been recently brought
out by Mr. G.E. Rutter, and the system has given very satisfactory
results with platform weighing machines. The engraving illustrates a
form of balance which may be applied to strength testing machines, or
for any work where an apparatus of the type of a Salter's balance
would be of use. It is simple in construction, and consists of a tube
A closed at the bottom and forming a reservoir for mercury. The body
which it is required to weigh is hung upon the hook B carried by the
crossbar C, which is connected by rigid rods to the upper part of the
tube, and by means of the internal rods D is attached to the cross
head E, which works freely inside the tube A. The top part of the tube
is, as will be clearly understood from the illustration, cut away to
allow of the descent of the rods. To the cross head E is attached the
piston F, which may be made of wood or of a hollow metal tube closed
at the end, or other suitable material. It will be easily understood
that when a weight is hung upon the hook B, the piston F is caused to
descend into the mercury which rises in the annular space between the
piston and the tube. The weight of the volume of displaced mercury is
proportional to the weight of the body hung upon the hook, and the
buoyancy of the piston in the mercury forms the upward force which
balances the downward pull of gravity. When the apparatus is at rest
the piston F descends into the mercury to such a distance as will
balance the weight of the rods, hook, and piston itself. If, now, the
cross bar G, provided with a pointer H, be fixed to the rods, it
should at that time register zero, upon the scale J fixed to the
outside of the tube, and as the descent of the piston into the mercury