to ascertain a Standard of Weight and Measure . 139 
by turning the micrometer screw n , and the divisions at 0 will 
give the difference, in loooths and io,oooths of an inch, -f- or 
— 6 inches. 
(§. 9.) Description of the Hydrostatic Balance. 
abed, (Tab. VI.) is a box, which contains the whole ap- 
paratus when not in use, and when used serves as a foot to 
the hollow brass pillar efg h, which is fixed into it by the four 
screws at the bottom e and/. This pillar contains another 
within it, which is raised up and down about inch, by 
means of the screw x. no is the beam, 27 inches long, and 
3,9 inches wide in its greatest diameter ; each arm of which is 
made hollow and conical, for strength and lightness : through 
the centre, at m, passes the axis of motion, the ends of which, 
when used, are suffered to fall gently upon two crystal planes, 
which are set horizontally by means of the spirit levels k, l, and 
the screws underneath the box, at c and b. The ends of this 
axis are of hardened steel, of a wedge-like shape, and reduced 
to a fine edge, viz. to an angle of about 40°, so as to move 
upon the planes with very little friction, and at the same time 
so hard as (with due care in using) to be in no danger of be- 
ing blunted : to prevent which, the inner pillar has a motion 
upwards, as has been said, by the screw x, and, by means of 
a semicircular arm at its upper extremity, lifts the beam off its 
bearings, when it is not used, or is greatly loaded. This axis 
is placed carefully at right angles to the beam; and, by means 
of two small brass springs that press gently at the ends, is. 
brought always to have the same bearing upon the crystal ; so 
that no error need be feared from a small deviation from the 
right-angular position of the axis to the beam, should any such 
T 2 
I 
^ *00:008$ 
