154 
DR. A. E. IT. TUTTOX ON THE ELASMOMETER 
junction to the same length as on the right side, but the taVmlar expansion, 
18 millims. wide, carrvino’ the ao’ate wedge, in which it terminates on that side, is 
curtailed in width to 8-2 milli3us. on the left side. This enables it to Ido reachlv 
inserted not only between tlie blocks which carry the platinum-iridium wedges, but 
also between the little spring taldes supporting the plate under investigation. At 
precisely the same distance, on this left side, from the fulcrum knife-edge, as is the 
pan-supporting knife-edge on the right, an upright agate point, the “ pressure-point,” 
is erected. It is mounted at the end of a short screw, passing through the beam-end 
from underneath, which serves the purpose of enabling it to be adjusted so that its 
point lies precisely in the same straight line with the agate fulcrum and the pan- 
supporting knife-edge. 
To compensate for the pan and its fittings on the right side of the balance, there 
is provided on the left, at 5'8 centims. from the pressure-point, a tabular expansion 
carrying an adjustable agate wedge, over which is suspended a stirrup with agate 
plate, similar to those on the right; a hollow cylinder is suspended from the stirrup 
by a short rod terminating in a hook, within which a load can be carried of the 
extent necessary for perfect compensation. The supporting frame is correspondingly 
modified on the left, being much shorter and curving upwards so as to support the 
top bar of the stirrup by a pair of inverted cones as on the right side. A transverse 
strensthenino- bar forms the extreme right end of the frame, and a similar bar is 
inserted on the left side, just before the frame begins to curve upwards. The load 
within the cylinder is a block of lead, made slightly heavier than recpiired, and 
sidjsefjuently pared until its weight has become exactly such as to bring the total 
weight on the left side of the balance equal to that on the right. The beam carries 
above its centre the usual adjustable gravity bob and vane, so that the last trace of 
adjustment can be effected by the latter. 
The balance is mounted on a rectangular gun-metal box which carries the raising 
and lowering gear of the supporting frame within it, the milled head for working 
which emerges in front. The box is arranged to l^e capable of 5 centims. of motion 
parallel to the front edge of the l:)asal slab, so that the pressure-point end of the 
beam may be advanced into or withdrawn from the recess between the blocks. This 
is achieved by providing the front and back bottom edges of the box with dove-tail 
flanges, which slide in corresponding guides screwed to the basal slab. The back 
guide has a clamping slip inserted between it and the flange, which can be driven 
rigidly against the latter by a strong inclined thumbscrew when it is desired to fix 
the balance base after adjustment. The motion is effected by means of a strong 
central rack screwed to the basal slab, and an equally strong pinion borne in bracket 
bearings projecting from the right side of the box and rotated by means of a winch 
handle. 
The balance is also provided with a fine azimuth adjustment to enable the 
pressure-point to be placed precisely in the veitical plane half-way between the 
