153 
NEW INTERFERENTIAL FORM OF ELASTICITY APPARATUS. 
top in order to luring the screws adequately nearer to each other than the knife- 
edges to enable them to clear the latter. To allow for varying compression of the 
springs, and consequent variable level of the plate, the screw-ends are expanded into 
little discs perpendicular to the length of the screws. 
The Balance. 
A specially constructed form of long-beam balance is employed, designed for 
maximum strength of beam and to take loads up to 500 grammes. The balance part 
proper Avas made by Mr. Oertling to the author’s design, and Messrs. Troughton 
and Simms subsequently mounted it on a movable base and fitted it with the control 
apparatus. Its general appearance will lie gathered from figs. I and 2. It is 
arranged on the right-liand part of the basal slab. 
The beam is 35’5 centims. long, has a horizontal top bar graduated as usual for a 
rider, and is rendered very ri^id bv four transverse struts. Tliis leno'th of beam was 
just adequate to admit of the insertion, on the left side, of the control apparatus and 
a counter])oise for the weight of the pan on the right side. 
The central agate fulcrum plane is 3'2 centims. long, and the knife-edge resting 
upon it, rigidly carried by the beam and about which the latter swings, is 4 centims. 
long and cut out of a single block; the swinging is consequently exceptionally 
steady. The central supporting pillar of the balance is shorter than usual, but 
adequately long for accuracy of the pointer’s readings on the ivory scale of the usual 
kind. 
The arrangements at the right-hand side of the column are of the type usually 
provided by Mr. Oertlixg in his most accurate form of long-beam balance for a load 
of 500 grammes. The thick wire suspending arrangement of the pan is shorter, 
however, corresponding to the sliortened colninn, and l)y means of the small hook at 
its summit the pan is liung from a large inverted stirrup, whose upper horizontal bar 
is provided with an agate plate by wdiich it rests on the adjustable agate knife-edge 
of the beam ; it is maintained in the proper central position thereon by two small 
cones carried by the supporting frame, fitting in corresjionding conical countersinkings 
under the bar, until tlie balance is released from support. This latter is achieved in 
the usual manner by rotation of a large milled head at the foot of the balance, which 
pulls down the inner shaft of the column and brings down with it the beam-supporting 
frame which it carries above ; three very rigid guides are provided, one on each side 
and a larger one behind, which also carries the agate fulcrum plate, to maintain the 
motion of the frame steady and certain. 
The arrangements at the left-hand side of the column are different. The free end 
of the right half of tlie hoiizontal bar of the beam, overhanging the pan-siqiport, has 
no counterpart on the left lialf, the beam passing down immediately into the steeply 
inclined end bar, by which it joins the lower bar. The end is carried out beyond tlie 
VOL. ccri. —A. 
X 
