RECENT PROGRESS IN PHYSICAL SCIENCE. 
105 
The construction of the apparatus will be understood from 
Fig. 1 and the following brief description as given by Messrs. 
Threlfall and Pollock: 
“A and B are two metallic rods, adjustable endwise along and around 
their common axis, but not free to move transversely to that axis. 
“ C is an elliptical-shaped spring, provided with a point H , to which 
the end of the quartz thread is fastened. The rod B , which is free to 
rotate about its axis, is rigidly attached to an arm carrying the vernier G, 
which in turn traverses a graduated arc, a sextant being, in fact, employed 
for this portion of the apparatus. 
“ The quartz thread has as nearly as possible a diameter of 0.0015 inch, 
and is very uniform. It is soldered up to the points prepared for it at 
K and 1 , and is stretched tight by the spring C. 
‘‘The length of the thread is 30.5 centims. (11.8 inches). D is a piece 
of gilded brass wire 5.3 centims. long and weighing 0.018 grms. It is 
attached to the thread by soldering, the thread lying in a little kink in 
the wire. The center of gravity lies a little to one side of the thread. 
11 E is a microscope attached to the frame of the machine so as to occupy 
an invariable position with respect to the supports A and B. 
“ The apparatus is thermally insulated with every possible care, and a 
platinum wire thermometer, wound upon very thin glass, lies alongside 
the thread. 
“ The gravitational moment of the lever is so adjusted that about three 
whole turns of each end of the thread are required to keep the lever hori¬ 
zontal. This adjustment is made once for all. During the process of ob¬ 
serving, the thread is twisted from one end only, viz., the end attached 
to the sextant arm. The equilibrium of the lever becomes unstable when 
its center of gravity rises a few degrees above the horizontal plane pass¬ 
ing through the thread. 
