22 KENNETH HARTLEY 
all that is desired. This is about ten times the accuracy of the best work done 
with the pendulum apparatus of the United States Geodetic Survey. 
Figure 1 is a simplified diagram of the essential parts of the apparatus. The 
main spring, which carries about 99.9 percent of the load, is formed from 
heavy wire and not stressed to more than 20 percent of its apparent elastic 
limit, so as to give the greatest possible dependability. A very light beam, 
hinged at the left side, is flexibly attached between the spring and the weight 
Lemp ; 
Reflecting Prism 
= 





Micrometer screw _ 


Weighing sp ring 
Lens 
ae 
4 
Scale beam 
ronda \ 
4 
—Js Mirrors 
es es LAA 
ee ixed support 
Fixed support 
Th 
Fig. 1. Schematic diagram of gravimeter. 


and carries two small mirrors at the free end. These mirrors act on the prin- 
ciple of the optical lever and are so mounted that a vertical displacement of 
the beam causes them to turn in opposite directions. This doubles the ampli- 
fication but a much more important fact is that it avoids the necessity for 
maintaining a fixed point of reference in the telescope; when the two mirrors 
are in the same plane the two images of t':e lamp filament as seen in the eye- 
piece will be in one straight line no matter how much the eye-piece and other 
optical parts may be displaced. A very light weighing spring is attached to the 
166 
