Nipher—Gravitational Repulsion. 179 
closed by a cylinder of copper wire gauze, between which 
and the outer screen was very loosely packed cotton 
fiber. A thin layer of cotton was placed at the ends of 
the screen, being also held in place by wire netting. This 
was designed to diminish convection effects. 
The body of the observer was wholly below the level 
of the table. Radiation of heat from the head to the 
Screen was cut off by sheets of heavy cardboard. 
The two rooms used for this work were wholly discon- 
nected from the heating system of the building. During 
the day the temperature of the large masses was usually 
slightly lower than that of the shield around the sus- 
pended masses. This temperature difference never ex- 
ceeded 1°.5 C. 
The large masses were placed at the ends of the 
screen or shield in line with the rod upon which the small 
masses were mounted, in order to determine their effect 
upon the time of vibration of this bifilar pendulum. The 
large masses and screen were in direct metallic connec- 
tion, and the air around them was electrified by a dis- 
charge from pin-points. A dise armed with 150 pins was 
Placed with the pin-points 4 em. from each of the large 
masses. <A noiseless discharge was delivered from the 
points of the pins. 
Let T represent the time of vibration when the masses 
were away. 
Let 7, represent the time of vibration when the masses 
are in place. 
Let T. = the time when the masses and screen are elec- 
trified. 
Then 
a ee 
Here m is the suspended mass, 2d the distance between 
the two silk fibers on which it is suspended, and / their 
length. I is the moment of inertia. 
