BRUSH— KINETIC THEORY OF GRAVITATION. 



47 



temperature changes due to outside weather conditions. The whole 

 lay-out is thirty feet from the nearest window, and the temperature 

 of the laboratory is very uniform and steady. The room selected 

 is an inside one and contains no heating apparatus. The floor is 

 thick concrete. The reading telescope shown in front and the car- 

 riage referred to are mounted on a massive table with thick marble 

 top, nowhere touching the bracketed slab or the walls to which it is 

 attached. The illuminated millimeter scale is two meters to the 

 right of the oscillating system and does not show in the picture. 



It will be noted that the tall brass tube containing the quartz 

 filament is loaded at the top with a hollow cone of metal. This is 

 found to increase the stability of the suspension apparatus very 

 greatly by so lengthening the period of free vibration of the upper 

 end of the tube that it can not respond to vibrations of the building 

 due to street traffic or other causes. Although the nearest street is 

 300 feet away, traffic vibrations often can be felt. 



The whole apparatus is protected from radiant heat of the scale 

 lamp, and one other more distant lamp used to light the room, also 

 from the heat and breath of the observer, by screens of cellular 

 paper (not shown). All air drafts in the room are avoided as care- 

 fully as possible. The rheostat on the wall in the upper part of the 

 picture has nothing to do with the apparatus, and never is used 

 durino; observations. 



Fig. I is a plan diagram of the essential parts. The suspended 

 silver balls are seen at A. B and C are cylinders of different metals, 



