482 PROCEEDINGS OF THE AMERICAN ACADEMT 



in. cm. in. 



11.8 4.65 The two ends 20.0 7.87 



The scale is uumbered from one end to the other, and as it takes the 

 bob just one second to traverse the full length, there is plenty of time 

 to measure any short exposure with accuracy. The string of the 

 pendulum is attached to a screw at the top, enabling us to raise the 

 bob. By this means we get several exposures on the same plate. 

 The slight change in the length of the pendulum (an inch or so) 

 makes no appreciable ditFerence in its rate of vibration. By these two 

 methods, involving no complicated apjiaratus, any exposure, no matter 

 how short, can be readily measured with the greatest accuracy. 



