OF ARTS AND SCIENCES. 481 



division marked 0. At a given signal the ball is dropped, and the 

 exposer released. A long black line is produced on the plate, and 

 the number of scale divisions that it covers measures the length of the 

 exposure. As the ball requires .3 sec. to reach the bottom, there is no 

 difficulty in catching it on some portion of its course. 



In the following table, which was calculated by the formula s=^ ^ , 



the first column gives the time required by the ball to fall in hun- 

 dredths of a second ; the second, the distance fallen in centimeters ; 

 and the third, the distance in inches. 



By painting the division marks at these distances, the length of the 

 line made by the ball as photographed on the plate with the scale will 

 give us at once the duration of the exposure in hundredths of a second. 

 Four or five exposures may be made on the same plate, moving the 

 camera slightly after each one. The results obtained will indicate the 

 uniformity of the exposure. 



For exposures longer than .05 of a second another method of meas- 

 urement has been devised. It consists it photographing a seconds 

 pendulum having a silvered glass ball for a bob. The pendulum is 

 placed in the sunlight, and is swimg before a painted scale spaced 

 as follows. The scale is constructed on an arc of a circle whose 

 radius is 39 inches, and it is symmetrical on both sides of the middle 

 point. The following distances are measured both ways, starting from 

 the middle, and each space save the last one represents the distance 

 traversed by the pendulum in .02 sec. To traverse the last space 

 requires .1 sec. 



VOL. XX. (X. S. XII.) 31 



