532 



Dr. A. F. Zahm on the Resistance of the 



been, for the most part, polished pine spheres. The light- 

 beams are 7 feet apart, the spacing being effected by 

 means of two iron slit bars, one on either side of the 

 trajectory. 



The camera, fig. 2, is a columnar cast-iron box, 5 feet high, 



Kg. 2. 



3 by 5 inches inside section, and provided with two interior 

 side grooves, down which a massive iron bar drops freely 

 carrying with it a photographic plate 2 feet high by 2J inches 

 wide. At the bottom of the camera is a cushion which yields 

 to the impact of the falling mass so as not to endanger the 

 plate; and in the back of the camera is a windlass which 

 draws the plate and holder up to their initial position, where 

 they are held by a firm latch ready for another drop. After 

 each fall of the plate the camera may be displaced sideways, 

 sa 7 1 3 6 °f an mcn 5 while the shutter and lens remain 

 stationary ; thus making it possible to take a dozen or more 

 records on one plate. Outside and behind the camera is a 

 trigger whose mechanism drops the plate, opens the shutter, 

 and fires the gun at preadjusted time-intervals. 



The records are measured on a dividing-engine by trans- 

 mitted light, and give very precise readings. The tracings 

 for a single shot consist of three fine straight lines 0*15 millim. 

 broad and 05 millim. apart from centre to centre, so that the 

 three can be seen at one time in the field of the dividing- 

 engine microscope. At the bottom of each line is a zero dot 

 formed by admitting the sunbeams to the sensitive plate for a 

 thousandth of a second while it is still suspended, the shutter 

 being adjusted to fly open only after the plate is under way. 

 The distance is measured along each line from the centre of 

 the zero dot to the middle of the interruption caused by the 

 sunbeam eclipse, thus giving the fall of the plate before the 

 bullet cuts the corresponding beam. It is found, in practice, 

 that this distance can be measured accurately to less than 

 0-005 millim. 



