OF THE NATIONAL RESEARCH COUNCIL m 



form and works in opposition to the pull of the motor. As it worked 

 out, this series of wave motions has been commented on most favor- 

 ably by all the experienced pointers who have observed it. With- 

 out exception they have said that it was the best artificial target 

 motion that they had ever seen on a dotter. By shifting the driv- 

 ing belt on the reducing pulleys, it is possible to imitate the move- 

 ments of a slowly moving gun-platform or a rapidly moving one at 

 will. Observers of experience name these speeds battleship roll, 

 merchantman roll, and destroyer roll. 



"Instead of moving the target platform directly across the 

 frame, the track on which it moves is laid at an angle of forty-five 

 degrees to the main frame. This makes the target travel the hypot- 

 enuse of a right isosceles triangle, of which the transverse leg would 

 give the apparent displacement of the target from the standpoint of 

 the trainer. The other leg in the direction of the axis of the gun 

 furnishes the conditions for the record of correlated movements of 

 target and gun. Thus a line traced by the recording lever which is 

 attached to each gun and writes on a sheet of paper laid on the tar- 

 get platform, will represent point for point each movement of the 

 target and the gun. If the gun remains stationary while the target 

 moves, this record line will be an oblique line at an angle of forty- 

 five degrees to the gun axis. If the gun moves while the target is 

 stationary, the record will show a straight line transverse to the 

 axis of the gun. If the gun moves exactly with the target, that is, 

 if absolutely accurate pointing is maintained throughout the move- 

 ment of the target, the record will be a straight line in the axis of 

 the gun. This latter is an impossible ideal of perfect pointing. 

 All the records show a greater or less number of irregularities inci- 

 dent to the reaction time of the pointer and the imperfections of 

 his coordinations. Improvement of the pointer in the essentials 

 shows itself in the gradual reduction of the lateral displacements 

 of the record from the theoretically perfect record of a single 

 straight line. The record of each performance may be analyzed for 

 the benefit of the recruit immediately after each trial. For this 

 purpose, it is probably expedient not to have the practice records 

 too long. Short records are less complicated and may be more 

 easily analyzed. Moreover, the recruit can remember better what 

 he did if the record is not too long. 



"Following the recommendations of Lieutenant Norton, 

 U. S. N., the instrument was adapted so that the targets 

 could be given a vertical motion for practice In pointing as 



