28 THE MECnANISM OF 



(those of us who are experienced in such things may, perhaps, 

 remark that he is a rather slow walker), and another assistant 

 then adjusts the contact- motor by altering the weights and fan 

 governor so that the contact-brush will pass over the first twelve 

 segments of the ring in about one and one-quarter seconds, 

 making a slight allowance for safety ; " because," the assistant 

 will tell us, " we would rather get the whole stride in only eleven 

 phases than run the risk of not getting it in twelve, if the horse 

 should decide to saunter a little." Next he gets the chronograph 

 ready for action, and the other assistant meantime having finished 

 setting exposors, brings the necessary plate-holders from the dark 

 room — twelve small ones for the lateral cameras and one large one 

 for each portable battery — and places them in the cameras. The 

 slides are drawn and we are all ready. 



The operator, with the contact-key in his hand, takes his 

 stand beside camera No. 1 of the lateral series, and his assistant 

 inside the camera-house starts the contact-motor and the chrono- 

 graph while the horse comes walking down the track. When he 

 reaches a point directly in front of camera No. 1 the operator 

 presses the contact-key ; we hear a slight humming sound, and 

 thirty-six diiferent photographs of the horse are made, three 

 different views of each twelve successive positions that he put 

 himself into while making a single stride. If now we step in- 

 side of the camera-house we can see on the drum of the chrono- 

 graph a record that was made simultaneously with the exposure ; 

 and if we are curious to know we can count the number of vibra- 

 tions in the lower line of the record lying between the first and 

 twelfth drop in the upper line, and ihus see exactly how many 

 hundredths of a second were spent in making the series of ex- 

 posures. Turning again to the diagram, Fig. 2, we will see how 

 a momentary contact of the key brought about these results. 

 The drum of the chronograph, the tuning-fork, and the shaft 

 through the collar of B are all in motion, while B is held in the 

 position indicated by the dotted lines by a catch in the armature- 

 lever of M, as before described. In this position the brush rests 

 on the insulation between segments 24 and 1, and therefore no 

 circuit for the electric current. Now contact is made for an in- 

 stant at the key A, a current passes through M, its momentary 

 magnetic attraction pulls the armature-lever down, thereby freeing 



