112 PROCEEDINGS OF THE AMERICAN ACADEMY 



distance from the revolving apparatus, and therefore to employ less 

 speed for the revolving mirror. There was no sensible aberration of 

 the image. Great care was taken to balance the mirror. Its large 

 size and weight made it very important, on account of the danger of 

 the apparatus flying apart, that it should revolve with uniformity. 

 The axis of the mirror was placed horizontally. This precaution 

 proved to be a wise one, for twice during the course of the many 

 runs which were made the mirror flew into pieces ; the excursions of 

 the fragments, however, were confined to a vertical plane. This lia- 

 bility to accident is perhaps inherent in a method which employs a 

 large plane mirror. The increased amount of light which results 

 from the use of a large mirror, however, forms a valuable compensa- 

 tion. The revolving mirror was driven by a gas engine. 



In order to determine the speed of the mirror at the instant the 

 spark passed, the following apparatus was devised. The same shaft 

 which carried the revolving mirror also carried a brass cylinder of 

 5 cm. in diameter and 21 cm. long. This cylinder was covered at each 

 trial with paper which was coated with lampblack. A stylus moving 

 along a stationary rod beside the shaft could be made to draw a spiral 

 upon the revolving cylinder. One terminal of a Ruhmkorf coil was 

 connected with the brass cylinder, and the other with the stylus. A 

 second pendulum was made to break the circuit of the primary of the 

 Ruhmkorf coil at intervals of one second, and at the middle point of 

 its swing. When the stylus was drawn along the stationary rod which 

 served to guide it, it was made to release automatically at the begin- 

 ning of the second another pendulum held up by an electro-magnet. 

 This latter pendulum, at the middle of its swing, discharged the air 

 condenser through the inductance circuit at the instant that the mirror 

 was in a suitable jiosition to reflect the image of the electric spark 

 into the photographic camera. While the stylus was being drawn 

 upon the revolving cylinder, the spark from the Ruhmkorf coil left 

 its trace upon the blackened paper. The record on the chronograph 

 consisted of a strongly marked spiral line of over fifty turns. The 

 two sparks from the Ruhmkorf coil left their trace upon the black- 

 ened paper or spires, which therefore measured the number of revolu- 

 tions of the cylinder between the swings of tlie pendulum, and thus 

 gave the rate at which the mirror was revolving. The chronograph 

 record enabled us to measure the time to ^^j^ of a second. 



In any operation which requires that an electrical spark should 

 make a record upon a disk or cylinder revolving at great speed, a 

 large Ruhmkorf coil and a strong battery are necessary, especially 



