OF ELECTRICITY AND THE DURATION OF ELECTRIC LIGHT. 587 



perfect judgement of the eye a revolving mirror, but more rapid in its motion and ac- 

 curate in its indications than any I had previously employed. The instrument I am 

 about to describe will, unless there be some error in the estimate which I have not 

 been able to perceive, measure beyond the millionth of a second ; and this degree of 

 minuteness may be yet far surpassed by more costly instruments and more careful 

 observations. 



But as it is only on the hypothesis of an actual transfer of a fluid from one end of 

 the wire to the other that a difference of time between the two sparks at its opposite 

 extremities might be expected to be observed, in order to render the proposed ex- 

 periment independent of this theoretical view, I took the necessary precaution of 

 bringing a third spark, formed by disconnecting the middle of the wire, near to and 

 in a line with the extreme sparks. For on the supposition of the transfer of two 

 fluids in opposite directions, the extreme sparks would be simultaneous, but the 

 middle spark later in its occurrence ; the same appearances would also accord with 

 the theory of one electricity, if we admit that a disturbance of electric equilibrium is 

 simultaneously propagated from each end, arising in the one case from successive 

 additions to, and in the other from successive subtractions from, the neutral quantity 

 in the conducting wire. 



The experiment was tried at the Gallery in Adelaide Street. The insulated wire, 

 the total length of which was half a mile, was disposed as in fig. 6. The parallel 

 portions of the wire were each 120 feet in length, and six inches apart, and were tied 

 to the balustrade with silk loops six inches long. The swagging of the wire was pre- 

 vented by silk cords extending across the gallery ; and to keep the lengths at their 

 proper distances apart they were tied to the cords wherever they crossed them. The 

 ends of the wire marked 2, 3, 4, 5, were continued to the similarly marked wires of 

 the spark-board, fig. "] , which was so fixed against the wall beneath the gallery, that 

 the balls between which the sparks were to pass were in the same horizontal line. 

 The striking-distance between each spark was the tenth of an inch, and the spark- 

 board itself was three inches and a half in diameter. The conducting wire I employed 

 was of copper, and its thickness the fifteenth of an inch. 



Fig. 8. represents the measuring instrument with its appendages; and fig. 10. 

 shows in a more distinct manner some of its essential parts. A B C D is a solid 

 board of well baked mahogany one foot in length, and eight inches in breadth. E is 

 a circular mirror of polished steel one inch in diameter, so fixed to the horizontal 

 axle F G, that the axis of rotation is in the plane of the mirror. The pivots of the 

 axle work in the uprights of the brass frame H I. Motion is communicated from the 

 wheel K to the axle by means of a thread passing over grooves made on the circum- 

 ferences of both ; and a band passing over the wheel L, on the same axis with K, 

 may be attached to the wheel of any machine capable of giving to it a rapid motion. 

 In the experiments I have made with this instrument the train of wheels was so 

 arranged that the axle carrying the mirror would have made 1800 revolutions 



