586 MR. WHEATSTONE ON SOME EXPERIMENTS TO MEASURE THE VELOCITY 



visual magnitude of the circle described increases with the distance of the object and 

 incMnation of the mirror. The flame of a candle presented to it appears as a broad 

 luminous ring ; the image of the sun is converted into a magnificent fiery belt, &c. 



A series of minute sparks made to pass between two points, or between a point and 

 the prime conductor of a machine, presents to the eye, from the rapidity of their suc- 

 cession, the appearance of a permanent star of light. When this star is placed in the 

 prolongation of the axis of the revolving mirror, the successive sparks of which it 

 consists are reflected to the eye each from a different part of the surface, and they 

 are exhibited arranged at regular distances in a circle. When the intermissions are 

 rapid the appearance is extremely beautiful. 



The brush of light which appears on a point when presented at some distance from 

 the conductor, is also by this means shown to be an intermitting action, notwith- 

 standing its permanent appearance ; its reflected images present, however, this re- 

 markable peculiarity, they are elongated in the direction of the motion, proving that 

 a brush is not so transient as a spark, and that the emissions which constitute it last 

 during an interval of time measurable by the motion of the mirror. 



But this instrument is not confined to observe merely the intermittences of electric 

 light ; whenever a rapid succession of alterations occurs in an object which does not 

 change its place, they may be separately examined by this means. Vibrating bodies 

 afford many instances for investigation ; one among these is perhaps worthy to be 

 mentioned. A flame of hydrogen gas burning in the open air presents a continuous 

 circle in the mirror ; but while producing a sound within a glass tube, regular in- 

 termissions of intensity are observed, which present a chain-like appearance, and 

 indicate alternate contractions and dilatations of the flame corresponding with the 

 sonorous vibrations of the column of air. 



Experiments have frequently been made with a view to determine the velocity of 

 the transmission of electricity through conducting bodies. In all the recorded trials 

 of this kind it was attempted to measure the interval of time supposed to occur 

 between two discharges made at opposite extremities of the wire, which were brought 

 near each other so that they might be seen at the same time. In one experiment, 

 performed at Shooter's Hill in 1747 under the superintendence of Dr. Watson, the 

 circuit was four miles in extent, two miles through wire, and two miles through the 

 ground ; but the discharges appeared, as in all similar experiments, to be perfectly 

 simultaneous. Nor need we feel surprised at this result, when we know that the eye 

 is unable to distinguish the succession of luminous objects which follow at the in- 

 terval of the eighth or tenth of a second, from their simultaneous appearance ; and 

 that, therefore, with a circuit even of four miles extent, the velocity of a few miles 

 per second would be the utmost observable by such means. 



I determined, therefore, to repeat a similar experiment, substituting for the im- 



