590 MR. WHEATSTONE ON SOME EXPERIMENTS TO MEASURE THE VELOCITY 



the velocity was low, the terminating points appeared to be exactly in the same ver- 

 tical line ; but when the velocity was considerable, and the mirror revolved towards 

 the right, the lines assumed this appearance, ; when it revo lved towar ds 



the left, they appeared thus, _, jn no case did I see them thus, ., 



or thus, , as required in the hypothesis of the actual transfer of a single 



fluid. I found it convenient to place at the side of and near the spark-board, the 

 flame of a taper or candle, to serve as a guide to the eye : the lines of electric light 

 in the mirror were immediately above and parallel to the constant line formed by the 

 reflection of this flame, and thus the eye could be more readily directed to them : 

 it also served to keep the focal distance of the eye properly adjusted. The spark- 

 board was in all the experiments placed at the distance of ten feet from the 

 mirror. 



The deviation between the extreme sparks and the middle one could not, I am 

 tolerably certain, have exceeded half a degree. 



Having obtained a considerable elongation of these sparks, I expected also to be 

 able to elongate the sparks or widen the lines in some of the various arrangements of 

 electric light described in § 2 ; but even with the extraordinary velocity now attained, 

 no alteration whatever could be observed in them ; they were still reflected as distinct 

 and unaltered as the objects themselves when directly looked at. The elongation of 

 the sparks at the interruptions of the wire above noticed were no doubt owing to this 

 circumstance, — that the diameter of the wire was not sufficiently great to allow the 

 charge of the jar to pass through it except in a successive manner. The duration of 

 the discharge in the cases of these sparks appeared to be longer than the time required 

 for the electricity to pass through many miles of wire. 



The sparks from the great magnet constructed by Mr. Saxton, which is at the 

 Gallery in Adelaide-street, ^vere considerably elongated even when the mirror was 

 moving with a comparatively low velocity. 



§5. 



For the purpose of increasing the chances of observing sparks, &c., when their ap- 

 pearance cannot be commanded at the moment the mirror is in the proper position to 

 reflect them to the eye, I propose to employ a mirror with polygonal faces symmetri- 

 cally placed with respect to the axis of rotation, a hexagon for instance, fig. 9, where 

 a, b is the moving axis, and c, d, e three of the reflecting surfaces. During one rota- 

 tion of the axis, if the object be continuously luminous, six luminous arcs will be suc- 

 cessively presented to the eye, all occupying the same position ; and if the light be 

 transient, we shall have six times the number of chances of observing its reflection 

 than if one reflecting surface only were employed. It is true the arcs are not circular 

 ones, but the difference is scarcely noticeable when the radius of the polygonal sec- 

 tion is very small compared with the distance of the luminous object, which would 

 be the case in all our experiments. 



