£372.| Electricity. 129 
S. H. Lockett, Professor of Engineering at Louisiana University, writing 
from Niagara Falls, relates the following phenomenon :—‘‘ While crossing the 
upper or new suspension bridge to-day, I had occasion, while conversing with 
a friend, to point toward the falls with my walking-cane. As soon as I did so, 
I heard distin@ly at the end of my cane a buzzing noise, like that made by 
electricity passing from a heavily charged battery to a sharp-pointed rod. 
Repeating the experiment, the same noise was heard. I stopped several 
passers, and tried their canes with the same result, except in one case where 
there was no ferule on the cane. I immediately supposed this might be an 
electrical phenomenon, and set to work to test the correctness of my supposi- 
tion. I took a key, and held it at arm’s length toward the falls, and heard the 
same sound. Finally, at dark, I returned to the bridge, and pointed my cane 
in the air, and had the satisfaction of seeing a clear beautiful electric brush on 
its end. The best point to observe this interesting and beautiful phenomenon 
is in the middle of the bridge, and the cane must be held at arm’s length, so 
that its end may be at some distance from any part of the bridge. The 
success of the experiment seems to depend a good deal on the direction of the 
wind and the amount of vapour blown over the bridge. To-day the wind is 
strong, and drives the mist directly from the falls to the bridge, but an 
occasional shifting or lulling of the wind would cause a cessation of the 
electrical noise or light. My explanation of the phenomenon is this :—As 
Franklin with his kite and key caught the lightning from the clouds of heaven, 
so here, from the suspension bridge, surrounded by vapours from the mighty 
falls, we may stand and gather on our walking-canes the electricity generated 
by the falling waters and contained in the floating mists. I think suitable 
arrangements might be made to collect enormous quantities of electricity from 
these mists, which might be used in producing grand and striking effects, thus 
adding another attractive feature to the sights at this wonderful place.” 
The duration of the spark of the Leyden jar has been measured by Professor 
Rood by an ingenious apparatus, by which intervals of time, measured by 
billionths of a second, areestimated. The wheel, painted black and carrying a 
distin& white point on its circumference, is provided with some means of 
giving it a uniform motion of rotation. Ifthe wheel makes one revolution in 
one-sixth of a second, the white point will appear as a continuous circle; for 
any impression produced on the eye remains during one-sixth of a second, 
therefore during one revolution of the wheel all the successive positions on 
the circumference occupied by the bright point remain impressed on the eye, 
and hence the circle appears unbroken. Now, if a flash of light in the place 
of the white point should last one-sixth of a second, the circle would appear 
complete; but if it lasted one-twelfth or one twenty-fourth of a second, then 
would the point describe one-half or one-quarter of the whole circle. Thus, 
by this simple means—remembering that the smaller the arc of the circle, the 
less the duration of the flash—we can readily measure from the length of this 
arc very minute portions of time. If, instead of having one white point on 
the wheel, we have too or more radial white bands drawn with the space 
between them equal to their breadth, then, if the wheél makes ten turns in a 
second, any radial white band will advance into the position previously 
occupied by an adjoining black band in one-thousandth of a second, and if the 
flash of light lasted one-thousandth of a second, all the white bands would, 
during that interval, have advanced into the position of the black bands, and 
vice versa, and the disk would appear without bands and covered with a 
uniform grey tint. We can thus readily and accurately measure one- 
thousandth of a second. With the above apparatus Arago about the year 
1835, first showed that a flash of lightning lasted less than one-thousandth of 
a second, but did not succeed in fixing the minimum limit to its duration. 
Professor Rood, however, was more fortunate ; for during the well-remembered 
remarkable display of lightning in August, 1869, with an apparatus similar to 
the above (extemporised from a piece of pasteboard and a shawl pin), he 
succeeded in measuring one five-hundredth of a second as the duration of 
those vivid and extensive flashes. It was soon found that the velocity of the 
revolving disk fell far behind that of the spark of the Leyden jar, for its flash 
VOL. Il. (N.S,) s 
