130 Progress in Science. (January, 
showed the revolving radial bars as absolutely at rest as when the disk was 
stationary. But Professor Wheatstone, in 1834, substituted for the revolving 
disk a mirror turning on a horizontal axis, and instead of the white point or bars 
he used the image ofthe spark reflected from the turning mirror. If the spark 
be instantaneous, then willit appear in the rotating mirror just as it is seen 
when reflected from the mirror at rest; but if the spark last during even an 
extremely minute fraction of a second, it will appear drawn into a line in the 
direGion in which the mirror turns. Wheatstone thus measured the one 
million one hundred and fifty thousandth of a second, and ascertained that 
the electricity from a Leyden jar goes over a copper wire at the rate of 
288,000 miles in a second, exceeding light itself in velocity. Professor Rood 
has now combined the two methods above given by viewing the appearance of 
stationary parallel and equidistant white and black bands reflected from the 
revolving mirror while the flash of the Leyden jar illuminated them. The 
direction of rotation of the mirror being across the length of the bands (which 
were only sixteen-thousandths of an inch apart), if the flash lasted during the 
time for the turning mirror to reflect a black band into the adjacent white 
space, then the bands would entirely disappear, and the plate on which they 
were drawn would appear of a uniform grey tint. By knowing the number of 
turns the mirror makes in a second, and the number of bands in the space of 
one inch, it is easy to calculate the time necessary for the obliteration of the 
bands. Thus has he produced, by this simple combination, an instrument 
surpassing in minuteness and accuracy of determination all that has gone 
before—an accomplishment which cannot but reflect much renown upon 
American science. He has succeeded (with a mirror making 350 turns in a 
second) in measuring accurately forty billionths of a second, and has shown 
that this is the duration of the flash of a Leyden jar having only 11 square 
inches of surface and one twenty-fifth of an inch striking distance—an 
interval of time just sufficient for a ray of light (going at the rate of 
190,000 miles in a second) to travel over 4o feet. The flash from a jar having 
II4 square inches of surface lasted four times as long as the smaller jar. 
Thus, for the range of electric flashes we have measures from the five- 
hundredth to the forty-billionth of a second. To enable the mind to form 
some idea of the minuteness of the spaces of time measured by Professor 
Rood, we may mention that the forty-billionth of a second bears nearly the 
same proportion to a second as a second does to a thousand years. 
Mr. George Gore, F.R.S., has, in pursuing his researches in thermo-electri- 
city, constructed a liquid thermo-electric battery. This battery is sufficiently 
powerful to give a deflection of 40° with a Thomson’s reflecting galvanometer, 
having a resistance of 3040°7 British Association units (=77872°327 miles of 
copper wire 1-16th of an inch in thickness). It consists of twelve glass tubes 
three-fourths of an inch in diameter and zo inches in length, one end of each 
tube containing a platinum wire hermetically fastened. Each tube is bent 
into the form of the letter J, the lower limb being the open one. The open 
end is closed by a cork, through which is passed a second platinum wire. The 
tubes are fixed vertically in a wooden stand. A tin box, the bottom of which 
has a long semicircular cavity so arranged as to cover the upper parts of the 
tubes as with a cap, when it is filled with boiling water forms the upper part of 
the apparatus and is the source of heat. A lamp placed under a projection 
from one side of the bath maintains the water at the boiling-point. When it 
is desired to charge the battery, alternate tubes are filled with a previously 
boiled and cooled mixture of 1 part of sulphuric acid to 76 parts of water. The 
remaining six tubes are filled witha similarly prepared solution of rro grains of 
hydrate of potassium dissolved in 19 ounces of water. The upper and lower 
wires are then conneéted to form an eleétrical series, the platinum wires in the 
hot acid being the negative, those in the hot alkali the positive elements of the 
upper ends; and the platinum wires in the cold acid the positive, in the cold 
alkali the negative elements at the lower ends of the tubes of the battery. 
The series is thus connected with acid to alkali and alkali to acid. Mr. Gore 
believes that the ele@tric currents produced by the dire@ influence of unequal 
temperature of copper and platinum electrodes, in conducting liquids which do 
