, ESSAY ON THE VELOCITY OF LIGHT. 137 
time by any means whatsoever, we have only to divide double the distance A B 
by the time found in order to obtain the velocity of the light. 
In proceeding as we have just indicated, no matter how great may be the 
distance of the two lamps A and B placed on the surface of the earth so that 
either may be seen from the place where the other is stationed, we always find 
that the time elapsed between the instant the screen is placed before the light 
A and the instant when the light B disappears to the observer stationed at A is 
absolutely inappreciable. It appears as if the extinction of the light of each 
of the two lamps was perceived at the same instant, without any delay, at 
the station of the other lamp; that is to say, as if the velocity of light was 
infinitely great. This happens because the velocity of light, although not 
infinitely great, yet is of a very high value; light going over in one second 
about three hundred thousand kilometres, (about 190,000 miles,*) or 74 times 
the circumference of the earth; it therefore does not employ, in the experi- 
ments we have just explained, to go from one lamp to the other but a fraction 
of a second, absolutely inappreciable by the ordinary means of observation. 
Every one has observed that sound requires a certain length of time to come 
from a distance. If we observe afar off a wood-cutter strike with his axe the 
trunk of a tree, we see each blow some time hefore we hear the sound which 
it occasions. On account of the extremely high velocity of light we can say 
that we perceive the blow at the very instant it is given by the wood-cutter; 
the interval of time which elapses between the instant when we see the blow 
and the instant when we hear it, is therefore the time employed by the sound 
to come from the tree to the place we occupy. This time, which is very appre- 
ciable, is so much the longer as we are at a greater distance from the wood- 
cutter. If we remove ourselves to such a distance that the interval is exactly 
one second, then we will have but to measure the distance to the tree in order 
to have the velocity of the sound. Instead of that, we can proceed as we indi- 
cated for light, by replacing the lamps A and B by cannon whose discharge 
we can instantly bring about. It is in operating thus that we have found that 
sound runs over 333 metres in one second of time.t There is the greatest 
analogy between the progressive transmission of light to adistance and the 
progressive transmission of sound through the atmosphere; only light is propa- 
gated with a velocity incomparably greater than sound. The clear conception 
of the progressive transmission of sound to a distance which we have arrived 
at from the preceding considerations renders it very easy to conceive the pro- 
gressive transmission of light; and in order to understand clearly all we are 
going to say of this last phenomenon we cannot do better than refer to the 
analogous phenomenon of sound, which is directly accessible to our senses. 
*Tn reducing the French measures of length to English, we have adopted Captain Kater’s 
value of the'metre, the French unit of length, as equal to 39.37079 English inches, or 3.280899 
English feet, or 0.6214 of an English mile. The brass scale made by Troughton for the 
United States Coast Survey has been declared the standard of the United States, and the 
value of the metre expressed in United States standard inches is given in the Coast Survey 
Report, far 1853, as equal to 39. 36850535 United States standard inches, or 3.28070878 
United States feet. Prof. Bache, in 1856, found that the Britisk bronze standard yard, No. 
11, is shorter than the American yard by 0.00087 inch.— Translator. 
+ It may be stated, in round numbers, that sound, in dry air and at the freezing tempera- 
ture, travels at the rate of 1,090 feet, or 363 yards, per second, and every additional degree 
of atmospheric temperature. on Fahrenheit’s scale, adds 1.14 foot to that velocity, so that at 
62° Fahrenheit (which is the standard temperature of the British metrical system) it runs | 
over 9,000 feet in’8 seconds, 122 British miles in a minute, or 765 miles in an hour, which 
is about three-fourths of the diurnal velocity of the earth’s equator. Hence, in latitude 424, 
if a gun be fired at the momenta star passes the meridian of any station, the sound will 
teach any other station exactly west of it at the precise instant of the same star’s arriving on 
tts meridian.—Sir J, Herschel. 
