Wave-lengths and Velocities of sound in Gases. 427 
lengths of different notes. The tubes, forming any one of these 
series, are used with the organ-pipe and resonator correspond- 
ing to their note; and as they are successively adapted to the 
resonator, they cause the serrations of its flame successively to 
coincide with and to bisect those of the organ pipe flame. 
Students after such exhibitions do not depart from the room 
with their usual skepticism as to the existence of an acoustic 
wave-length, but look upon the tubes as measurés of actua 
entities, 
3. The Acoustic Pyrometer—Having devised this simple 
arrangement of apparatus for measuring the number of wave- 
lengths contained in a given tube, the idea occurred to me that 
T could use the method in determining the variation in the num- 
ber of wave-lengths contained in this tube, caused by a change 
in the temperature of the air which it contained; and thus 
Succeed in readily determining any temperature to which the 
tube might be exposed. 
The accuracy of this (as far as I know) entirely new method 
of pyrometry, and the facility of its application can be judge 
of by the following discussion. 
The formula v= jon (1+ at) id gives the velocity of 
sound in air of a known temperature. This formula, as is well 
known, is reduced numerically to V= 333™ / 1+ 00367 én 
which, V=the velocity of sound at the temperature ¢ centigrade ; 
338= the velocity of sound in meters, at 0° C.; and 00367 the 
coefficient of expansion of air under a constant pressure. We 
will suppose that we have outside of the furnace, whose tem- 
perature we would measure, an UT’, organ pipe; that we have 
laced opposite its mouth an UT, resonator; and that tubes 
rom pipe and resonator lead to contiguous gas jets placed before 
the revolving mirror. We will also assume that the air in and 
around the organ pipe is at 0° C., and that the serrations of the 
ames of pipe and resonator are brought to coincide when 13 
meters of metal tube, connecting the resonator with its mano- 
metric capsule, are placed in a furnace which also has the tem- 
ature of 0° ©. Therefore the length of a wave in the 
urnace tube is 3% =0™-65, and it will contain 20 wave-lengths. 
Now gradually raise the temperature of the furnace to 820° C. 
As the temperature rises we will see the serrations of the 
resonator flame gradually slide over those of the organ pipe 
flame, and when the temperature has reached 820° C. we will 
have observed that the serrations of the resonator flame have 
glided over 10 times the distance separating the centers of two 
contiguous serrations of the flame of the organ pipe: for at 
