376 Propagation of Pulses in Elastic Media. 
the particles in which a pulse originates must be great in order to 
produce the sensation of sound in the most delicate ear. We 
also know that in the case of the heavier sounds, as the report of 
cannon, the condensation of the particles in which the pulse 
originates is very intense. 
his view of the subject may throw some light upon the dis- 
crepancy between the theoretical velocity of sound, as determin- 
ed by Newton and others, and its real velocitysas ‘found by ex- 
periment. 'The velocity according to that theory should be about 
944 feet in a second; varying slightly from this according to the 
state of the barometer, thermometer, and i gener 
Roberts 300 
Boyle, F : : 1200 
The velocities found } Merse . 1474 
by experiment, by Fikiwsoed and Halley, ‘ 1142 
Florentine Academy, . 1148 
French Academ 
my; 
Newton adopts the smallest of these experimental velocities, 
viz. 1142 feet per second, as the true practical velocity of sound ; 
and to this he reconciles his theory ; in part, by a hypothesis that 
the ag Soneieits to a certain extent, of solid particles, throug 
which the pulse is transmitted instantaneously ; and in part by 
aie: hypothesis, that the pulse does not give motion to the 
foreign matter which the air contains, and so is transmitted so 
much the faster through the true air as there is less of it in a giv- 
en space. This explanation of the matter has not been satisfac- 
tory to those who have followed Newton in investigating this 
peg They have justly thought that such causes mig 
loss, the propelling force derived from the reéxpansion of the pat- 
ticles must be not less than their retarding force in being com- 
pressed ; and in order to this, whatever ‘heat is evolved during 
their compression, must be reabsorbed in their reéxpansion. If 
then any dime is required either for the evolution of the heat, or 
for its reabsorption, so that the specific heat of the particle does 
hot instantaneously conform to the change of density ; or if any 
the heat evolved is radiated and lost, so as not to 
Present to be reabsorbed, then the evolution of heat must tee 
Otherwise it ‘cannot affect the velocity of its or 
